Identify the different ways that the energy of a system can be changed a…

The idea

A system stores energy, and that store changes only when energy is transferred into or out of it. Three routes are required here. First, a force does work when it moves its point of application along its own line of action: pushing a box across a floor transfers energy mechanically, and the work done equals force × distance moved along the force. Second, electrical equipment changes a system's energy because a current flowing through a potential difference transfers energy: a motor lifts a load, a heater warms a room, a lamp radiates. Third, heating transfers energy from a hotter region to a cooler one by conduction, convection or radiation, with no force and no current needed. Recognise each route from a described situation and name the store that gains or loses energy.

What earns the marks

• Names work done by a force as a route that changes a system's energy when a force moves an object along its line of action.
• Identifies electrical equipment as a route in which a current flowing through a potential difference transfers energy to or from the system.
• Describes heating as a transfer from a hotter region to a cooler one by conduction, convection or radiation.
• States the store that gains or loses energy in a described situation, such as gravitational potential store for a lifted load.
• Distinguishes the three routes by the mechanism involved rather than by the device named.

Common mistakes and how to put them right

• Confuses energy with force.

  Put it right: State that a force does work and energy is transferred, so energy is measured in joules and force in newtons.

• Treats heating as requiring a force.

  Put it right: State that heating transfers energy by a temperature difference alone.

• Assumes electrical equipment creates energy.

  Put it right: State that it transfers energy from a supply to another store.

• Ignores the direction of transfer.

  Put it right: State which store gains and which loses energy.

Exam tips

• Read the stem for the mechanism named (force, current or temperature difference) before choosing an option.
• Reject any option that says energy is used up or created rather than transferred between stores.
• Check units in options: energy in joules, force in newtons, power in watts.
• Where two options seem similar, choose the one naming both the route and the store that changes.