Energy stores and transfers: the named energy stores, the ways energy is transferred, and drawing and interpreting energy transfer diagrams for everyday systems.

What this dot point is asking

Edexcel statements 3.3 to 3.5 want you to draw and interpret diagrams that represent energy transfers, and to analyse the changes in the way energy is stored when a system changes, such as an object projected upwards, a moving object hitting an obstacle, an object accelerated by a constant force, a vehicle slowing down, or water brought to the boil in a kettle.

Energy stores

Thinking in terms of stores keeps energy accounting tidy: at any instant the energy of a system is shared among its stores, and a change moves energy from one store to another. For example, a stretched catapult has energy in its elastic store; releasing it transfers that energy to the kinetic store of the projectile.

Pathways: how energy is transferred

It is worth separating the store (where energy is) from the pathway (how it moves). Heating a kettle uses the electrical pathway to fill the thermal store of the water; a falling ball uses the mechanical pathway (gravity doing work) to fill its kinetic store. The same change can involve several pathways at once.

Energy transfer diagrams

To analyse a system change, name the store that loses energy, the store that gains the useful energy, and any store that gains wasted energy. For a car accelerating, the chemical store of the fuel falls, the kinetic store of the car rises, and the thermal store of the surroundings also rises (wasted energy from the engine and friction).

How Edexcel examines this

This dot point is examined on both tiers, usually as a short description or a "draw the energy transfer diagram" question worth two or three marks, and the ideas underpin the conservation, dissipation and efficiency statements that follow. Mark schemes reward the modern store-and-pathway language, so practise naming the kinetic, gravitational potential, elastic, thermal and chemical stores precisely, and describing transfers as mechanical, electrical, by heating or by radiation. A reliable full-mark technique for any system change is to name the store that loses energy, the store that usefully gains energy, and the store (almost always thermal, in the surroundings) that gains the wasted energy. Edexcel sets these on the specific examples listed in the specification, an object projected upwards, a moving object hitting an obstacle, an object accelerated by a constant force, a vehicle slowing down, and water boiling in a kettle, so rehearse the energy story for each. Avoid vague phrases such as "energy is lost"; energy is transferred to a named store, and saying so secures the marks.

Try this

Q1. Name the energy store that increases as an object is lifted higher. [1 mark]

• Cue. The gravitational potential store.

Q2. State the pathway by which energy is transferred when a current flows through a heater. [1 mark]

• Cue. Electrically (the current does work), then by heating.