Give examples that illustrate the definition of power eg comparing two electric motors that both lift the same weight through the same height but one does it faster than the other.
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GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Power
Energy changes in a system, and the ways energy is stored
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the specified scientific knowledge of power to a relevant example.
Analyse an unfamiliar example of power using the specified scientific ideas.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
Physics
- What Is Power?
- Power describes how quickly energy is transferred, meaning two scenarios can involve the same energy transfer but different amounts of power if the time taken differs.
- A person running up stairs and a person walking up stairs both transfer the same amount of energy (e.g.
- 10 J), but the runner has greater power because they complete the task in less time.
The Two Equations for Power
- Power can be calculated using: P = E / t, where E is energy transferred in joules (J) and t is time in seconds (s).
- The same relationship can be written as: P = W / t, where W is work done — in physics, work done means exactly the same thing as energy transferred.
- Neither of these equations is provided on the GCSE equation sheet, so both must be memorised.
The Unit of Power: Watts
- The unit of power is the watt (W), which is equivalent to one joule per second (J/s).
- The abbreviation for watts is a capital W, and it tells you how many joules of energy are transferred every second.
Time Must Be in Seconds
- When using the power equations, time must always be in seconds (s), so you may need to convert minutes or hours into seconds before calculating.
- To convert minutes to seconds, multiply by 60; to convert hours to seconds, multiply by 3,600.
Power Ratings of Appliances
- The power rating of an appliance tells you how many joules of energy it transfers per second.
- A 100 W speaker transfers 100 J of energy every second, whilst a 50 W lamp transfers 50 J every second.
- An appliance with a higher power rating transfers more energy per second than one with a lower power rating.
Worked Example 1: Straightforward Power Calculation
- A motor transfers 20 J of energy in 10 s; using P = E / t, the power is frac{20} {10} = 2 W.
- This example requires only one equation and is a typical one-mark or two-mark GCSE question.
Gravitational Potential Energy (GPE)
- When an object is lifted, it gains gravitational potential energy (GPE), calculated using: Egpe = mgh, where m is mass in kg, g is gravitational field strength in N/kg, and h is height in metres (m).
- The value of g is often given as 10 N/kg in GCSE questions, though it may also appear as 9.8 or 9.81 N/kg.
- This equation must also be memorised for the GCSE exam.
Worked Example 2: Two-Step Power Calculation
- A 5 kg mass is lifted 1.2 m in 30 s (g = 10 N/kg); first calculate GPE: E = 5 × 10 × 1.2 = 60 J.
- Then use the power equation: P = 60 / 30 = 2 W, showing that two-step questions require linking the GPE equation with the power equation.
- This type of question is worth more marks because it requires two separate equations and two calculation steps.
Key Takeaways on Power
- Power depends on both the amount of energy transferred and the time taken, so the same task done more quickly requires greater power.
- Power is measured in watts (W), equivalent to joules per second, and both power equations (P = E / t and P = W / t) must be learnt by heart.
- The concept of power will be revisited in the electricity topic, where power ratings of electrical appliances are explored further.