How to Show Working in IGCSE Physics Calculations

Use this guide to present calculation working that earns maximum method marks in Cambridge IGCSE Physics 0625 exams.

Why showing working matters

In a 3-mark calculation, the marks are typically split:

• 1 mark for writing the correct formula
• 1 mark for correct substitution
• 1 mark for the correct final answer with unit

If you skip the formula and substitution, then make a calculator error, you score 0 out of 3. If you show full working, the same calculator error costs you only the final mark: you still get 2 out of 3.

The four-step method

Step 1: Write the formula

Write the equation from memory or the equation sheet. Use standard symbols.

Example: v = s / t

Step 2: Substitute the values

Replace each symbol with the number from the question. Include units at this stage.

Example: v = 120 m / 30 s

Step 3: Calculate

Perform the arithmetic. Show intermediate steps if the calculation involves rearrangement.

Example: v = 4 m/s

Step 4: State the answer with its unit

Write the final answer clearly. Circle it or underline it so the examiner can find it.

Example: v = 4 m/s

Worked example: Specific heat capacity (4 marks)

Question: A 2.0 kg block of aluminium is heated from 20 degrees C to 70 degrees C. The energy supplied is 90 000 J. Calculate the specific heat capacity of aluminium.

Working:

E = m c (change in T)

change in T = 70 - 20 = 50 degrees C

c = E / (m x change in T)

c = 90 000 / (2.0 x 50)

c = 90 000 / 100

c = 900 J/(kg degrees C)

Mark allocation: formula (1), substitution (1), change in T correct (1), final answer with unit (1).

Worked example: Electrical power (3 marks)

Question: A lamp has a resistance of 12 ohm and a current of 2.5 A flowing through it. Calculate the power dissipated by the lamp.

Working:

P = I^2 x R

P = (2.5)^2 x 12

P = 6.25 x 12

P = 75 W

Common mistakes in showing working

• Forgetting the unit in the final answer. This is typically a separate mark.
• Using the wrong version of the formula and not showing the rearrangement.
• Mixing up units (using grams instead of kilograms, minutes instead of seconds).
• Writing the answer without enough significant figures or with too many.

Significant figures

The general rule: give your answer to the same number of significant figures as the data in the question, or to 2 or 3 significant figures, whichever is appropriate. Never round intermediate steps; only round the final answer.