Physics; The Basics
Radiation passes heat on as an electromagnetic wave called infra-red radiation. Infra-red is often called heat radiation or thermal radiation. All the heat from the Sun reaches us as electromagnetic radiation. (We will look at the electromagnetic spectrum in another topic.) You can feel the infra-red from the Sun. If you concentrate the rays of the Sun using a magnifying glass, you can set fire to paper.
Thermal radiation behaves just like light:
- It travels in straight lines;
- Its speed is 300 million m/s in a vacuum;
- It can be reflected and refracted.
- Thermal radiation heats objects up by making the molecules move faster.
Our eyes cannot see infra red, but a digital camera can. Here is a picture of a hot plate that appears much brighter than it actually is because of the infra red radiation.
All objects can absorb or emit radiation. Infra red cameras pick up radiation emitted by hot objects. This allows wildlife cameramen to film animals at night. The animals cannot see infra-red, so are not disturbed by bright infra-red lights. The images are in black and white; colours cannot be seen.
Burglars are detected by security cameras because they give off infra-red. They cannot prevent that. A burglar giving off no heat is dead; dead burglars are not a threat.
The hotter the object, the more the radiation it emits. A heat sensitive camera can show hot spots. The cat’s nose in the picture below is the hottest part. It is quite warm in the cat’s ears as well.
You can also use a heat sensitive camera to show where heat is escaping from a house.
Dark (especially black) surfaces absorb infra-red well. Dark coloured cars get hot in the sunshine.
White and silver surfaces reflect radiation.
This experiment is called Leslie’s Cube. Hot water is poured into the cube, and heat passes from the sides by radiation, which is picked up by a detector called a thermopile. The thermopile is connected to a galvanometer, a very sensitive ammeter which can detect tiny currents. The more infra red that is given out, the further the galvanometer moves across.
Another experiment you will have seen is this one where infra red is shone onto a silver surface and a matt black surface at the same time.
Some uses for Infra Red
TV remote controllers use an infra-red LED. You can see it flashing if you shine it into a digital camera. Old remote controllers used ultrasound, which had the unfortunate effect of sending the cat up the curtains when you changed channels.
The shape of an object affects the rate at which it absorbs or emits heat. The heat-sink in an electronic circuit is:
- made of metal so that it conducts well;
- fin-shaped so that the area for cooling by convection is increased;
- painted black so that heat can pass by radiation.
Dull black surfaces absorb more radiation than white shiny surfaces. White surfaces reflect radiation. Some important uses:
- Cooling fins on heat exchangers are best coloured matt black to emit more thermal radiation.
- Houses in hot countries are often pained white to reflect thermal radiation.
- A fell-runner suffering from hypothermia (has got too cold) is wrapped up in a space blanket which is made of highly reflective foil.
- Thermal radiation is an important factor in global warming. The ice on the poles reflects a lot of heat while the dark blue ocean absorb a lot of heat. Loss of the ice caps will make global warming more serious.
Kinetic theory is about how molecules move about and change state from solid to liquid to gas and back. Kinetic comes from the Greek word “kinein” which means “to move”.
Water, H20, is one of the most common substances in the universe. On planets that are close to their stars, it exists as steam, a gas. On distant planets it is a solid, ice.
For example, Saturn’s Moon, Titan, is thought to consist of water ice, with a dense atmosphere of nitrogen, with rivers and lakes of liquid methane (CH4). At the temperatures found, the water ice is as hard as rock. Volcanic activity occurs, with liquid water as the lava.
The liquid water will rapidly freeze, to form a hard rock.
On Titan the temperature is -170 oC. Although it is cold, there is weather. It rains methane.
Fortunately for us on Earth, water is in a state that allows life to exist. It can exist as a solid, a liquid, or a gas. These states are called phases. Liquid water is essential for life.
A solid has long range order. The molecules cannot move about. However the bonds in a molecule can vibrate. They vibrate more if the temperature is higher. If the molecules vibrate enough, the bonds between molecules break, and the long-range order breaks down. We say that the substance is melting.
The material changes phase and becomes a liquid. It melts.
Small groups of molecules can move past each other. A liquid adopts the shape of its container. If there is no container, it will spread out into a thin layer. In the liquid phase, some molecules can escape. The liquid evaporates. In very volatile liquids, lots of molecules escape. The liquid evaporates quickly. If you spill some ethanol on the bench, it disappears quickly.
If we continue to put energy into the liquid, the molecules move about faster. Also intermolecular bonds break more, and single molecules are given off as a gas. All liquids have a boiling point at which the liquid becomes a gas.
The molecules are now on their own. The space between molecules is large, and the molecules are free to move about. They collide against the walls of the container.
The force from these collisions is called the pressure. Since there is a lot of space between molecules, the gas can be squashed together, or compressed. If we compress the gas enough, we can actually make it liquid again. Under extreme conditions, hydrogen, H2, can exist as a solid metal.
Some materials, for example carbon dioxide, can change state from solid to gas without passing a liquid phase. This is called sublimation.
Notice that the temperature does not change at all while the substance is changing state. The energy used up in changing state is called latent heat, and it’s the heat required to turn 1 kg substance from solid to liquid, or liquid to gas. You are not expected to know anything more than that; you will study it in detail at A2 level.
When a gas turns back into a liquid, it condenses. When a liquid turns to a solid, it freezes. Both changes of state give out energy, as does a gas, or liquid as it cools. Gaseous water (steam) can hold a lot of energy, which makes it useful for a steam engine.
Water freezes at 0 oC. At that temperature, it can exist in all three states, solid, liquid, or gas. Hence 0 oC is called the triple point of water.
The boiling point of water depends not just on the temperature, but also on the pressure. At normal atmospheric pressure (1 atmosphere) it boils at 100 oC.
When you go up a mountain or in an aeroplane, the atmospheric pressure falls. That is why your ears “pop” as you climb.
The pressure in a gas is the result of movement of gas molecules. Gas molecules move about randomly. They collide with each other, and the walls of the container. They move at a range of speeds, about 300 to 500 m/s.
The movement of molecules in a substance results in the material having an internal energy. The higher the internal energy, the higher the temperature.
Objects get hot if heat energy (thermal energy) passes into them. We know they get hot because the temperature rises. It is important that we know the difference between temperature and heat:
- Temperature – the measure of how hot something is, measured in degrees Celsius;
- Heat – a flow of energy from a hot object to a cold object. Heat is measured in joules (J).
Temperature is not heat. Temperature represents internal energy, the energy that is the result of molecules vibrating.
Be careful. A big heat flow does not mean a high temperature. If we heat up a beaker of ice, there is a big heat flow, but the temperature does not rise above 0 oC until all the ice has melted.
Heat always flows from hot to cold. Heat does not flow from a cold object to a hot object. For example the heat from a hotplate passes into a cold pan to heat up the water in the pan.
The greater the temperature difference between the hot object and the cold object, the greater is the flow of heat.
How Heat Flows
There are three methods for heat to flow:
If you put a metal bar into a candle (or bunsen) flame, it gets hot quickly. Soon you can’t hold it. If you put a glass bar into a candle, it won’t get too hot to hold. But if you touched the end that was in the flame, you would find that it was really hot!
The process in which heat passes through a solid substance is called conduction. Metals are good conductors of heat. Non-metals are generally bad conductors of heat. Liquids and gases are bad conductors of heat as well. A bad conductor of heat is called an insulator.
Your duvet traps air which is a good insulator.
Conduction works like this. You will know that atoms and molecules in a solid are in a fixed lattice like this. They are bound with bonds (like springs). The molecules vibrate about a central point.
If we apply heat, the molecules vibrate with bigger vibrations and set their neighbours vibrating with bigger vibrations. These pass on the vibrations to their neighbours in turn. The bigger the vibrations, the hotter the material. If the vibrations are passed on easily, the material is a good conductor. (In reality the situation is more complicated than this, but this is all you need to know at this level.)
The better the conductor, the quicker the blue on the thermometer strip changes from dark blue to yellow.
Convection occurs only in liquids and gases. We call liquids and gases fluids. It cannot happen in solids. It needs particles to be free to move about.
When a liquid is heated, the molecules at the bottom move about with bigger vibrations. They take up more space which means that the density goes down. The less dense fluid rises. It gives its energy to the fluid above, and cools down. It becomes denser and falls back to the bottom. A convection current is set up. You can see this happening in a pan of peas.
A radiator in a room heats up the room by convection (NOT radiation).
You can see how the hot air expands, and rises to the top of the room. It loses its heat to the ceiling and becomes dense. It then falls back towards the floor. In very tall rooms, you can have a fire belting out the heat, but you still feel cold. All the hot air is at the top of the room.
We have looked at the mechanism of radiation in Topic 1. All hot objects transfer heat by radiation. They give out infra red radiation, which we can feel with our skin, or observe using an infra-red sensitive camera. Black objects emit and absorb infra red radiation well. White objects reflect infra red.
If three young men are competing in a running race.
Each young man’s body temperature will rise as he runs. It is important that the body does not get too hot, otherwise enzymes will stop working which could be fatal to the athlete. The body detects the rise in temperature as the athlete runs. To prevent that, sweat gland secrete water onto the surface of the skin. It takes energy to evaporate water, and the outward energy flow cools the body.
If the athlete does not have enough water, the sweat glands no longer produce water, and the body can overheat. It is vital for the athlete to keep hydrated.
The young man to the front of the picture is quite skinny. He has a low body mass, less than 50 kg. He also has quite a large surface area. When he stops running, he will lose heat quickly. It is possible that he could over-cool, and risk hypothermia. An aluminium blanket could be used to reflect the heat that he has lost through radiation.
His friend is less skinny, with a body mass of about 65 kg. The second athlete has a slight lower surface area to volume ratio, so he will lose heat less quickly.
Let us look at how volume and surface area are related. Consider a cube of 1 cm in every direction.
The volume is 1 cm3. The area is 6 cm2, so the surface area to volume ratio is 6:1. Now we double the size.
The volume is 2 × 2 × 2 = 8 cm3. The area of each face is 4 cm3 so the total area is 2 × 4 = 24 cm2. Therefore the surface area to volume ratio is 24:8 = 3:1.
The shape with the lowest surface area to volume ratio is a sphere.
The shape and size of animals is important in whether they can survive in cold climates. Big animals have a lower surface area to volume ratio, so they lose heat less than smaller animals. The polar bear has the following features to enable it to survive in the cold Arctic:
A large body size (it has a mass of up to 800 kg);
- Hollow hairs to enable heat to be trapped;
- Light guides in the hair to pick up what little infra-red there is;
- Black skin to absorb the heat;
- Small ears that lose little heat.
This animal is a Fennec Fox that lives in the desert. The background is sand, not snow.
- Heat flows from hot objects to cold objects.
- Heat flow depends on the temperature difference.
- Heat is not temperature.
- Heat passes by conduction, convection, or radiation.
- Radiation is caused by infra-red radiation
It costs a lot to heat a house. And it is getting more expensive year-on-year. Here is a typical house, and we will look at the various measures that we can take to reduce heat loss, and reduce fuel bills.
Even in an old house it is possible to improve the insulation.
Building engineers use the U-value to measure the effectiveness of the insulation. The U-value measures how well a building component like a wall, a double glazed window, or a door keeps the heat inside a building. In hot countries, it indicates how well it keeps heat out; people want their houses to be cool. The lower the U-value, the better the insulation.
the energy in watts that flows through a building component of 1 square metre across a temperature difference of 1 Kelvin.
A temperature difference of 1 Kelvin is the same as a temperature difference of 1 degree Celsius.
The units for U-values are Watts per square metre per Kelvin (W m-2 K-1).
Consider this wall which has a high U-value.
This wall will let a lot of heat out. This will give rise to a number of problems:
The fuel bills will be very high;
The wall is likely to become damp, especially in an old house, where the damp proof course may be ineffective or non-existent;
The cold wall will cause condensation, especially if there is cooking going on in the kitchen.
Condensation causes moulds and health problems.
Condensation causes decorations to deteriorate and become unpleasant.
Poor people often have poorly built nineteen-sixties houses are difficult to heat, and they cannot afford the heating either. The cold houses suffer badly from condensation, leading to many health problems.
The U-value can be lowered by adding a layer of insulation, covered with plasterboard. Builders call this dry-lining. It is effective and not very expensive to install. However it does reduce the size of the room.
Using building components with low U-values (i.e. good) has these advantages:
- It saves money
- It helps preserve the environment
- It is good for your health
- It gives better indoor comfort.
The house built with low U-values uses a lot less energy and the energy bills are less.
Components with low U-values increase the temperature on the inside. This prevents growth of mould and fungus, and improve the health of the people living there.
A good U-value for a wall is 0.2 W m-2 K-1 and for a window, it’s 1 W m-2 K-1.
The physics of heating homes depends on the heat transfer processes of conduction, convection, and radiation.
The earliest form of heating was the open fire, which passed most heat by radiation. Convection took the smoke up the chimney, and a good amount of the warm air from the room. Open fires are very inefficient. Having the fire enclosed in a stove is more efficient. The room would be heated by convection. The hot stove would also pass heat by radiation. Stoves have the advantage that:
They use a range of fuels; The fuels can be obtained quite cheaply and from sustainable sources.
The disadvantages are:
- They have to be lit;
- They are messy, because they produce a lot of ash. Wood-ash can be used on the garden, but coal ash cannot.
In modern houses it is possible to have permanently mounted gas or electric heater in each room. The gas heaters have a flue (exhaust) that goes through the wall of the house. These have the attraction that you only have to pay for heating one room, rather than every room in the house.
Central heating driven by a boiler was introduced in the late nineteenth century. The heated water circulates by convection to radiators in the rooms. In modern installations, the convection is assisted by a pump which assists the convection. Modern central heating systems are powered by gas, oil, or (more rarely) solid fuel.
Central heating systems have two functions:
- to heat the rooms;
- to heat the hot water for sinks, wash-basins, and the bath.
Many modern houses have no hot-water cylinder. Instead the combi-boiler heats the water instantly, ready for use.
Modern boilers extract more heat by condensing water from the flue-gases. The condensed water is drained away by an external pipe. Unfortunately, in many houses, the drain-pipe froze in the very cold winters of 2009 and 2010, making the boilers shut down.
Electric central heating works by using night-storage heaters.
These work by electrical elements heating up large storage bricks. The elements are switched on during the night when electricity is cheap. They store the energy as internal energy, releasing it as heat during the day.
The advantages are:
- It’s cheap to install;
- Little maintenance is needed.
The disadvantage is that:
- The heat is released during the day when most people are out of the house;
- The house is cold in the evening.
- Many rural areas have no gas supply. Therefore the choice is between oil and electric central heating.
Domestic heating bills are getting more and more expensive. The prices go up like a rocket when fuel is expensive, but drift down on a parachute when the fuel is cheap. The Government defines fuel poverty as being when a household spends more than 10 % of its annual income on fuel.
Ask your parents about your family fuel bills.
Many people are now putting solar panels on their houses. These can be used to:
- Generate electricity;
- Heat water.
In the Summer the Sun delivers energy (its intensity) at a rate of 550 watts per square metre. However solar panels are not very efficient. They convert no more than about 10 % of that energy into electricity or heat for heating water. That means that a 1 m2 solar panel can capture about 55 watts. However there are some other things to think about:
A large solar panel array on a roof can give out 3 kilowatts (3000 watts) peak. That means when the Sun is at its highest on a brilliantly sunny day in high Summer.
If the Sun is lower in the sky, the power is rather less.
If it’s cloudy, little energy is harvested from the array.
In Winter, when you need the heat or electricity the most, the panels produce least.
To get the best results, you need a south-facing roof.
You can have arrays of batteries to store the energy.
Photovoltaic panels are very expensive. Government subsidies have been available for people to pay for these arrays, and for the excess electricity these produce. However these have recently been cut.
A householder may earn about £500 a year from the electricity sold to the national grid. We can work out the time taken for the householder to get his or her money back. This is called the payback time.
Payback time = cost ÷ money saved each year
A solar panel can be made to heat water. You can make one yourself from recycled materials. An old central heating radiator painted black and mounted in a glass-fronted box will make a reasonable solar heater. The the hot water from the solar heater is pumped to the hot water cylinder. However it takes a lot of energy to heat water.
Water can hold a lot of energy because it has a high specific heat capacity.
the amount of energy to raise 1 kg of water through a temperature difference of 1 degree Celsius.
The formula is
Energy (J) = mass (kg) × specific heat (J kg-1 K-1) × temperature change (K)
You will notice from the formula that the temperature change is in Kelvin (K). It can also be written in oC as a temperature change of 1 K = temperature change of 1 oC.
1 K is not written 1 oK.
In Physics code, the formula is written:
E = mcDq
(E – energy in J; m – mass in kg; c – specific heat capacity in J kg-1 K-1; Dq – temperature change in K or oC)
The strange looking symbols are:
- D – “delta”, a Greek capital letter ‘D’, meaning “change in”;
- q – “theta”, a Greek letter ‘th’, meaning “temperature”.
Water has a very high specific heat capacity. This enables it to carry a lot of energy and is useful in:
- cooling systems;
- keeping food warm;
Some substances have higher specific heat capacities than water:
Substance Specific Heat Capacity (J kg-1 K-1)
- Ammonia 4700
- Lithium 4400
- Hydrogen 14300
No, that last figure is NOT a typo. This makes hydrogen a very useful coolant in a power station generator.
When we do a job of work, we transfer energy. Some energy forms can be stored, while others cannot.
Electricity cannot be stored as such. A battery stores energy as chemical energy. The chemical reaction produces electrons as it proceeds. When the current is turned off, the reaction stops. When the chemical reaction is complete, the battery goes flat.
The current produced by the chemical reaction can be used to light a bulb or run a motor. We can trace an energy chain, which you will have done at Key Stage 3. In this case:
chemical ⇒ electrical ⇒ kinetic
The Law of Conservation of Energy.
This law is an important rule of Physics:
Energy can neither be created nor destroyed. It is turned from one form to another.
What this means is that you cannot get something for nothing. Nor can you lose it. Every joule of energy has to be accounted for. This is what this topic is about.
Useful Energy and Waste Heat
Think of a light bulb. For every 100 J of energy contained in the electricity:
- 98 J is lost as heat;
- 2 J is turned into light.
- In a car, 100 kJ of energy comes from the petrol. Of this:
- 60 kJ is lost as heat through the radiator and the exhaust;
- 40 kJ is used to move the car.
- Of that 40 kJ:
- 35 kJ goes to driving the wheels
- 0.5 kJ to power the electrics;
- 1 kJ to power the steering;
- 3.5 kJ to power the air conditioning.
If you add up all the numbers, you will find that they add up to 100 kJ.
Of the 35 kJ to drive the wheels, some will be lost in friction. Less than 35 % of the energy we put into the car actually ends up in moving the car along the road. However none of the energy has been destroyed. It has simply been turned into other forms of energy.
So what happens to the energy that a car uses to go along the road? It is used to:
- overcome friction from the road surface;
- overcome drag from the air;
- to provide kinetic (movement) energy for the car.
So most the energy to make the car move is being turned into heat from friction.
The Sankey diagram is a way of showing the way the energy is lost, and how much energy is lost. It is quantitative, which means that it is drawn to scale.
The arrow pointing to the right shows the useful energy. The arrows pointing downwards show where the energy is being lost.
And when the brakes are applied to slow down, the kinetic (movement) energy is turned into heat. You can see the brakes of this racing car glowing red hot.
All the of energy from the petrol we use on a journey is eventually turned into heat. On the way we have diverted a little of that energy to do a useful job for us, namely to go somewhere!
Whenever we have any machine to do a job of work, some energy is wasted. This traction motor from an electric locomotive needs to be cooled by a fan, otherwise it gets hot. The cold air comes in through the square pipe and leaves through vents at the far end of the motor.
Any energy that is not useful is wasted. It is possible to harvest some of the waste energy to make it useful, for example:
the heater in a car keeps us warm on cold days;
the turbo-charger uses hot exhaust gases to pump extra air into the cylinders.
You can see the huge amount of waste heat (and smuts) coming from this plane as it takes off.
This waste heat is simply heating up the air.
Low Grade Heat
There are ways of recovering waste heat. Many devices have heat exchangers which use waste heat going out to warm up air or water going in. In the end all energy ends up as low grade heat. The further along the energy process, the lower the grade of heat. This low grade heat is hard to extract energy from.
Reducing the Waste
One way of reducing wasted energy from brakes is to use regenerative braking.
This idea has been used with electric trains for many years. When this locomotive goes down a hill, its motors act as generators and puts current into the wires. This provides extra current for locomotives going up the hill in the opposite direction.
You can’t do that with a petrol or diesel engine, but there are now hybrid vehicles available.
No energy converting process ever gives out as much energy as is put in. You always have to put in more energy than you get out. An efficient machine is one that uses as little energy as possible to do a particular job. Little energy is wasted.
We can measure the energy efficiency of a device using this simple equation:
Efficiency = useful energy output
total energy input
Efficiency can be expressed as a fraction, a decimal, or a percentage. Often the efficiency is given as a percentage:
Efficiency = useful energy output × 100 %
total energy input
You cannot ever get more than 100 % efficient.
A diesel engine gives out 350 J of kinetic energy for every 1000 J of chemical energy put in. It is 35 % efficient. A petrol engine gives out 300 J of kinetic energy for each 1000 J put in. It is 30 % efficient.
The energy efficiency is always a fraction less than 1, which is multiplied by 100 to give a percentage. (If you get 2/3 correct in your exam, you get 67 %.)
Examples of efficiency:
- Steam engine 10%
- Car 35%
- Power station 45%
- Electric motor 70%
- Transformer 95%
You never get devices that are 100 % efficient.
No machine is ever 100 % efficient. If it were, it would be a perpetual motion machine. Some toys are sold as “perpetual motion machines” but actually have a small battery to power the machine (usually through a small electromagnet). If the battery runs out, the machine stops.
Electrical transformers are among the most efficient machines, about 95 %.
Why should we worry about waste heat?
There are currently several pressing issues:
- Global warming with all its related issues;
- Rising price of oil (Petrol is £1.50 (€1.80) a litre and may well rise further);
- Dwindling energy resources;
- Pollution and environmental damage.
We need to think through ways in which we use energy, especially in transport:
Cars (especially 4 x 4 vehicles) use a lot of energy. May be there should be more effort to move to hybrid vehicles. Super fast top speeds and accelerations are possibly luxuries we cannot afford any longer.
More travel by public transport, although many people dislike buses and there are many places in this country that are no longer accessible by rail.
How much longer will cheap flights be available or acceptable?
Electric vehicles are quiet and non-polluting. But we have to generate the electricity somewhere, and that does cause pollution. We need to think about ways of generating electricity:
- Coal-fired power-stations cause pollution and acid rain;
- Gas-fired stations are running out of gas. They also emit greenhouse gases;
- Wind farms cause no pollution, but don’t generate on a calm day;
- Nuclear powers stations do not give out greenhouse gases, but nuclear waste is dangerous.
We have to think about the efficiency of the energy process.
The Wider Picture
It’s essential to use less energy, as there are serious problems affecting the environment, such as global warming. Various small things can help:
- Switching off appliances that are not in use;
- Turning heating down;
- Walking or cycling for short journeys instead of using a car;
- Going by bus or train for longer journeys;
- Insulating our homes.
If everyone did these, large amounts of energy could be saved.
We can make our homes use less energy by:
- Installing double glazing;
- Insulating the loft;
- Using energy-saving light bulbs;
- Cavity wall insulation;
- Draught-proofing doors and windows.
Each of these will cost us money, but we will get the money back in the savings we make. This is the payback time:
Payback time = cost of insulation ÷ saving made per year.
Loft insulation may cost £250 to install, and saves £50 a year on heating bills. The payback time is 250 ÷ 50 = 5 years.
When we put energy saving measures into our homes, the argument about payback time is not the only thing we need to consider. Energy is used by the factories that make the products in the first place.
Also some draughts are needed, otherwise our homes would have a stale atmosphere and running gas appliances could be dangerous, because carbon monoxide could build up. Carbon monoxide is a highly toxic gas, which has no smell. Exposure to it can be rapidly fatal.
Energy saving bulbs might give out more light and less heat, but they are ugly and the light given out is like that in an office or classroom. Additionally they require much more energy to make, they are expensive, and contain toxic materials like heavy metals. Also an ordinary light bulb helps to heat the room, so less heat is needed from the central heating.
- Energy never created nor destroyed; it is turned from one form to another.
- When we do useful work, some energy is always wasted as heat.
- This heat goes to warm up the surroundings.
- Eventually all the energy ends up as low grade heat.
- Low grade heat is hard to get further energy from.
- The fraction of energy that does useful work is the efficiency.
- We never get 100 % efficiency.
Why is Electricity so Useful?
We take electricity for granted in our homes. It is very easy to plug in an appliance or switch on a light. Electricity is clean, and, if used properly, safe. It can do all sorts of jobs that other energies can do, and more. It is possible to heat and light with gas. It could be possible to use a gas-powered hoover, but a TV or computer can only work with electricity.
To do a useful job for us, electrical energy has to be converted into other forms of energy. For example:
Electrical Energy ⇒ Movement (Kinetic) Energy
Electrical Energy ⇒ Sound Energy
Energy Transformations in Everyday Appliances.
This motor operates at a voltage of 230 V and at mains frequency of 50 Hz (50 cycles per second). Its power is 250 watts. That means it turns 250 joules of electrical energy into movement energy every second.
The formula that gives you the right answer is shown in the box below:
energy transferred (kWh) = power (kW) × time (h)
In Physics Code:
E = Pt
Often appliances have their power marked in kilowatts (kW):
1 kW = 1000 W
1 W = 1/1000 kW
So our 250 W motor will have a power of 250 ÷ 1000 = 0.25 kW
How much does it cost to use an appliance?
When we work out the cost of using an appliance, we pay for the energy that has been used. We could pay for the number of joules used, but the joule is only a small unit. So we need a bigger unit. This is called the kilowatt-hour (kWh) or unit.
- 1 kilowatt hour is the amount of energy used by a 1 kW appliance running for 1 hour.
- To work out the amount of energy used by an appliance:
- Work out the power in kilowatts (kW);
- Work out the time used in hours (h);
- Multiply the two numbers together.
The formula that gives you the right answer is shown in the box below:
energy transferred (kWh) = power (kW) × time (h)
In Physics Code:
E = Pt
To work out the cost we simply multiply the number of kilowatt-hours by the cost per unit.
total cost = number of kilowatt-hours × cost per kilowatt-hour
Electrical energy typically costs about 11 pence per kilowatt-hour.
- Appliances convert electrical energy into useful forms of energy
- Power is measured in watts or kilowatts;
- Electrical energy is purchased in kilowatt hours (units);
- energy transferred (kWh) = power (kW) × time (h);
- total cost = number of kilowatt-hours × cost per kilowatt-hour;
Fossil Fuels are the remains of plants and animals that lived many millions of years ago. They captured energy from the Sun while alive. When they died they were buried and compressed to form either coal or oil.
Fossil fuels are very concentrated forms of energy, which means that we can get out lots of energy from a small amount of the fuel. 50 litres (40 kg) of petrol will take a car 800 km.
A 100 kg battery will take a car about 200 km. Although electric cars are marketed as very modern, they were actually on the roads before the petrol car. Some bright spark had the idea of detaching the horse, sticking a battery under the carriage, and running the vehicle with an electric motor. Their range was limited. All in all, a horse was cheaper.
Fossil fuels will run out. They are non-renewable. Although new reserves are being found, there will eventually be a time when there are not enough to meet the demand.
Fossil fuels give out carbon dioxide, which contributes to global warming.
Coal-fired Power Stations
Electricity is made in huge quantities in Power Stations. The most common type of power station is powered by coal.
Coal (1) is crushed in a mill. It is then blown in a stream of air (2) to be burned in a large boiler (3) which produces large amounts of steam. The steam turns a turbine (4) which turns a generator (5). The generator is connected to the step-up transformer by very heavy wires which can carry 100 000 A at 25 000 V.
To make the power station more efficient, the steam is cooled in a condenser (6). The waste heat is carried to the cooling tower (7), where it returns as cool water (8). The clouds you see around cooling towers are not smoke; they are water vapour.
Coal fired power stations have the following advantages:
- They use coal which is relatively plentiful;
- They are less expensive to build and run compared to nuclear power stations;
- They do not produce dangerous waste.
They have disadvantages:
- They need trainloads of coal every day;
- They produce carbon dioxide which is a greenhouse gas, involved in global warming;
- They produce sulphur dioxide which makes acid rain.
Nuclear Power Stations
A nuclear station is identical to a coal-fired station in the way that it uses steam to turn a turbine. The boiler is different and is called a reactor.
The fuel is uranium whose nuclei split by a process called fission. Fission releases lots of energy. 1 kg nuclear fuel is equivalent in energy to 25 tonnes of coal.
- There are no waste gases, therefore no pollution;
- Much less fuel is needed;
- Some reactors can generate material that can be used in other reactors.
- Nuclear waste is dangerous;
- Nuclear power stations are very expensive to build;
- They are even harder to dismantle;
- The radioactive waste remains dangerous for a long time.
Nuclear power remains controversial. There were high hopes that they would produce electricity so cheaply that it would be free. However they did not live up to those hopes.
Although nuclear power stations have a good safety record, there have been some well-known accidents such as Chernobyl. In this case an unauthorised experiment was carried out to find out what would happen in a worst case failure; they found out. The reactor “ran away” and there was a chemical explosion that blew the lid of the reactor and tipped it on its side, spilling out a large quantity of radioactive muck.
Another major accident occurred at Fukoshima Daiichi (Fukoshima No 1), in which the reactors shut down in response to an earthquake. As it was cooling, a tsunami swamped the emergency diesel generators and the reactors overheated.
It may be that nuclear power is used in the future when fossil fuels run out. There will be a lot of debate about it.
Alternative Energy Sources
These are often called renewable or sustainable energy sources. They do not depend on fossil fuels. They mostly capture energy from the Sun as it arrives.
The UK is one of the windiest places in Europe. Therefore it makes sense to tap energy from the wind. However lots of turbines are needed to replace one conventional power station. Also on a calm day, they don’t work.
Large numbers of wind-farms are being built, many out at sea. Although wind turbines are clean, many people don’t want them in their area.
The energy from falling water can produce a lot of electricity in a hydroelectric power station.
Switzerland generates most of its electricity by hydroelectric power.
These power stations can only be built in hilly areas with lots of rainfall. Big dams need to be built, which is every expensive. Large reservoirs destroy large areas of countryside and wildlife habitats. The failure of dam is a major disaster.
Many people are now setting up small hydro-electric schemes that might provide the energy needs of a house. That is nothing new. The engineer and businessman, William George Armstrong, built one such installation in his new house at Cragside in Northumberland in 1890. There are some small-scale hydro-electric power stations that take water from a river or a millpond.
Pumped storage schemes use large amounts of electricity at night to make the generators act as huge motors to pump water back into the reservoir. At times of peak demand they can quickly reverse to allow water to drive the turbines as generators.
The rise and fall of the tides are driven by the moon. Large estuaries can be dammed and the flowing water can be used to drive a turbine.
Solar panels capture energy directly from the sun to be used in two different ways:
- to heat water in pipes (you often see them on roofs);
- to generate electricity with photovoltaic cells;
A solar-powered car uses electricity from the Sun to drive a small motor. Some road-signs are lit by electricity generated by a solar cells and stored in a battery.
Solar panels are not very efficient. To make reasonable amounts of electricity, you need large panels.
Crops can be grown to make oil or alcohol which can in turn be used to run an engine.
Some rapidly growing trees can be harvested to burn as fuel to heat water.
Biological waste (e.g. food scraps and the material you flush away when you sit on the lavatory) decompose to form methane gas, which can be burned in machine like a large car engine which drives a generator. Or the gas can be used to cook with.
This uses energy from hot rocks below the Earth’s surface. Water is pumped down to the rocks and turned to steam. It then comes through another pipe to drive a turbine. Waste heat is often used to heat people’s houses.
- Most of our energy comes from fossil fuels;
- Coal is burned in power stations to make electricity;
- Nuclear power is used to make electricity;
- Nuclear power produces dangerous waste;
- Fossil fuels will run out;
- Therefore alternatives will need to be found;
- Alternative (renewable) energy sources include wind, hydroelectric, tidal, solar, biofuels, and geothermal power.
How do we get electricity to our homes?
Electrical energy cannot be stored as electricity. In a battery it is converted to chemical energy. We are constantly using electricity, so it has to be constantly generated. Electricity is generated in large quantities in power stations.
Electricity is passed from the power station to our homes through a complex network of wires called the National Grid. When you see pylons, these are part of the National Grid. The Grid is constructed in such a way that if one line of wires (a transmission line) fails, other lines can take over so that the electricity supply to our homes does not fail.
Generally the system is very reliable, although overhead power lines are vulnerable to:
- lightning strikes;
- high winds;
- heavy snowfall.
Underground cables are less vulnerable, but are much more expensive to put in.
Mains electricity comes in to our homes at a voltage of 230 Volts (V). However it is not generated at 230 V because the current needed would be huge. Big currents need heavy cables that get hot. In the power station the electricity is generated at 25 000 V (at a current of 100 000 A).
Outside the power station there are huge step-up transformers that take the voltage from 25 000 V to 275 000 V. The voltage can be as high as 415 000 V in the super grid.
Power = current (A) × Voltage (V)
In Physics Code:
P = IV
As the voltage goes up ten times, the current comes down ten times.
Obviously 275 000 V is far too high a voltage to use, so step-down transformers are used to reduce the voltage as follows:
- Local distribution 33 000 V
- Railways 25 000 V
- Heavy industry 11 000 V
- Light Industry 415 V
- Homes 230 V
The reason for voltage being so high is that the current is lower. Therefore the wires can be thinner, and don’t get so hot. Therefore less energy is wasted. Even so there is a fair amount of energy lost in heating up the wires which heats up the countryside.
Features of Waves
Any wave transfers energy from a point where there is a disturbance. Drop a stone into water, and you will see the waves moving away from where the stone fell in (the disturbance).
All waves have a frequency. This means the number of waves per second. Frequency is measured in Hertz (Hz).
All waves have a wavelength. This is the length of one complete wave, the distance between two successive peaks. It is measured in metres (m).
Waves travel at different speeds as shown in the table:
- EM waves 300 000 000 (m/s)
- Sound in air 340 (m/s)
- Sound in water 1500 (m/s)
- Sound in steel 5000 (m/s)
The kind of wave that we are going to look at transfers energy from one point to another. However only the disturbance travels. The particles move up and down (or forwards and backwards). But they do not travel. We call these waves progressive waves.
Transverse and Longitudinal Waves
A mechanical wave needs a material (e.g. water) to travel in.
A water wave is called a transverse wave. The disturbance is perpendicular to the direction of travel.
However a sound wave is not like a water wave, but travels as a series of pulses of high and low pressure.
Note the following:
- The regions of high pressure are called compressions.
- The regions of low pressure are called rarefaction.
Rarefaction is NOTHING to do with refraction.
A sound wave is a longitudinal wave. The disturbance is parallel to the direction of travel.
The Electromagnetic Spectrum
Electromagnetic waves are transverse waves. They transfer energy from a source as waves. They have an electrical component and a magnetic component, but you don’t need to know the details of them at this stage.
All electromagnetic waves travel at the speed of light.
Speed of light (Physics code c) = 300 000 000 m/s = 3 × 108 m/s
(Many very big or very small numbers in physics are written in standard form or scientific notation. Make sure you know what this means. Ask your physics or maths teacher if you don’t. Make sure you know how to enter standard form on your calculator.)
Electromagnetic waves travel in straight lines.
Unlike other types of wave, electromagnetic materials do not need a material to travel through. They travel in a vacuum, which is why we see light from the Sun, but don’t hear its roar.
Light forms a small part of a large family of electromagnetic waves. You will know how light splits into the colours of the rainbow. The scientific term for this is a spectrum.
You can see that the colours run into each other. There are no distinct boundaries.
The rest of the electromagnetic spectrum is like this as well.
Notice these features:
- The boundaries on the diagram are not distinct. Waves of wavelength 0.01 nm may be called X-rays or gamma-rays.
- The shortest wavelengths are on the left and the longest wavelengths are on the right.
- The most energetic waves are on the left, while the least energetic are on the right.
Therefore the shorter the wavelength, the more energetic the wave is.
Uses of Electromagnetic Waves:
- Long Wave Radio
- Medium Wave Radio
- Short Wave Radio
- FM Radio
- UHF Radio
- TV transmissions
- Heating up food
- Infra red
- Communication in optical fibres
- Remote Controllers
- Ultra violet
- Sun tanning
- Gamma rays
- Scientific research
Note the following conversions:
- 1 cm (centimetre) = 1 × 10-2 m
- 1 mm (millimetre) = 1 × 10-3 m
- 1 mm (micrometre) = 1 × 10-6 m
- 1 nm (nanometre) = 1 × 10-9 m
The boundaries between various radio frequencies are agreed internationally. This is to stop stations from interfering with each other. For example FM radio broadcasts occupy the frequency band 88 to 108 MHz. Above 108 MHz the band is occupied by the aviation industry for communication and navigation aids for aeroplanes.
Note these conversions:
- 1 kHz (kilohertz) = 1000 Hz
- 1 MHz (megahertz) = 1 × 106 Hz
All waves can be:
Although we usually show these effects with light, we can also show them with other wave types, for example, water waves in a ripple tank.
When light strikes a plane (flat) mirror, it is reflected.
We should note the following:
- There is an incident and reflected ray;
- The angle of incidence and the angle of refraction are measured from the normal line;
- The normal line is a construction line that is at 90 degrees to the surface of the mirror.
- The angle of incidence = angle of reflection.
- You must remember to measure from the normal.
If we draw accurately two rays coming from the object and hitting the mirror at an angle. Since angle of reflection = angle of incidence, the two rays will be reflected as shown. We can then extend the rays back. Where the two rays meet, that is where the corresponding part of the image is found.
You can see where this is done at the top and bottom of the image.
There are two points to note about the image in a mirror:
- It laterally inverted. This means that, although the image is the right way up compared with the object, left is swapped with right.
- It is virtual. This means that if you look behind the mirror, you won’t find the image there.
In lateral inversion, left is right and right is left. However the image is the right way up.
In refraction, a ray passes the boundary between air and a transparent material like glass.
When light hits an air-glass boundary, there are three things that happen to it:
- Some light is reflected;
- Some light is absorbed;
- Most of the light is transmitted.
If the light strikes the boundary at 90 o, the ray carries on in a straight line. No refraction occurs. If we shine a ray of light at an angle, we find something a little strange. The ray does not carry on in a straight line as you might expect. Instead it bends inwards. This is called refraction.
Note the following:
- All angles are measured from the normal.
The angle of incidence is greater than the angle of refraction. The ray therefore bends towards the normal.
When the ray emerges from the glass, it bends away from the normal. The angle of refraction in this case is bigger than the angle of incidence.
Refraction occurs because the speed of light in air is greater than the speed of light in glass.
For a prism, the ray diagram is like this, using a ray of monochromatic (single colour) red light. White light is a mixture of colours.
If we use a ray of white light, we see that the light ray gets split into the colours of the rainbow (a spectrum). This is because different wavelengths refract by different amounts.
This is called dispersion.
If we pass waves through a single slit, we observe that the waves spread out due to diffraction.
- If the slit is narrow, the diffraction is more marked.
- The wavelength remains the same.
Diffraction does not need a slit. Waves can bend round a barrier by diffraction. Radio signals can be picked up behind hills for this reason.
The longer the wavelength, the more the waves will diffract.
All waves diffract.
If the slit is less than one wavelength, no diffraction will occur.
A radio can pick up signals even though it’s not in direct line of sight of the transmitter. Immediately behind the hill there is a radio shadow where no signals can be picked up (or they are of very poor quality). We see the same behind the tall building. If we go far enough away from the tall building, we come out of the radio shadow, and we can pick up diffracted waves. The longer the wavelength, the more the waves diffract.
How Electromagnetic Waves Behave
When an electromagnetic wave (radiation) hits a material, it can be:
- reflected, like light in a mirror;
- absorbed, like heat absorbed by a black surface;
- transmitted, or passed on, like light passing through glass.
Sometimes two, or even all three processes can occur. Substances behave differently depending on what kind of radiation is falling on them.
When energy is absorbed by a surface, it heats up. For example microwaves are absorbed by water molecules and warm up. This is how a microwave oven works. Light waves simply pass through water.
A dark painted metal surface absorbs radio and light waves. However X-rays and gamma rays can pass straight through.
Wax can transmit microwaves, but absorbs light waves.
Radio waves can pass through the atmosphere, but longer wave radio waves are reflected by the ionosphere. They pass through walls and our bodies.
These pass through the atmosphere and the ionosphere. Water molecules absorb them and gain energy. In early experiments with powerful radars, birds flying across the beams dropped out of the sky partially cooked.
Absorbed to a limited depth by human skin. Light Strongly absorbed by our bodies, but any heating effect is very slight.
UV light is absorbed. Some ionisation can occur, leading to damage to cells (sunburn) and DNA.
Soft tissues are transparent to X-rays, but they are absorbed by bones. Ionisation can occur, leading to tissue damage.
Pass through virtually anything. They can ionise atoms in our tissues, leading to damage.
The Wave Equation
There are three measured quantities in electromagnetic waves:
- The speed;
- The wavelength;
- The frequency.
They are linked by the following simple equation:
Learn this for the exam:
wave speed (m/s) = frequency (Hz) × wavelength (m)
In Physics Code:
c = fl
The strange looking symbol that looks like an upside-down “y” is lambda, a Greek letter “l”. It is the physics code for wavelength. The other codes are:
c – wave speed (for electromagnetic waves c = 3 × 108 m/s)
f – frequency.
Hazards of Electromagnetic Waves
We have seen that the shorter the wavelength, the more energetic the wave. This can produce hazards, and we must take precautions to prevent these hazards from causing us harm.
- Electromagnetic waves form a large spectrum of which light is a small part;
- Electromagnetic waves travel at 3 × 108 m/s in a vacuum in straight lines;
- EM waves can be absorbed, reflected, or transmitted;
- Their behaviour depends on the material;
- Radiation energy increases as the wavelength gets shorter;
- Energetic radiation has hazards to life;
- Low energy EM radiation can be used for broadcasting and communication;
Nature of Sound
Sound is a mechanical wave. This means that it needs a material to travel in. A sound wave has a source and spreads out from that source. Sound waves arise because of the vibration of a source. One such source is a loudspeaker.
A water wave also needs a material (water) to travel in. A water wave is called a transverse wave.
However a sound wave is not like a water wave, but travels as a series of pulses of high and low pressure.
Note the following:
- The regions of high pressure are called compressions.
- The regions of low pressure are called rarefaction.
- A sound wave is a longitudinal wave.
There are all sorts of different ways of measuring the speed of sound. In air the speed of sound is usually considered to be 340 m/s. In water it is about 1500 m/s, while in steel the speed is 6000 m/s.
Like all waves, the wave equation applies:
Wave speed (m/s) = frequency (Hz) × wavelength (m)
In physics code:
c = fl
The human ear can pick up sounds that have a frequency of 20 Hz to 20 000 Hz (20 KHz). Outside this range we have:
Infrasound (below 20 Hz). This is often used by very large animals (e.g. elephants) for communication over long distances.
Ultrasound (above 20 kHz). This is used by bats for navigation by SONAR (sound navigation and ranging). It is also used by a non-destructive investigation equipment. We will look at this in the next topic.
The human ear is most sensitive to 3000 Hz, the frequency of a human scream. Babies cry at this frequency and alarms sound at this frequency. 3000 Hz sounds are very penetrating and will make you feel very uncomfortable. As you get older, the upper limit of hearing goes down. Young people can hear bats; middle aged people cannot.
Sound needs a material to travel in. It cannot travel through a vacuum.
Properties of Sound
Sound has a number of properties that we can investigate using a CRO.
The microphone turns the sound waves into electrical waves, which the CRO displays on the screen.
There are three main properties that we can display on the screen:
- The frequency (pitch), the number of waves per second.
- The amplitude (volume), the size of the waves.
- The quality (timbre), the shape of the waves.
We will look at how these properties look on a CRO screen. The CRO plots a voltage-time graph with the voltage on the vertical axis and time on the horizontal axis.
The amplitude or loudness (volume) is represented by the size of the wave. The bigger the wave, the louder the sound.
The frequency (pitch) is shown by the number of waves on the screen. The more waves shown, the higher the frequency.
These sounds have the same loudness, but different frequency. A high pitched note has a high frequency, NOT a loud volume.
Quality of Sound
The pictures above show sine waves, which give pure sounds. However these are very boring to listen to. Musical instruments give more complicated waves, which makes them sound more interesting. The quality of the sound is shown as different wave shapes. The quality of the sound allows us to tell what instrument is playing the note.
Reflection and Refraction of Sound
Like all waves, sound waves can be reflected and refracted.
Echoes are examples of reflected sound. A ship using SONAR (sound navigation and ranging) sends pulses (pings) of high frequency sound to the bottom of the sea. It has microphones that pick up the echoes, and the depth of the water can be worked out from the time taken between sending the pulse and receiving the echo.
Many animals navigate using pulses of sound. They make a sound picture in their brains. It’s an effective way of hunting when conditions are murky underwater, or dark. The most obvious animals that do this are dolphins and bats.
Geophysicists can tell the structure of rocks by setting off explosions and listening to the echoes with a network of microphones. The sound waves both reflect and refract as they pass through different rock types.
The pattern of reflections is quite complicated to interpret, but nowadays there are computer programs that can give a detailed read out of the rock structure within seconds.
- Sound is a mechanical longitudinal wave.
- It needs a material to travel in.
- It has wave properties. It can be reflected or refracted.
- The frequency is the pitch
- The amplitude is the loudness
- The quality allows us to know the source of the sound.
- Humans can hear in the range from 20 Hz to 20 kHz
Exploring the Universe
The exploration of space is called astronomy.
Astronomy has NOTHING to do with astrology which deals with horoscopes (the daily drivel you get in some popular newspapers telling you that Venus will make it a good day for financial speculation).
Space is big. Although we have sent men to the Moon and probes to the planets of the solar system, the distances are so massive that the probes have hardly got anywhere! So most of our observations in space have come from using telescopes of different kinds. These include:
- radio telescopes;
- Light telescopes;
- UV telescopes;
- X-rays and gamma rays are not easy to observe through a telescope.
Ground-based light telescopes have advantages:
- They are relatively cheap to install;
- Observers can use them on any night;
- Observers don’t have to use very sophisticated equipment to make their observations.
There are disadvantages:
- No observations can be made on a cloudy night;
- The atmosphere is turbulent, making stars twinkle;
- Light pollution from cities degrades the images;
- Dust in the atmosphere degrades the images.
These problems can be overcome by flying the telescopes very high in the atmosphere in a large aeroplane. This is expensive, and the quality of the images depends on the skill of the pilot.
Some telescopes are sent into orbit as satellites, giving images of stunning quality. The most famous is the Hubble Space Telescope. Space telescopes give astronomers the opportunity to observe in the infra-red, UV, X-ray, and gamma ray regions.
You may well have been on a street when an ambulance has gone past with its siren blaring. The note of the siren has a high pitch as it approaches, and the pitch goes down as it moves away. This change in frequency is called the Doppler Effect.
The Doppler effect also works with light.
About a hundred years ago, astronomers observed an interesting result. If you analyse elements with an instrument called a spectroscope, you find that each element has a characteristic pattern of coloured lines.
If you put the coloured lines against a spectrum of visible light, the coloured lines show up as black lines, but the pattern is the same. Astronomers found this to be a good way of working out what chemicals there were in stars.
They found that when they analysed elements in distant stars, the patterns were there, but moved over towards the red end of the spectrum (longer wavelength).
You can see how the pattern is the same, but the colours of the lines have changed. For example the blue line has become green; its wavelength has increased.
This could be explained by the Doppler Effect. The longer wavelength (lower frequency) is called Red-Shift, and shows that the star is moving away from us. The same effect is seen with radio waves or microwaves.
The further the star is away from us, the more the red-shift, which means the faster the star is travelling away from us.
How the Universe Began
Astronomers have observed that stars and galaxies are moving away from us. Therefore they believe that everything was once contained in one place. All the matter in the universe was contained as energy in the space of a pinhead.
The energy was released in a titanic explosion called the Big Bang. (This term was first used in a radio broadcast about fifty years ago by the then Astronomer Royal, Sir Fred Hoyle, who didn’t believe a word of it and was pretty sarcastic about the theory.)
The temperature was billions of Kelvin. Then within seconds, some of the energy turned into matter, making electrons, protons, and neutrons. These in turn started to make the simple atoms like Hydrogen (1 proton and 1 electron).
In the massively high temperatures of the Big Bang (1032 K), a whole zoo of particles was made, for example:
Particle Physicists are using massive machines to produce the conditions after the Big Bang. They have got within 10-30 s after the Big Bang.
Conditions cooled rapidly after the Big Bang. After 15 000 million years, the temperature of Outer Space is 3 Kelvin (-270 oC), not very warm.
The Big Bang is thought to have occurred about 18 000 million years ago.
What is the evidence for this?
Galaxies are moving away from us as shown by red-shift;
The further they are away from us, the faster they are going;
Cosmic background microwave radiation is described as “the echoes of the Big Bang”.
The background temperature of space is 2.73 K (= -270 oC), a higher temperature than expected. But still pretty cold.
Light travels at 300 million m/s. It takes 8 minutes for the light of the Sun to get to us. The nearest star other than the Sun is 4 light years away. A light year is the distance that light travels in 1 year, which is about 1016 m. So the light that left that star did so in 2004. If the star went out now, we wouldn’t know about it for 4 years.
Light from distant galaxies takes thousands of millions of years to get to us. Therefore we can say that what we see now is what the galaxies were like all that time ago. Many of them might well not exist now.
Cosmic Microwave Background Radiation (CMBR) has also been discovered, which is considered to be the afterglow of the Big Bang. This is microwave radiation out at the very edges of the universe, giving us an insight into what the universe was like 400 000 years after the Big Bang. It explains why the Universe has a temperature of 2 K, not 0 K.
Material was thrown out as the Universe expanded, and in places came together under the influence of gravity to form galaxies, stars, and planets.
It is a mind-boggling thought that the light reaching us from the most distant galaxies left those galaxies not long after the big bang. There are many thousands of millions of stars. Latest evidence suggests that most have planets, and some of these are thought to have conditions that are ideal for life. Who knows; there may be life.
Many astronomers believe in other theories, e.g. the Steady State theory. However the Big Bang theory is the only one that can explain the evidence.
The fate of the Universe is of interest to scientists. There are three possibilities:
- The force of gravity may overcome the expansion, which will slow down. Then it will stop, and all the galaxies will come together again. This is the opposite to the Big Bang, and is called the Big Crunch. It’s like a cosmic bungee-jump.
- The force of gravity is not strong enough to overcome the expansion, which will continue. The Universe will be a cold, lonely, and bleak place.
- The force of gravity and the expansion will balance out, leaving the Universe in a stable state.
It all depends on how much material there is in the Universe. We can see a lot of it, but there is thought to be even more that we can’t see. It is called dark matter. Until scientists know exactly how much there is, which one of the three will be the fate cannot be known for sure.
What is certain is that it will happen eons after we are all long dead, gone, and forgotten.
- Telescopes are used to observe the universe;
- Satellites and probes are also used;
- Ground based telescopes are limited in use;
- Space telescopes give very clear images;
- Telescopes can be made to use any part of the electromagnetic spectrum;
- Most astronomers believe that the Universe was formed by the Big Bang.