Explain what is meant by conservation of energy

The idea

Conservation of energy is the principle that energy is never created or destroyed; it is only transferred between stores or transformed from one form to another. In a closed system the total energy stays constant. For example, a ball dropped from rest transfers energy from its gravitational potential store to its kinetic store as it falls, and on impact some energy is transferred by heating and by sound to the surroundings, but the total amount of energy is unchanged. Recognising this lets you account for energy in processes such as a pendulum swinging, a battery lighting a lamp, or a falling object. In multiple-choice questions, look for the option that states energy is transferred or conserved, not one that claims energy is used up or lost.

What earns the marks

• States that energy cannot be created or destroyed, only transferred between stores or transformed from one form to another.
• Identifies that the total energy of a closed system remains constant.
• Recognises that energy transferred by heating or sound to the surroundings is not destroyed but dissipated.
• Applies the principle to a named process, such as a falling object transferring energy from a gravitational potential store to a kinetic store.

Common mistakes and how to put them right

• Claims that energy is used up or disappears when a device stops working.

  Put it right by explaining that energy is transferred to the surroundings, often by heating.

• Confuses energy with force or power.

  Put it right by stating that energy is measured in joules and is a property of a system, not a push or a rate.

• Assumes energy can be created by a machine.

  Put it right by stating that a machine only transfers energy from one store to another.

Exam tips

• Read all options before choosing, because several may mention energy but only one states conservation correctly.
• Reject any option that says energy is lost, used up or destroyed.
• Link the chosen option to a simple example, such as a falling ball, to check it makes sense.