Physics - Energy


Energy is transferred between stores

When energy is transferred to an object, the energy is stored in one of the object's energy stores.

The main energy stores are:

  • Thermal energy stores
  • Kinetic energy stores
  • Gravitational Potential energy stores
  • Elastic potential energy stores
  • Chemical energy stores
  • Magnetic energy stores
  • Electrostatic energy stores
  • Nuclear energy stores

Energy is transferred mechanically (by a force doing work), electrically (work done by moving charges), by heating or by radiation (e.g. light and sound)

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When a System Changes, Energy is Transferred

A system means a single object (e.g the air in a piston) or a group of objects (e.g. two vehicle colliding) that you are interested in.

When a system changes, energy is transferred. It can be transferred into or away from the system, between different objects in the system or between different types of energy stores

Closed systems are systems whee neither matter nor energy can enter or leave. The net change in the total energy of a closed system is always zero.

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How Energy is Transferred


Take the example of boiling water in a kettle - with water as the system. Energy is transferred to the water (from the kettles heating element) by heating, into the water's thermal energy store causing the water temperature to rise.

The kettle's heating element and the water could be considerd as a two-object system. Energy is transferred elictrically to the thermal energy store of the kettle's heating element, which transfers energy by heating to the wate's thermal energy store

Work Done

Work done is another way of saying energy transferred. Work can be done when current flows (work is done against resistance in a circuit) or by a force moving an object.

E.g. The Friction between a car's brakes and its wheel does work as it slows down. It causes an energy transfer from the wheels' kinetic energy stores to the thermal energy store of the surrounding.

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Kinetic Energy

Any that is moving has energy in its kinetic energy store. Energy is transferred to this stoe when an objects speeds up and is transferred away from this stre when an object slows down.

The energy in the kinetic energy store depends on the object's mass and speed. The greater its mass and the faster it's going, the more energy there will be in its kinetic energy store.

Formula: Kinetic Energy(J) = 0.5 x mass(kg) x speed ^2 (m/s^2)

                K = 0.5mv^2

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Gravitational Potential Energy Stores

Lifting an object in a gravitational field requires work. This causes a transfer of energy to the gravitational potential energy (GPE) store of the raised object. The higher the object is lifted, the more energy is transferred to this store.

Formula: GPE(J) = mass(kg) x gravitational field strength(N/kg) x  Height (m)

                GPE = mgh

When something falls, energy from its gravitaional potential energy store is ransferred to its kinetic energy store.

For a falling object when there's no air resistance:

Energy lost from GPE store = Energy gained in the kinetic energy store

In real life, air resistance acts against all falling objects, it causes some energy to be transferred to other energy stores, e.g. the thermal energy stores of the object and surroundings.

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Elastic Potential Energy

Stretching or squashing an object can transfer energy to its elastic potential energy store.

So long as the limit of proportionality has not been exceeded, energy in the elastic potential energy store of a tretched spring can be found using: 

Elastic Potential Energy (J) = 0.5 x spring constant (N/m) x Extension^2 (m)

EPE = 0.5ke^2

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Specific Heat Capcity

More energy needs to be transferred to the thermal energy store of some materials to increads their temperature than others. E.g. you need 4200 J to warm 1 kg of water by 1 degrees celcius, but only 139 J to warm 1kg of mercury by 1 degree celcius.

Materials that need to gain lotsof energy in their thermak energy stores to warm up also transfer loads of energy when they cool down again. They can stre a lot of energy.

Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 degree celcius.


Change in thermal energy (J) = mass(kg) x specific heat capacity(J/kgC)x temperature change (C)

ΔE = mcΔθ

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Conservation of Energy Principle

'Energy can be transferred usefully, stored or dissipated, but can never be created or destroyed'

When energy is transferred between stores, not ll of the energy is transferred usefully into the store that you want it to go to. some energy is always dissipated when an energy transfer takes place.

Dissipated energy is sometimes called 'waste energy' because the energy is being stored in a way that is not useful (useually energy has been transferred into thermal energy stores).

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Power is the rate of energy tranfer, or the rate of doing work. 

Power is measured in watts. One watt = 1 joule of energy transferred per second.

Power(W) = Energy Transferred(J)/ Time(s)                                 P=E/t

Power(W) = Work Done(J)/time(s)                                                 P=W/t

A powerful machine is not necessarily one which can exert a strong force. A powerful machine is one which transfers a lot of energy in a short space of time.

E.g. Take two cars that are identical in every way except from the power of their engines. Both cars race the same distance along a straight track. The car with the more powerful engine will reach the finish line faster. It will transfer the same amount of energy as the other car but over less time.

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Conduction is the process where vibrating particles transfer energy to neighbouring particles.

Energy transferred to an object by heating is transferred to the thermal energy store of the object. This energy is shared across the kinetic energy stores of the particles in the object. 

The particles in the part of the object being heated vibrate more and collide with each other. These collisions cause energy to be transferred to the other side of the object. It's then usually transferred to the thermal energy store of the surroundings (or anything else touching the object)

Thermal conductivity is a measure of how quickly energy is transferred through a material in this way. Materials with a high thermal conductivity transfer energy between their particles quickly.

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Convection is where energetic particles move away from hotter to cooler regions.

Convection can happen in gases and liquids. Energy is transferred by heating to the thermal store of theliquid or gas. This energy is shared across the kinetic energy stores of the gas or liquid's particles.

Unlike in solids, the particles in liquids and gases are able to move. When you heat a region of a gas or liquid, the particles move faster and the space between individual particles increases. This causes the density of the region being heated to decrease.

Because liquids and gases can flow, the warmer and less dense region will rise above denser, cooler regions. If there is a constant heat source, a convection current can be created.

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Convection Current - Radiators

Heating a room with a radiator relies on creating convection currents in the air of the room.

Energy is transferred from the radiator to the nearby air particles by conduction (the air particles collide with the radiator surface).

The air by the radiator becomes warmer and less dense as the particles move quicker.

This warm air rises and is replaced by cooler air. The cooler air is then heated by the radiator.

At the sam time, the previously heated air transfers energy to the surroundings. It cools, becomes denser and sinks.

This cycle repeats, causing a flow of air to circulate around the room - this is a convection current.

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Reducing Unwanted Energy Transfers


Whenever something moves, there's usually at least one frictional force acting against it. This causes some energy in the system to be dissipated, e.g. air resistance can transfer energy from the falling object's kinetic energy store to its thermal energy store.

For objects that are being rubbed together, lubricants can be used to reduce the friction between the objects' surfaces when they move. Lubricants are usually liquids (like oil), so they can flow easily between objects and coat them.

Thermal Insulation:

Thermal insulation is important to reduce the rate of energy transfer by heating.

Ways to prevent energy loss through heating:

  • Thick walls from a material with a low thermal conductivity
  • Have cavity walls with insulation (foam) in the middle, loft insulators
  • Double glazed windows
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Useful devices are only useful because they can transfer energy from one store to another.

Some of the input energy is usually wasted by being transferred to a useless energy store - usually thermal.

The less energy that is wasted, the more efficient the device is.

Efficiency can be improved by insulating objects, lubricating them or making them more streamlined.

Efficiency = Useful output energy transfer     x 100

                      Total input energy transfer

Efficiency = Useful power output           x 100

                      Total power input

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100% Efficiency

No device is 100% efficient and the wasted energy is usually transferred to useless thermal energy stores.

Electric heaters are the exception to this. They're usually 100% efficient because all the energy in the electrostatic energy store is transferred to 'useful' thermal energy stores.

Ultimately, all energy ends up transferred to thermal energy stores. E.g. if you use an electric drill, its energy transfers to lots of different energy stores, but quickly ends up all in thermal energy stores.

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Energy Resources

Non-Renewable Energy Resources:

Non renewable energy resources are fossil fuels and nuclear fuel (uranium and plutonium). Fossil fuels are natural resources that form underground over millions of years. They are typically burnt to provide energy.

The three main fossil fuels are: coal, oil and natural gas.

However these will all run out eventually and they do damage to the environment and atmosphere, but they provide most of our energy.

Renewable Energy Resources:

Renewable energy resources are: the sun (solar), wind, water waves, hydro-electricty, bio-fuel, tides, geothermal.

These will never run out - the energy can be 'renewed' as it is used. They do less damage to the environment than non renewable resources. However they don't provide much energy and some are unreliable because they depend on the weather.

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Energy Resources used for Transport

Non-Renewable Energy Resources:

  • Petrol and deisel powered vehicles (most cars) use fuel created from oil
  • Coal is used in some old-fashioned steam trains to boil; water to produce steam

Renewable Energy Resources:

  • Some vehicles run on bio-fuel which is renewable
  • Some vehicles run on a mixture or biofuel and petrol/deisel which is partially renewable.

Some vehicles can be powered from electricity (mainly trains but an increasing number of cars). However, the electricty could be generated from either renewable or non-renewable resources. 

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Energy Resources used for Heating

Non-Renewable Energy Resources:

  • Natural gas is the most widely used fuel for heating homes in the UK. The gas is used to heat water, which is then pumped into radiators throughout the home.
  • Coal is burnt in fireplaces.
  • Electric heaters usually use electricity produced fron non-renewable resources.

Renewable Energy Resources:

  • A geothermal heat pump uses geothermal energy to heat buildings.
  • Solar water heaters work by using the sun to heat water water which is then pumped into radiators in the building.
  • Burning bio-fuel or using electricity generated from renewable resources can also be used for heating.
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Wind Power

This involves putting up lots of wind turbines up in exposed places like on moors or round coasts.

Each turbine has a generator inside it - the rotating blades turn the generator and produce electricity.

There's no pollution except during manufaction

They are very noisy and can spoil the view as it would take 1500 turbines to replace one coal fired power station which would take up a lot of space.

There's a problem of the turbines stopping when the wind stops or if the wind is too strong, and it's impossible to increase supply when there's extra demand. On average, wind turbines produce electricity 70-85% of the time.

The initial costs are high but there are no fuel costs and minimal running costs.

There's no permanent damage to the landscape, except if they are in the sea as they can damage coral reefs and other habitats.

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Solar Cells

Solar cells generate electric currents directly from sunlight. Solar cells are often the best source of energy to charge batteries in calculators and watches which don't use a lot of electricity.

Solar power is often used in remote places where there's not much choice and to power electric road signs and satellites.

Theres no pollution except during manufaction.

In sunny countries solar power is a very reliable source of energy  but only in the daytime. Solar power can still be cost effective even in cloudier countries like England.

Power output cannot be increased when there is extra demand.

Initial costs are high but after that the energy is free and running costs are almost nothing.

Solar cells are usually used to generate electricity on a relatively small scale.

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Geothermal Power

This is possible in volcanic areas or where hot rocks like quite near to the surface. The source of much of the energy is the slow decay of various radioactive elements, including uranium, deep inside the Earth.

This is free energy that is reliable and does very little damage to the environment.

Geothermal power can be used to generate electricity, or to heat buildings directly.

The main drawbacks with geothermal power are that there aren't very many suitable locations for power plants, and that the cost of building a power plant is very high compared to the amount of energy it produces.

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Hydro-Electric Power

Hydro-electric power usually requires the flooding of a valley by building a big dam. Water is allowed out through turbines. There is no pollution.

However, there is a big impact on the environment due to the flooding of a valley (rotting vegetation releases methane and carbon dioxide) and possible loss of habitat for some species and loss of villages. The reservoirs can look very unsightly when they dry up. Putting hydroelectric power stations in remote valleys tends to reduce their impact on humans.

It can provide an immediate response to an increased demand for electricity.

Very reliable - not likely to have droughts especially in the UK.

Initial costs are high but no fuel costs and minimal running costs.

Good way to produce electricity on a small scale in remote areas.

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Wave Power

This includes lots of small wave powered turbines located around the coast. The turbines are connected to generators.

There is no pollution but it can damage the seabed and the habitats of marine animals and can be a hazard to boats.

They are unreliable as waves die down when the wind drops.

Initial costs are high but no fuel costs and minimal running costs.

Unlikely to be used on a large scale. 

They are very useful on small islands.

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Tidal Barrages

Tides are used in lots of ways to generate electricity. the most common method is building a tidal barrage.

Tidal barrages are big dams built across river esturaries, with turbines in them. As the tide comes in it fill up the estruary. The water is then allowed out through turbines at a controlled speed. 

Tides are produced by the gravitational pull of the sun and moon.

There is no pollution. However it prevents free access by boasts and it alters the habitat of wildlife.

Tides are reliable as they happen twice a day and always near to the predicted height. The height of the tide is variable so lower, neap, tides will provide significantly less energy than the bigger, spring, tides. They also don't work when the water level is the same either side of the barrage which happens four times a day.

Initial costs are moderately high, but there is not fuel cost and minimal running costs. Even though it can be only used in some estuaries it has the potential of generating a significant amount of electricty.

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Bio-fuels are renewable energy resources created from either plant products or animal dung. They can be solid, liquid or gas and can be burnt to produce electricity or run cars in the same way as fossil fuels.

They are carbon neutral if the plants are grown at the same rate that they are being burned.

Bio-fuels are fairly reliable, as crops take a relatively short time to grow and different crops can be grown all year round. However, they cannot respond to immediate energy demands.

The cost to refine bio-fuels is very high and growing crops specifically for bio-fuel could mean that there isn't enough space or water to meet the demand of crops for food.

In some regions large areas of forests have been removed to grow bio-fuels resulting in a loss of habitat for many animals. 

The decay and burning of this vegetation increases carbon dioxide and methane emissions.

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Non-Renewable Resources

Fossil fuels and nuclear energy are reliable. There's enough fossil and nuclear fuels to meet the current demand, and they are extracted from the Earth at a fast enough rate that power plants always have fuel in stock. This means that power plants can respond quickly to changes in demand.

These fuels are slowing running out so if no new resources are found some fossil fuel stocks may run out within 100 years.Set up costs are high but low running costs and low fuel extraction costs. Using fossil fuels is a cost effective way to produce energy.

Environmental Problems:

  • Fossil fuels release carbon dioxide which contributes to the greenhouse effect and global warming.
  • Burning coal and oil releases sulphur dioxide which causes acid rain - harmful to trees and soils and effects ecosystems. Can be reduced by remove sulphur before burning or clearing emmisions.
  • Oil spillages cause serious environmental problems.
  • Nuclear waste is very dangerous and difficult to dispose of. Nuclear power carries the risk of a major catastrophe like Fukushima and Chenobyl
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