Energy transfers

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Energy is measured in the unit joule (J) and is needed by all objects to change shape, size or position.

The principle of conservation of energy states that energy cannot be created or destroyed. Energy can only be transferred from one energy store into another.

As objects cannot create their own energy, it must be supplied from elsewhere.

For example, humans need energy to move, grow and stay alive and we get our energy from the food we eat. This food contains a store of chemical energy. When we digest the food, this chemical energy is converted into thermal energy to keep us warm and kinetic energy to allow us to move. Some of the energy is also stored in our cells, ready to be converted as and when needed.

Energy can be found in eight different types of store. These are listed below including examples of their use:

chemical – used to hold the energy in batteries
kinetic – used to hold the energy in a moving object
gravitational – used to hold the energy for an object suspended at height
elastic – used to hold the energy in a stretched elastic band
thermal – used to hold the energy in a source of heat
magnetic – used to hold the energy in a magnet
electrostatic – used to hold the energy in an electrical field
nuclear – used to hold the energy in a radioactive source

Energy transfers may occur mechanically, electrically, by heating or by radiation:

Mechanical energy transfer – this type of energy transfer occurs through the action of a force. For example, a book being held at a given height has a store of gravitational potential energy. If the book is dropped, it moves towards the ground. The energy in the gravitational potential energy store is transferred to kinetic energy.
Electrical energy transfer – this type of energy transfer occurs when there is a movement of charges, i.e. from a current. For example, when an electric fan is plugged in, the movement of the electrons (current) carries the energy from the electrostatic store to the fan where it is transferred to the kinetic energy store as the fan moves.
Energy transfer by heating – this type of energy transfer occurs when energy is transferred from a thermal energy store to a system causing the system to heat up. For example, if a pan of cold water was placed on an electrical hob, the hob heats up. The thermal energy stored in the hob is then transferred to the thermal energy store of the water in the pan. This transfer of thermal energy causes the water to heat up and its temperature to increase.
Energy transfer by radiation – this type of energy transfer occurs between a source and the surroundings and often results in the conversion of energy to heat, light or sound energy. For example, the energy stored in a battery powered torch is held in the chemical store of the battery. When the torch is switched on, the energy is transferred from the chemical store and is converted into light and heat energy which are both radiated away from the torch bulb into the surroundings to allow us to see.

Efficiency

Energy conversions occur when the energy is transferred from the store and converted into other forms of energy. Most devices used in our everyday lives, transfer energy from a chemical or electrical store into other forms of energy. Some of these energy conversions are useful and indeed necessary for life. We need energy in the form of heat for our working and living environments. We also need energy in the form of light for our homes, buildings, vehicles and roads. Most of the heat and light energy we use has been converted from electrical energy.

However, as well as providing us with useful energy, most energy conversions also provide us with energy which is not useful to us. This non-useful energy is considered to be waste energy.

For example, when an electrical lamp is used, the electrical energy is transferred to heat and light. The light energy is useful to us whereas the heat energy is not. As the heat and light energy are dissipated into the surroundings, they cannot be useful. The heat energy is considered as waste energy.

In your exam, you may be asked to calculate the efficiency of a device. The efficiency of any device can be assessed by comparing the output of the useful energy with the total energy output (or total energy input), using the equation:

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy output}} \times 100$

You must know and be able to use this equation as it will not be provided in the exam.

Example

An electric lamp has a total energy output of 100 J. The lamp produces 10 J of light energy and 90 J of heat energy.

Calculate the efficiency of the lamp.

Useful energy output = 10 J

Total energy output = 100 J

$\text{efficiency} = \frac{10 J}{100 J} \times 100$

$\text{efficiency} = 10\%$

This tells us that only 10% of the energy is transferred as light energy so only 10J of light energy is produced for every 100 J of electricity used. The higher the value calculated, the more efficient the device, and the more useful energy is produced. The efficiency of a device will always be less than 100%. This is because the device cannot convert all of the energy into useful energy and there will therefore always be some waste energy transferred.

You may also see this equation presented as:

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100$

Remember, the total energy input and total energy output for the device should be equal, as none of the energy is destroyed. Both equations can be used to calculate efficiency and both versions would give the same result.

Sankey diagrams

Sankey diagrams are used to provide a visual summary of the efficiency of a device. In Sankey diagrams, arrows are drawn to scale to show the distribution of the energy types. The thicker the arrow, the higher the amount of energy. To represent the waste energy in a Sankey diagram, the arrow is drawn pointing downwards and the useful energy is represented using an arrow pointing straight along.

The Sankey diagram illustrating our previous example of the electric lamp is shown below:

As you can see from the diagram, the width of the arrow representing energy input and the combined widths of the arrows for energy outputs are equal. This shows that the total energy input and output is equal at 100 J.

The bigger arrow points down to show how much energy is ‘wasted’ as it is converted into heat energy and escapes into the surroundings. The amount of energy that escapes is 90 J per 100 J of energy.

The smaller arrow represents the amount of useful light energy that is produced as the electrical energy is converted. The Sankey diagram clearly shows that the arrow for the heat energy is far larger than that for the light energy. This shows that the amount of wasted energy is far larger than the useful energy and that the lamp has a poor efficiency.

The efficiency calculation and Sankey diagrams can be used to assess the efficiency of a variety of everyday and scientific devices as shown in the following examples.

Example

An energy-saving light bulb has a total energy input of 100 J. 75 J of this energy is transferred to useful light energy. The Sankey diagram for the light bulb is shown below:

Calculate the efficiency of the bulb.

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100$

$\text{efficiency} = \frac{75}{100} \times 100$

$\text{efficiency} = 75 \%$

This shows that energy-saving light bulbs are more efficient than the normal filament bulb used previously. Here only 25 J of energy is transferred as wasted heat energy compared to the 75 J transferred from the filament bulb.

Example

A generator has a total energy output of 1,750 kJ per second. 200 kJ of this energy is wasted energy. Calculate the efficiency of the generator.

$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100$

$\text{Total energy output} = 1750 kJ$