Energy transfers in a system: Students should be able to describe with examples where there are energy transfers in a closed system, that there is no net change to the total energy.

The idea

A closed system is one where no energy enters or leaves, so the total energy stored inside stays constant. Energy moves between stores, but the total is conserved. For example, a pendulum swinging in a vacuum: gravitational potential energy transfers to kinetic energy and back again, while the total energy remains the same. In a closed system, energy transfers between kinetic, gravitational potential, thermal and other stores, but no energy is added or removed. 'No net change' means the sum of all energy stores before and after any transfer is equal. Describe specific transfers, such as a falling object where gravitational potential energy decreases as kinetic energy increases, and state that the total energy is unchanged.

What earns the marks

• Defines a closed system as one in which no energy enters or leaves, so the total energy stored remains constant.
• Describes a named example, such as a pendulum or falling object, identifying the stores involved and the direction of transfer.
• States that energy is conserved: the total energy before a transfer equals the total energy after, with no net change.
• Uses the idea that energy can transfer between stores but cannot be created or destroyed within a closed system.
• Applies the principle to a simple calculation or comparison, showing that the sum of energy stores is unchanged.

Common mistakes and how to put them right

• Saying energy is 'used up' or 'lost' in a closed system.

  Put it right by stating energy is transferred to other stores and the total remains constant.

• Assuming no energy transfers occur in a closed system.

  Put it right by explaining transfers still happen between stores inside the system, but no energy enters or leaves.

• Thinking that energy dissipated to the surroundings can happen in a closed system.

  Put it right by remembering that if energy transfers to the surroundings, the system is not closed.

Exam tips

• Name the system and state clearly that it is closed before describing the transfers.
• Use the phrase 'no net change to the total energy' to show understanding of conservation.
• Give a specific example with named stores and describe the direction of each transfer.
• Check that your example does not allow energy to leave the system, otherwise it is not closed.