AQA GCSE Conservation of Energy

Law of Conservation of energy

Law of conservation of energy states that energy cannot be created or destroyed.

Under this law, it is possible that energy can be transferred from one energy store to another, using one of four transfer methods:

1. Heating

2. Radiation

3. Mechanically

4. Electrically

Law of conservation of energy applies to a closed system. In this case there is no change to the total energy overall. However, you can have energy transfers between stores within the system.

Total energy at the start = Total energy at the end.

Energy can be:

1. Transferred between energy stores

2. Stored in energy stores

3. Dissipated to the surroundings, this means transferred to less useful energy stores.

Example 1

The battery contains a chemical energy store, this is transferred electrically into kinetic energy store of the fan, this is the useful energy output.

However, energy is also transferred from the kinetic energy store by radiation to sound waves which is dissipating the energy.

Some of the kinetic energy store of the fan is transferred by radiation to the thermal energy store in the surroundings, this is also known as dissipating the energy.

Some of the chemical potential energy of the cell is transferred to thermal energy energy store in the surroundings by radiation.

The total input energy will equal the total output energy

Example 2

The fuel contains a chemical energy store which is mechanically converted into the kinetic energy store of the moving car, this is a useful energy output.

Kinetic energy store of the car is converted by radiation into sound waves. This would be dissipated energy.

The kinetic energy store of the car is transferred to the thermal energy store of the surroundings by radiation. This would be a dissipated energy store.

The total input energy = total output energy.

Practice Questions

1. Draw an energy transfer diagram for the following examples:

Include initial energy store, final energy stores (useful and wasted), how energy is transferred

a) Battery powered torch

b) Wood being burnt on a fire

2. A saucepan of soup is being heated on a cooker hob. The internal energy store of the metal saucepan gains 2000J of energy, whist the liquid soup increases its energy store by 1500J whilst being heated. Calculate the size of the thermal energy store of the heating element in the hob.

3. A runner has Y amount of energy stored in his muscles as chemical energy. The runner goes for a run, 80% of the chemical energy is mechanically transferred into kinetic energy, whilst 20% of the energy is transferred to the thermal energy store of the surroundings by radiation. The thermal energy store of the surroundings increases by 500J during the run.

a) Calculate the gain in the kinetic energy store

b) Calculate the size of Y (chemical potential energy store.)

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