Transformers: Class 12 practice
For an ideal transformer V_s/V_p = N_s/N_p = I_p/I_s. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Ratios
The idea behind transformers
For an ideal transformer V_s/V_p = N_s/N_p = I_p/I_s. Step-up transformers raise voltage and lower current; power in = power out (ideal).
Step-by-step method
- Write the turns ratio.
- V_s = V_p × N_s/N_p.
- I_s = I_p × N_p/N_s; efficiency = output/input.
Worked example 1 (Basic)
A transformer has 300 primary and 30 secondary turns on 220 V. Find the secondary voltage.
- V_s = 220 × 30/300
- = 22 V (step-down)
- Answer: 22 V
Worked example 2 (Standard)
With primary current 9 A in the same ideal transformer, find the secondary current.
- I_s = I_p × N_p/N_s
- = 90 A
- Answer: 90 A
Worked example 3 (Challenge)
A transformer draws 1540 W and is 95% efficient. Find the output power.
- P_out = 95% of 1540
- = 1463 W
- Answer: 1463 W
Common mistakes and misconceptions
- Inverting the current ratio
- Expecting transformers to work on DC
Practice questions
- (Basic) A transformer has 600 primary and 300 secondary turns on 220 V. Find the secondary voltage.
- (Basic) A transformer has 500 primary and 250 secondary turns on 110 V. Find the secondary voltage.
- (Standard) With primary current 7 A in the same ideal transformer, find the secondary current.
- (Standard) With primary current 1 A in the same ideal transformer, find the secondary current.
- (Challenge) A transformer draws 1680 W and is 95% efficient. Find the output power.
- (Challenge) A transformer draws 1100 W and is 80% efficient. Find the output power.
Answer key with full working
- Q1: V_s = 220 × 300/600 → = 110 V (step-down) → Answer: 110 V
- Q2: V_s = 110 × 250/500 → = 55 V (step-down) → Answer: 55 V
- Q3: I_s = I_p × N_p/N_s → = 1.75 A → Answer: 1.75 A
- Q4: I_s = I_p × N_p/N_s → = 0.5 A → Answer: 0.5 A
- Q5: P_out = 95% of 1680 → = 1596 W → Answer: 1596 W
- Q6: P_out = 80% of 1100 → = 880 W → Answer: 880 W
Syllabus note
This skill is listed under "Alternating Current" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is transformers in?
"Alternating Current", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.