Work, Power & Efficiency (Edexcel GCSE Physics): Revision notes

Section 1

What is work done, and how is it related to energy?

Work done is a measure of the energy transferred when a force causes something to move.

Equation: E = F × d (energy transferred, or work done, in joules = force in newtons × distance moved in the direction of the force, in metres).
Energy transferred (in joules) is equal to work done (in joules) — they are the same quantity, measured the same way.
Work is only done on an object when a force causes it to move; if there is no movement in the direction of the force, no work is done by that force.

Key termswork done

Example

A force of 20 N pushes a box 3 m. Work done = F × d = 20 × 3 = 60 J.

Section 2

How can the energy of a system change?

The energy of a system can be changed in three main ways:

Through work done by forces (mechanical work, e.g. lifting, pushing, stretching)
Through electrical equipment (electrical energy transferred to other stores)
Through heating (thermal energy transferred directly, without doing mechanical work)

Energy transfer diagrams can be drawn and interpreted to represent how energy moves between stores during any of these changes. In a closed system, energy transfers occur between stores but there is no net change to the total energy in that system.

Two key related equations:

Change in gravitational potential energy: ΔGPE = m × g × Δh
Kinetic energy: KE = ½ × m × v²

Exam tip

When a question asks 'how is the energy of the system changed', pick from the three routes: work done by forces, electrical equipment, or heating — name the correct one for the scenario.

Section 3

Why is not all work done useful?

In all system changes, energy is dissipated, meaning it ends up stored in less useful ways — typically spread out into the thermal store of the surroundings.
Mechanical processes become wasteful when they cause a rise in temperature, dissipating energy in heating the surroundings (for example, friction between moving parts).
This wasted energy is not destroyed (energy is always conserved) but becomes far less useful for doing further work.

Key termsdissipated

Section 4

What are power and efficiency?

Power is defined as the rate at which energy is transferred (or work is done).

Equation: P = E ÷ t (power in watts = energy transferred in joules ÷ time in seconds).
One watt is equal to one joule per second (1 W = 1 J/s).

Efficiency compares how much of the total energy supplied to a device is usefully transferred:

efficiency = useful energy transferred by the device ÷ total energy supplied to the device

Efficiency can be expressed as a decimal (0 to 1) or as a percentage. No real device is 100% efficient, since some energy is always dissipated.

Key termspowerwattefficiency

Example

A motor transfers 300 J of energy in 5 s. Power = E ÷ t = 300 ÷ 5 = 60 W.

Must Know

Work done = force × distance moved in the direction of the force: E = F × d. Work done (J) = energy transferred (J).
A system's energy can be changed by: work done by forces, electrical equipment, or heating.
In a closed system, energy transfers between stores but the total stays the same.
Energy is always dissipated in system changes, ending up less useful — mechanical processes waste energy when they cause heating.
Power = energy transferred ÷ time: P = E ÷ t. 1 watt = 1 joule per second.
Efficiency = useful energy transferred ÷ total energy supplied; no device is 100% efficient.

That's the notes covered.

Carry on to the next subtopic.