Power

OCR

GCSE

Power is rigorously defined as the rate at which energy is transferred or the rate at which work is done. It quantifies the speed of energy conversion between stores, measured in Watts (W), where one Watt represents one Joule per second. The concept bridges mechanical systems, where power relates to force and velocity, and electrical systems, where it relates to potential difference and current. Mastery involves applying formulae for mechanical and electrical power, converting between units such as kilowatts and Watts, and evaluating the efficiency of energy transfer in practical contexts.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Subtopics in this area

Power

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that Power is the rate at which work is done or energy is transferred
Credit the correct substitution of values into $P = I^2 R$, specifically looking for evidence of squaring the current
Award 1 mark for converting time into seconds (e.g., 5 minutes to 300s) before using $P = E/t$
Allow 1 mark for the correct unit: Watts (W) or Joules per second (J/s), but reject 'Joules' alone
Award 1 mark for explicitly defining power as the 'rate of energy transfer' or 'work done per second' — vague references to 'energy over time' are often insufficient
Credit the correct substitution of values into $P = E/t$ or $P = W/t$ before any rearrangement attempts
Award 1 mark for converting non-standard units (e.g., minutes to seconds, kilowatts to Watts) prior to calculation
In electrical contexts, credit the selection of $P = I^2R$ over $P = IV$ when voltage is not explicitly given but resistance is known

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You have correctly identified the formula, but check your units — remember that time must always be in seconds for Watts"
"Your definition is incomplete; you must specify that power is the *rate* of energy transfer, not just the transfer itself"
"Good calculation of the total energy, but the question asked for Power. Remember to divide by time as the final step"
"When rearranging $P=I^2R$ to find current, don't forget the final step is to square root the result"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that Power is the rate at which work is done or energy is transferred
Credit the correct substitution of values into $P = I^2 R$, specifically looking for evidence of squaring the current
Award 1 mark for converting time into seconds (e.g., 5 minutes to 300s) before using $P = E/t$
Allow 1 mark for the correct unit: Watts (W) or Joules per second (J/s), but reject 'Joules' alone
Award 1 mark for explicitly defining power as the 'rate of energy transfer' or 'work done per second' — vague references to 'energy over time' are often insufficient
Credit the correct substitution of values into $P = E/t$ or $P = W/t$ before any rearrangement attempts
Award 1 mark for converting non-standard units (e.g., minutes to seconds, kilowatts to Watts) prior to calculation
In electrical contexts, credit the selection of $P = I^2R$ over $P = IV$ when voltage is not explicitly given but resistance is known

Examiner Tips

Expert advice for maximising your marks

💡When a question asks to 'Show that' the power is a specific value, you must write down the unrounded calculation result before the final answer to secure the mark.
💡Memorise that 1 Watt is strictly defined as 1 Joule per second; stating this relationship often earns a definition mark even if the wording is imperfect.
💡Check the power rating prefixes carefully; 'kW' requires multiplication by 1000, and 'MW' by 1,000,000 before substitution.
💡When asked to 'Define Power', always use the phrase 'rate of' or 'per second'; stating 'amount of work done' without the time factor earns zero marks
💡For 4-5 mark calculation questions, write down your known variables (e.g., $R=10\Omega$, $P=5W$) to help select the correct equation immediately
💡If a question involves a motor lifting a load, calculate the Gravitational Potential Energy ($E_p = mgh$) first, then divide by time to find the useful power output

Common Mistakes

Pitfalls to avoid in your exam answers

Failing to square the current term when applying the electrical power equation $P = I^2 R$
Confusing energy (Joules) with power (Watts), often by stating 'Power is the energy used'
Neglecting to convert time from minutes or hours into seconds, resulting in calculation errors by a factor of 60 or 3600
Incorrectly rearranging $P = E/t$ to find time, often multiplying Power by Energy instead of dividing Energy by Power
Confusing the units for Energy (Joules) and Power (Watts), often interchanging them in final answers
Failing to square the current ($I$) when using the formula $P = I^2R$, or forgetting to take the square root when rearranging to find $I$
Neglecting to convert time from minutes or hours into seconds when applying $P = E/t$
Calculating efficiency as a value greater than 1 (or >100%) by dividing input by output instead of output by input

Key Terminology

Essential terms to know

Rate of energy transfer and work done (P = E/t)

Electrical power relationships (P = IV, P = I²R)

The Watt as a derived unit (1 W = 1 J/s)

Power ratings and efficiency

Rate of energy transfer and work done (P = E/t)

The Watt (W) as a derived SI unit (1 J/s)

Electrical power relationships (P = IV, P = I²R)

Efficiency and power dissipation mechanisms

Likely Command Words

How questions on this topic are typically asked

Calculate

Define

Explain

Show that

Suggest

Compare

Practical Links

Related required practicals

•{"code":"P3","title":"Investigation of Resistance","relevance":"Calculating power dissipated in components using P=IV and P=I^2R"}
•{"code":"P1","title":"Specific Heat Capacity","relevance":"Using the power rating of an immersion heater to calculate energy transferred (E = P x t)"}
•{"code":"PAG P5","title":"Investigation of I-V characteristics","relevance":"Calculating power dissipation in components using measured V and I values"}
•{"code":"PAG P1","title":"Specific Heat Capacity","relevance":"Using the power rating of an immersion heater ($P=IV$) to determine energy supplied ($E=Pt$)"}

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