The Physics Question Bank

Ramped Questions for GCSE Physics

Transformer Turns Ratio (Vp/Vs = np/ns)

✏️ Paper first! Work out every question on paper before you tap Show solution. Write down every step – the equation, the numbers with units, the rearranging and the answer with its unit. In the exam, if your final answer is wrong you can still get marks for correct working, but only if the examiner can see it.

Course: Separate Physics only | Tier: Higher only | Equation sheet: given in the exam

The equation

p.d. across primary ÷ p.d. across secondary = turns on primary ÷ turns on secondary V_p / V_s = n_p / n_s

Quantity	Symbol	Unit
primary p.d.	V_p	volts (V)
secondary p.d.	V_s	volts (V)
turns on primary coil	n_p	no unit
turns on secondary coil	n_s	no unit

Rearranged: V_s = V_p × n_s ÷ n_p | n_s = n_p × V_s ÷ V_p | n_p = n_s × V_p ÷ V_s | Step-up: more secondary turns; step-down: fewer.

Use FIFA for every answer

F – Formula: write the equation as it appears on the sheet.
I – Insert: put in the numbers (both p.d.s in the same unit).
F – Fix: rearrange to make the unknown the subject.
A – Answer: calculate and give the unit (turns have none).

Higher tier only: work through all 10. Most questions need rearranging.

Questions

Q1 A transformer has 200 turns on the primary and 20 on the secondary. The primary is connected to 230 V. Calculate the secondary p.d.

Show solution

F: V_p / V_s = n_p / n_s
I: 230 ÷ V_s = 200 ÷ 20 = 10
F: V_s = 230 ÷ 10
A: 23 V

Q2 A transformer steps 230 V down to 12 V. The primary has 1150 turns. How many turns are on the secondary?

Show solution

F: V_p / V_s = n_p / n_s
I: 230 ÷ 12 = 1150 ÷ n_s
F: n_s = 1150 × 12 ÷ 230
A: 60 turns

Q3 A transformer gives 6 V from its 30-turn secondary when the primary is connected to 240 V. How many turns are on the primary?

Show solution

F: V_p / V_s = n_p / n_s
I: 240 ÷ 6 = n_p ÷ 30
F: n_p = 30 × 240 ÷ 6
A: 1200 turns

Q4 A power station transformer steps 25 kV up to 400 kV. The primary has 500 turns. How many turns are on the secondary?

Show solution

F: V_p / V_s = n_p / n_s
I: 25 ÷ 400 = 500 ÷ n_s (both p.d.s in kV)
F: n_s = 500 × 400 ÷ 25
A: 8000 turns

Q5 A doorbell transformer has 2300 primary turns and 80 secondary turns. It is connected to the 230 V mains. Calculate the p.d. for the bell.

Show solution

F: V_p / V_s = n_p / n_s
I: 230 ÷ V_s = 2300 ÷ 80
F: V_s = 230 × 80 ÷ 2300
A: 8 V

Q6 A substation transformer steps 400 kV down to 132 kV. The secondary has 3300 turns. How many turns are on the primary?

Show solution

F: V_p / V_s = n_p / n_s
I: 400 ÷ 132 = n_p ÷ 3300
F: n_p = 3300 × 400 ÷ 132
A: 10 000 turns

Q7 A phone charger transformer changes 230 V to 5.0 V. Its secondary has 40 turns. How many turns are on the primary?

Show solution

F: V_p / V_s = n_p / n_s
I: 230 ÷ 5.0 = n_p ÷ 40
F: n_p = 40 × 230 ÷ 5.0
A: 1840 turns

Q8 A step-up transformer has 2.0 × 10³ primary turns and 3.2 × 10⁴ secondary turns. The primary p.d. is 2.5 × 10⁴ V. Calculate the secondary p.d.

Show solution

F: V_p / V_s = n_p / n_s
I: 2.5 × 10⁴ ÷ V_s = 2.0 × 10³ ÷ 3.2 × 10⁴
F: V_s = 2.5 × 10⁴ × 3.2 × 10⁴ ÷ 2.0 × 10³
A: 4.0 × 10⁵ V

Q9 A transformer with 460 primary turns and 24 secondary turns is connected to 230 V. The secondary powers a 6 Ω lamp. Calculate the current in the lamp.

Show solution

Step 1 – secondary p.d.
F: V_p / V_s = n_p / n_s
I: 230 ÷ V_s = 460 ÷ 24
F: V_s = 230 × 24 ÷ 460 = 12 V
Step 2 – current: I = V ÷ R = 12 ÷ 6
A: 2 A

Q10 A transformer has 1500 primary turns and 85 secondary turns. It is connected to 230 V. Calculate the secondary p.d., to 2 significant figures.

Show solution

F: V_p / V_s = n_p / n_s
I: 230 ÷ V_s = 1500 ÷ 85
F: V_s = 230 × 85 ÷ 1500 = 13.03 V
A: 13 V (2 s.f.)