Electrical calculations for GCSE Engineering

GCSE Engineering, Systems. Usually met in years 9 to 11.

Calculate voltage, current, resistance and power, and combine resistors in series and parallel.

What you practise

Ohm’s law: V = I × R
Power: P = V × I
Series and parallel resistors and running costs

Starts with Ohm’s law and builds to parallel resistors and the cost of running a device. Practice moves up a level when a round goes well and down when it does not.

Sample questions

Each sample comes with the method and the reason every wrong option is wrong. Real practice rounds are ten questions at your level.

Sample question, level 2 of 5

A 0.4 V supply is connected across a 4 Ω resistor. What current flows?

1.1 A
1 A
0.1 A
1.6 A
10 A

Show the answer and method

Answer: C, 0.1 A

The method: I = V ÷ R = 0.4 ÷ 4 = 0.1 A.

Why the other options are wrong:

A, 1.1 A: Close, but check your working: this is a small slip away from the answer.
B, 1 A: The decimal point is one place out.
D, 1.6 A: The voltage was multiplied by the resistance. I = V ÷ R.
E, 10 A: The division is upside down. I = V ÷ R.

Sample question, level 4 of 5

Resistors of 15 Ω and 33 Ω are connected in series. What is the total resistance?

96 Ω
48 Ω
495 Ω
10.313 Ω
33 Ω

Show the answer and method

Answer: B, 48 Ω

The method: In series, resistances add: 15 + 33 = 48 Ω.

Why the other options are wrong:

A, 96 Ω: The total was doubled.
C, 495 Ω: The resistances were multiplied. In series, add them.
D, 10.313 Ω: This is the parallel formula. In series, just add them.
E, 33 Ω: This is only the largest resistor. In series, add them all.

See where this fits: every GCSE option topic and Key Stage 3.