Quick answer: An alternating current reverses direction regularly, and its rms value, V₀/√2 for a sine wave, gives the equivalent steady heating effect. This Physics 11-12 test on transformers has 8 questions worth 20 marks, to do in 45 minutes: defining rms, peak to rms and power in a resistor. The mark scheme is on a separate page; free printable PDF.
This test covers rms values, mean power, the turns-ratio equation and transmission losses. Show formulas, substitutions and units. Calculators are allowed. Treat all transformers as ideal unless a question says otherwise.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.7.5
Question 1 Defining rms (2 marks)
Also, explain what is meant by the rms value of an alternating current.
Question 2 Peak to rms (2 marks)
In addition, the peak value of an alternating voltage is 340 V. Calculate the rms value.
Question 3 Power in a resistor (3 marks)
A sinusoidal voltage of peak value 18 V is applied across a 12 Ω resistor. Moreover, calculate the mean power dissipated.
Question 4 Transformer turns (3 marks)
Also, an ideal transformer has 800 primary turns and is connected to 230 V rms. Its secondary voltage is 11.5 V.
- In addition, calculate the number of secondary turns. (1 mark)
- Moreover, the secondary supplies 23 W. Also, calculate the secondary and primary currents. (2 marks)
Question 5 Why high voltage? (3 marks)
In addition, explain why electrical energy is transmitted at very high voltage over long distances.
Question 6 Reading a trace (3 marks)
The graph shows an alternating voltage.
- Read the period and calculate the frequency. (2 marks)
- Calculate the rms voltage. (1 mark)
Question 7 Efficiency (2 marks)
A transformer takes 250 W from the mains and delivers 235 W. Calculate its efficiency.
Question 8 Laminated core (2 marks)
State one way of reducing eddy-current losses in a transformer core and explain how it works.
