Explain and calculate electrical power using P = VI and P = I²R, relating it to energy transferred over time.
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GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Power
Energy transfers
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the scientific explanation of power to a relevant example.
Analyse a new situation involving power and explain the scientific reasoning.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
Physics
- What Is Power?
- Power is the rate at which energy is transferred, meaning it takes into account both the amount of energy transferred and the time taken to do so.
- Two people climbing the same stairs transfer the same amount of energy (e.g.
- 10 J), but the one running has greater power because they complete the task in less time.
- Power is different from energy — the same task can be done with different amounts of power depending on how quickly it is completed.
The Unit of Power: The Watt
- The unit of power is the watt (W), named after James Watt, where 1 watt equals 1 joule per second (1 W = 1 J/s).
- Power is measured in watts (W), which tells us how many joules of energy are transferred every second.
- The abbreviation for watts uses a capital W to distinguish it from other units.
Power Equation 1: Energy Transferred
- The first equation for power is: P = E / t, where P is power in watts (W), E is energy transferred in joules (J), and t is time in seconds (s).
- Time must always be in seconds when using this equation, so you may need to convert minutes or hours into seconds before calculating.
- This equation is not provided on the GCSE equation sheet, so it must be memorised.
Power Equation 2: Work Done
- The second equation for power is: P = W / t, where W is work done in joules (J) and t is time in seconds (s).
- Work done and energy transferred mean exactly the same thing in physics — both describe the transfer of energy from one store to another.
- Like the first equation, this formula is not given on the GCSE equation sheet and must be memorised.
Power Ratings of Appliances
- The power rating of an appliance tells you how many joules of energy it transfers every second.
- A 100 W speaker transfers 100 joules of energy per second, while a 50 W lamp transfers 50 joules per 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 seconds; using P = E / t, the power is P = 20 / 10 = 2 W.
- Always include the unit (watts, W) in your final answer when calculating power.
Gravitational Potential Energy: A Linked Equation
- Gravitational potential energy (GPE) is the energy an object gains due to its height above the ground, calculated using: Egpe = mgh, where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).
- This equation is needed when a question gives you mass, height, and gravitational field strength instead of directly giving you the energy transferred.
- The value of g is typically given as 10 N/kg in GCSE questions, though it may also appear as 9.8 or 9.81 N/kg.
Worked Example 2: Two-Step Power Calculation
- When a 5 kg mass is lifted 1.2 m in 30 s (g = 10 N/kg), first calculate GPE: Egpe = 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 and power equations together.
- Two-step calculations like this are worth more marks in an exam because they require you to apply two equations in sequence.
Key Reminders & Exam Tips
- Both power equations (P = E / t and P = W / t) must be memorised as they are not provided on the GCSE equation sheet.
- Always check that time is converted into seconds before substituting values into the power equation.
- Power will be revisited in the topic of electricity, where it is applied to electrical appliances and circuits.