Physics
Cambridge IGCSE

TOPIC 1B: ENERGY and PRESSURE

# 1.7a Energy transfers

Why is the car in figure 1 attached to an electrical cable?

Figure 1. An electric car recharging

The answer is that this car is an all-electric car. it does not need petrol or diesel fuel. It gets all of the energy needed from electrical energy, which charges up the car batteries. The large batteries store energy, and this energy can then be transferred to make the car move quickly, climb hills, turn on the heater or lights, play music and a range of other features found in the car. However, it is vital that the batteries can store a large quantity of energy to keep it going before the next re-charge.

In this section, we will learn how we can store energy, and how it can then be transferred from one store to another.

## Energy stores

What exactly is an energy store? Any energy store has the ability to do work once the energy is transferred. A battery 'holds on' to this energy until it is needed. Once the energy is transferred as electrical energy, work is being done.

Physicists say that "work is done when energy is transferred." Work and energy are both measured in joules (J).

There are a few different classes of energy stores that you need to know for the exam, as described here:

### 1. Chemical energy

Batteries contain a combination of chemicals. When the battery is charged up, there is a reversible chemical reaction which stores energy. The energy can then be released again later on when needed.
Batteries are an example of the use of chemicals to store energy. Plants use sunlight to produce a store of energy in glucose - another chemical energy store. Coal, oil, gas and wood are all fuels that will burn, releasing their store of energy through a chemical reaction with oxygen in the air.

All of these examples are classed as chemical energy stores.

### 2. Elastic (strain) energy

When an elastic band is stretched, it stores energy. A spring can be stretched or compressed, and this also stores energy. Bows and arrows and spring loaded toys work by using the stored energy in an elastic material.
All of these examples are classed as elastic energy stores.

### 3. Magnetic and electrostatic energy

If you hold 2 magnets together as shown in figure 2, they will repel. Once released, both magnets will move away from each other. This is another way to store energy, using the magnetic energy stored between magnets. You can do the same with the repulsion (or attraction) between two or more charged objects, called stored electrostatic energy.

Figure 2. Magnetic and electrostatic stored energy

### 4. Nuclear Energy

Nuclear power stations use the energy store trapped inside atoms, usually uranium atoms. You will learn more about this in topic 7. Nuclear energy is different from chemical energy which uses reactions between atoms, rather than the energy trapped inside them.

### 5. Internal (thermal/heat) Energy

A hot kettle will store thermal energy for some time. However, over time the heat is lost into the surroundings. The air around the kettle warms up and some hot water evaporates. We say that the stored thermal energy dissipates into the surroundings. For this exam course, thermal or heat energy stored by a substance is called Internal Energy.

### 6. Kinetic energy

This is the energy stored in moving objects. Section 4.3 deals with this in more detail, but much of our transport system relies on us converting various stores of energy into kinetic energy.

### 7. Gravitational energy

Of all the energy stores, this is perhaps the hardest to understand. When we climb a flight of stairs, we work hard and tap into our chemical energy stores to make our muscles work. Once at the top of the stairs, we have stored gravitational energy. Any object lifted up against gravity has this form of energy. It can be released from this store by simply dropping the object!

Hydroelectric projects like the one shown in figure 3 rely on having a large mass of water being stored at a significant height above the river below. This becomes a vast store of gravitational energy, that can be transferred by letting water flow downhill through electricity generators. You will learn more about gravitational energy in section 4.3.

Figure 3. Stored gravitational energy in a hydroelectric power station (Maxpixel CC-O licence)

## Transferring energy

What happens if you blow up a balloon, then release it without tying the end? In this example, it will zoom off around the room - all of the stored elastic energy in the stretched, inflated balloon is turned into kinetic energy. However, it does this by producing a force on the air. This is a mechanical process involving moving parts (air) and forces, and is called a mechanical transfer of energy.

Figure 4. The transfer of stored energy when an inflated balloon is released.

There are four key processes that transfer energy - learn these for the exam (along with the 8 energy stores listed above).

1. Mechanical transfer - involving moving parts and forces.

2. Electrical transfer - as the name suggests, involving electricity!

3. Transfer by heating - a gas cooker converts stored chemical energy in the gas to stored thermal energy in food, through the process of heating.

4. Transfer through waves - this could involve light or sound waves radiating outwards, or any other form of electromagnetic radiation or other wave.

Note: Different exam boards and web sites often use slightly different lists, making this topic confusing if you are researching on-line. Stick with the lists above from Cambridge International - the exam questions will be marked on this.

## The law of conservation of energy

The energy transfer shown in figure 4 describes how one store of energy is changed to another. You can see from the diagram that the total energy stored remains the same. This rule is called the law of conservation of energy. In precise language, it says:

"Energy cannot be created or destroyed, only transferred from one form to another".

A common mistake is to describe energy being lost, for example when energy is transferred through an electrical cable. However, the energy is still there - it may have been transferred to heat in and around the cable. This may be an unwanted process, but the energy is not lost or destroyed, just transferred to a different form. In practice, the balloon in figure 4 will not transfer all of the elastic energy to kinetic energy - some will transfer away as heat and some radiated away as sound.

We will learn more about calculating the useful energy that has been transferred in the next sections.

Questions:

1. A ball rolls down a hill. State the energy store at the top and at the bottom of the hill.

At the top of the hill the ball will have stored gravitational energy. At the bottom, the ball will have accelerated and have kinetic energy.

2. A catapult is used to fire a small stone, using a strong elastic band.

• i) State the energy store in the catapult just before, and after firing.
• ii) Which of the following energy transfer processes is involved in the firing of the stone?

a) energy transferred electrically
b) energy transferred mechanically
c) energy transferred by waves
d) energy transferred by heating

i) Before firing, there is a store of elastic energy. Afterwards, the stone has a store of kinetic energy.

ii) The answer is (b) transferred mechanically, as the catapult involves the movement of the elastic and forces on the stone.

Tier

## Please choose a tier of entry

 Extended Tier (Core and Supplementary content, Grades A* to G) Core Tier (Core content only, grades C to G) Remember my choice