In AQA GCSE Physics you are given the equation sheet, so you don’t have to memorise every equation. But that creates a new problem: with over 20 equations in front of you, which one do you use? Here is a method that works every time.
Step 1: list what you know
Read the question and write down every number with its symbol and unit. Units are a huge clue. A number in watts is a power (P). A number in coulombs is a charge (Q). A number in N/kg is gravitational field strength (g).
Step 2: write what you want
Find the word straight after “calculate” and write its symbol with a question mark, for example E = ?
Step 3: find the equation with exactly those letters
Scan the equation sheet for the one equation that contains all your symbols and nothing else you don’t know. That’s your equation.
Example 1
A 2.0 kW kettle is switched on for 3 minutes. Calculate the energy transferred.
Know: P = 2.0 kW = 2000 W, t = 3 min = 180 s
Want: E = ?
Equation with P, t and E: E = P t
Answer: E = 2000 × 180 = 360 000 J
Example 2
30 C of charge flows through a lamp and 360 J of energy is transferred. Calculate the potential difference across the lamp.
Know: Q = 30 C, E = 360 J
Want: V = ?
Equation with E, Q and V: E = Q V
Answer: 360 = 30 × V, so V = 360 ÷ 30 = 12 V
Both questions are about energy transferred, but they need different equations. That’s why you match the other quantities, not just the one you’re looking for.
What if no single equation fits?
Then it’s a multi-step problem. Use one equation to find a missing quantity, then use that answer in a second equation. For example, to find the speed of a falling ball you might find the energy with Ep = mgh first, then use Ek = ½mv². These are common in Higher papers and are worth lots of marks.
Practise choosing
Practising one equation at a time is great for learning it, but in the exam the questions are mixed up. Our mixed sets make you choose, just like the real thing.
- Which equation? Mixed questions by topic
- Multi-step problems
- The equation sheet: get to know where everything is before the exam
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