Conservation of Energy (AQA A Level Physics): Revision Note

The principle of conservation of energy

• The principle of conservation of energy states that:

  Energy cannot be created or destroyed, it can only be transferred from one form to another

• This means the total amount of energy in a closed system remains constant, although how much of each form there is may change

• The following table shows different types of energy:

Energy dissipation

• No energy transfer is 100% efficient

• When energy is transferred, some of the energy is dissipated to the surroundings

• Dissipated energy is usually regarded as wasted energy because it cannot easily be used for a specific purpose

• Any energy not transferred for a useful purpose is wasted energy

• These are commonly in the form of thermal (heat), light or sound energy

• What counts as wasted energy depends on the system

• For example, in a television:

electrical energy ➝ light energy + sound energy + thermal energy

• Light and sound energy are useful energy transfers, whereas thermal energy (from the heating up of wires) is wasted

• Another example, in a heater:

electrical energy ➝ thermal energy + sound energy

• Thermal energy is useful, whereas sound is not

Applications of energy conservation

• Common examples of energy transfers are:

  • A falling object (in a vacuum): gravitational potential energy ➝ kinetic energy

  • A battery: chemical energy ➝ electrical energy 

  • Horizontal mass on a spring: elastic potential energy ➝ kinetic energy

• There may also be work done against resistive forces such as friction

• For example, if an object travels up a rough inclined surface, then

Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

Energy equations

• The most common equations used in the conservation of energy calculations are:

$\mathrm{Kinetic}\mathrm{energy}=\frac{1}{2}m{v}^2$

$\mathrm{Gravitational}\mathrm{potential}\mathrm{energy}=mg∆h$

$\mathrm{Elastic}\mathrm{potential}\mathrm{energy}=\frac{1}{2}k{\left(∆L\right)}^2$