Power: power as the rate of energy transfer or work done, the power equation, the watt as a joule per second, and the core practical measuring personal power.

What this dot point is asking

Edexcel statements 8.12 to 8.14 want you to define power as the rate at which energy is transferred (or work is done), to recall and use the power equation, to recall that one watt is one joule per second, and to apply this in the core practical measuring personal power (for example running up stairs).

What power means

Power is about speed of energy transfer, not the total energy. Two motors might transfer the same total energy, but the one that does it faster has the greater power. This is why a powerful engine accelerates a car more quickly: it transfers energy to the kinetic store at a faster rate.

The power equation

The equation rearranges to find energy ($E = P \times t$) or time ($t = \dfrac{E}{P}$). Because work done equals energy transferred, you can use either the work done or the energy transferred in the top of the equation, which is how stairs-climbing and lifting problems are solved.

The personal power core practical

This practical brings together work done and power. The work done is the weight multiplied by the vertical height (the gain in GPE), and dividing by the time gives the power. Repeating and averaging, and measuring the height and time carefully, improve the result; the vertical height (not the distance along the stairs) is the key measurement.

How Edexcel examines this

This dot point is examined on both tiers, both as a direct power calculation and as a two-stage problem combining work done with power. The mark scheme for the direct calculation rewards the equation, substitution and the unit watt, so write $P = \frac{E}{t}$ first. The classic two-stage question gives a person's weight, a vertical height and a time and asks for the power developed; the full-mark route is to find the work done (weight times height, equal to the gain in GPE) and then divide by the time. Examiners reward both steps and penalise using the distance along the stairs instead of the vertical height. The core practical may be examined as a method question, rewarding measuring weight, vertical height and time, calculating the work done and dividing by time, and improving accuracy by repeating. Defining the watt as a joule per second is a frequent one-mark recall, and rearranging the equation to find energy or time also appears. Keep every quantity in SI units, converting kilowatts to watts where needed.

Try this

Q1. State what one watt is equal to. [1 mark]

• Cue. One joule per second ($1\,\text{J/s}$).

Q2. A device transfers $400\,\text{J}$ in $8\,\text{s}$. Calculate its power. [2 marks]

• Cue. $P = \dfrac{E}{t} = \dfrac{400}{8} = 50\,\text{W}$.