Energy stores

Energy is not a physical thing that we can see or touch.

Different objects store energy in different ways. These are called energy stores and are
listed below:

Name Description Examples

Thermal Any object with a temperature has a thermal A hot cup of tea, hot
energy store. The higher the temperature of an bath water, a hot
object, the more energy in the thermal energy radiator.
store.

Kinetic Any object that is moving has a kinetic energy A car or train that is
store. The faster the movement, the more energy moving, somebody
in the kinetic energy store. walking or running.

Gravitational Objects at a height above the ground have a A book on a high shelf,
potential gravitational potential energy store. The larger the a bird standing on a
height above the ground, the more energy in the high branch of a tree.
gravitational potential energy store.

Elastic When objects such as a spring or elastic band are A stretched elastic
potential stretched or compressed they store elastic band/spring/hair tie.
potential energy. The greater the extension or
compression, the larger the elastic potential
energy store.

Chemical Different chemicals store energy in their chemical Food, fuel, batteries.
energy store.

Nuclear Energy that is stored in the nucleus of an atom is Fuel in nuclear power
in the nuclear energy store. plants. For example,
uranium or plutonium.

Magnetic When two magnets are held close together, they Two magnets near
potential have a magnetic potential energy store. each other.

Electrostatic When charges are close together, they have an Protons, electrons, any
potential electric potential energy store. charged particle.
 Energy transfers
The principle of conservation of energy tells us that energy is never created or destroyed;
it is only transferred from one energy store to another.
The unit of energy is the Joule (J).
There are four pathways for energy to be transferred from one store to another. Energy
can be transferred:

1. Mechanically. Mechanical work is done on an object when a force is applied over a

distance.
2. Electrically. Electrical work is done when charges move through a circuit.
3. By heating. The transfer of energy from a hotter object to a cooler object.
4. By radiation. This includes energy transferred by light (or any electromagnetic wave)
and sound.
Note the phrase “work” has been used in some of the pathways. “Work done” is just
another way of saying energy transferred from one store to another.
The diagram to the right shows a pendulum swinging from side to side.
In position 1, the pendulum is at its maximum height and it has a gravitational potential
energy (GPE) store of 10 J.
As the pendulum falls into position 2, gravity does mechanical work on the
pendulum. The pendulum speeds up and becomes lower in height. Its GPE store empties,
and its kinetic energy (KE) store fills to 10 J. Note how the overall amount of energy is the
same, due to the principle of conservation of energy.
The pendulum then continues swinging to position 3. As it does so, its KE store empties and
its GPE store fills back to the original 10 J. The pendulum is stationary at position 3 so it has
no KE. It is also at its maximum height, so it has maximum GPE. The total energy is still
unchanged.
However, the pendulum will eventually come to a stop. This is primarily due to air
resistance. Generally, energy transfers are not 100% efficient. Energy is “wasted” by raising
the thermal energy store of the surroundings. The temperature of the pendulum and the
nearby air will be raised slightly.
 Power
Power is defined as the rate at which energy is transferred. The unit of power is the Watt
(W).
There is an equation that relates power, energy and time:
Power = Energy ÷ Time or in symbols P=E÷t
Remember, the unit of energy is the Joule (J) and the unit of time is the second (s).
We can also rearrange this equation to give:
Energy = Power × Time or in symbols E=P×t

Example question 1:
A torch has a power of 5 W and is used for a time of 30 s. Calculate the energy used by the
torch.
Step 1. Write down equation: E=P×t
Step 2. Insert variables into equation: = 5 × 30
Step 3. Calculate answer. Remember units: = 150 J

Example question 2:
A toaster uses 20 000 J of energy in a time of 100 s. Calculate the power of the toaster.
Step 1. Write down equation: P=E÷t
Step 2. Insert variables into equation: = 20 000 ÷ 100
Step 3. Calculate answer. Remember units: = 200 W

Sometimes power is given in units of kilowatts (kW). One kilowatt is equal to one thousand
watts.
To convert from kilowatts to watts you need to multiply the number of kilowatts by one
thousand: kW × 1000 → W
To convert from watts to kilowatts you need to divide the number of watts by one
thousand: W ÷ 1000 → kW
 Energy resources
We generate electricity through two categories of energy resource: those that are
nonrenewable and those that are renewable.
A non-renewable resource is one which cannot be replaced once it has been used. One day,
non-renewable resources will run out.
The most common non-renewable resources are the three fossil fuels (oil, coal and gas)
and nuclear power.
As well as being used in power plants, fossil fuels can be used for transport and heating.
For example, petrol and diesel (which are both made from oil) are used in cars. Gas is
commonly used in central heating systems, and coal is burnt in some fireplaces.
The main advantage of using a non-renewable resource in a power plant is that they all
produce a reliable output.
However, the main disadvantage is that burning fossil fuels increases the amount of carbon
dioxide (CO2) in the atmosphere. Carbon dioxide is a greenhouse gas. Greenhouse gases
are responsible for global warming (a raising of the average temperature of Earth).
Burning coal and oil also produces sulphur dioxide which contributes to acid rain.
Nuclear power plants do not have carbon dioxide or sulphur dioxide emissions but they do
produce radioactive waste which is difficult to dispose of safely. However, they do
generate a reliable and large output.
A renewable resource will not run out. Three renewable resources are listed below:

Resource Advantage Disadvantage

It is not always windy, so the output is

Wind No carbon dioxide emissions, so unreliable. Noisy & spoils the view.
they do not contribute to global
Hydro- warming. Requires the flooding of a valley with
electric Once built, no fuel costs and so a dam. This causes a loss of habitat.
are cheap to run.
Only work in direct sunlight, so do not
Solar generate electricity at night.

We are using more renewable resources for generating electricity. In 2022, there was an
“energy crisis” due to shortages of oil and gas. This shortage increased the price of
electricity. Renewable resources can help reduce our reliance on fossil fuels.
 Electricity bills
We’ve already learnt, on page 8, that one unit of energy is the Joule (J). Two other units of
energy are:

1. The kilojoule (kJ). One kilojoule is equal to one thousand joules.

2. The kilowatt hour (kWh). One kilowatt hour is equivalent to an electrical device with
a power of 1 kW (equal to 1000 W) being used for a time of one hour. As there are
3600 seconds in one hour, one kilowatt hour is therefore equal to 3 600 000 J of
energy.
We use kilowatt hours for electricity bills. This is because electrical devices in the average
UK household transfer over 10 billion joules of energy a year. It’s more convenient to use
kilowatt hours as this brings the numbers down to a more manageable size.
Electrical devices in the average UK household transfer 3000 kWh of energy each year.
To calculate the amount of energy in kWh that a device transfers you need to multiply the
power of the device (in kW) by the time that it is used for (in hours):
Energy (in kWh) = Power (in kW) × time used (in h)
As of April 2022, the price for each kWh in the UK was 28 p. To calculate the cost of an
electricity bill we use the equation:
Total cost (in pence) = Energy transferred (in kWh) × price per kWh (in pence)
Example question:
Using the meter readings to the right, calculate the cost of the monthly electricity bill. The
price of each kWh is 28 p.
Step 1. Calculate the total number of kWh used.
To do this, we need to look at the difference in meter readings:
2210 – 2031 = 179 kWh
Step 2. Calculate cost by multiplying total number of kWh by cost per kWh.
179 × 28 = 5012 p
There are 100 p in a pound. To convert from pence to pounds you need to divide the
number of pence by one hundred: p ÷ 100 → £
In other words, the 5012 p we calculated is equal to £50.12.
 Thermal energy transfer
There are three ways that thermal energy can be transferred:

1. Conduction.
2. Convection.
3. Radiation.
Thermal energy is transferred from hotter objects to colder objects.

Conduction occurs primarily in solids. As particles in a solid are heated, they vibrate more.
These vibrations cause collisions between particles and the vibrations are transferred along
the material.
If a material is able to conduct heat well, it is called a conductor. Metals are one example of
a good conductor.
If a material is not able to conduct heat well, it is called an insulator. Plastics are an
example of a good insulator. Gases (like air) are very good insulators, as their particles are
far apart from each other.
There are no particles in a vacuum. Conduction is not possible through a vacuum.
Convection happens in both liquids and gases. As a liquid or gas is heated, the particles
move faster and become more spread out. Due to this they become less dense and
therefore rise. Colder liquids or gases are more dense and sink. This is a convection current.
This is shown in the image on the left with a pan on a gas stove. The bottom of the pan is
hot and so water at the bottom of the pan heats up and rises. Meanwhile, water at the top
of the pan cools and sinks.

All objects emit radiation in the form of infrared waves. This can be referred to as thermal
radiation or infrared radiation. As infrared radiation is an electromagnetic wave, it can even
travel through a vacuum. This is how thermal radiation reaches the Earth from the Sun.
Matt black objects absorb the most infrared radiation, while shiny silver surfaces reflect
infrared radiation.
 Reducing unwanted thermal energy transfers
To reduce the size of electricity and fuel bills, it is important to reduce thermal energy
losses from a home:

More thermal energy escaping from home > more energy/fuel needed > higher heating
costs

In the average home:

• 35% of thermal energy loss is through the walls. This can be reduced with cavity wall
insulation. A cavity wall is made of two separate walls with a gap in between them.
This gap can then be filled with an insulator. This is shown in the diagram to the right.
• 25% of thermal energy loss is through the roof/attic. This can be reduced with loft
insulation.
• 25% of thermal energy loss is through windows and doors. This can be reduced with
double glazing, closing the curtains and with a draught excluder that stop draughts
coming in through the bottom of the door.
• 15% of thermal energy loss is through the floor, this can be reduced by improving the
insulation in the floor.

Vacuum flasks are also designed to limit thermal energy transfers. If a hot liquid is inside a
vacuum flask, the following features keep the liquid hot for as long as possible:

• The silvered surfaces reflect infrared radiation back into the liquid.
• The vacuum does not allow for conduction or convection as there are no particles.
• The plastic lid is an insulator, limiting conduction. It also prevents evaporation of
liquid.

They also work keep a cold liquid cooler for longer. The silvered surfaces now reflect
infrared radiation away from the liquid. The vacuum prevents conduction and convection
and the plastic lid also limits conduction.