Energy Transfers and Stores

A key concept in Physics is that energy cannot be created or destroyed, only transferred from one form to another.

Energy is measured in joules, .

It can exist in different forms, such as:

• Kinetic energy, found in moving objects. The faster an object is moving, the more kinetic energy it has. For example, a Ferrari will have more kinetic energy than a snail (if we could imagine them having the same mass).

• Elastic potential energy, found in objects that are stretched or compressed, such as an elastic band or a stress ball.

• Thermal energy, found in hot objects. A cup of tea will have more thermal energy than a glass of water.

• Chemical energy, found in food and fuel.

• Gravitational potential energy, found in objects due to their height above the ground. For example, an apple in a tree will have more gravitational potential energy than a flower on the ground (if we could imagine them having the same mass).

Energy can change from one store to another. For example, when a ball is thrown up, the kinetic energy due to the ball moving is converted into gravitational potential energy as the ball gets higher above the ground.

Kinetic energy gravitational potential energy.

The image below shows a boy playing with a slingshot. The elastic potential energy stored in the stretched rope is converted into kinetic energy as the rock resting in the slingshot is accelerated forward.

Elastic potential energy kinetic energy

Energy can be transferred between the different stores of energy by :

• heating, energy is transferred from the hotter to the cooler object

• mechanically, when there is a force acting on an object, such as the force needed to compress/squeeze a stress ball

• electrically, when current flows, such as the current that moves from a battery to a light bulb, transferring the chemical energy from the battery into the light bulb's thermal energy (the light bulb heats up when on).

• radiation, when energy is transferred by electromagnetic waves, such as the energy from the Sun that warms the Earth.

When water is boiled in a kettle, energy is transferred electrically from the mains supply to the kettle's heating element. Energy is then transferred from the heating element's thermal energy store to the water's thermal energy store by heating.

Sometimes it is not obvious where the energy is transferred to, and this usually happens when energy is transferred to the thermal energy of the surroundings. This is known as dissipation. For example, if a fast car suddenly stopped, it might be confusing to think about where all its kinetic energy went. But if you feel the tyres, you will notice they have warmed up, or you might see smoke around them. Energy is thus transferred from the kinetic store to the thermal energy store of the surroundings, mechanically, as a force has to be applied to stop the car.

When a car starts moving, chemical energy stored in the fuel is converted to kinetic energy. However, some energy is dissipated (lost/transferred to the surroundings) due to the frictional force acting between the car tyres and the road, which tries to oppose the car's motion. Not all of the car's chemical energy will be converted to kinetic energy, as some is lost/transferred to the surroundings as thermal energy. A more efficient car will transfer more of its chemical energy into kinetic energy with less energy wasted (transferred to less useful forms). For example, if the car had of chemical energy to begin with, and has of kinetic energy when moving, clearly of energy were dissipated to the surroundings.