| Date | May 2017 | Marks available | 4 | Reference code | 17M.2.HL.TZ1.8 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 8 | Adapted from | N/A |
Question
The diagram shows a sketch of an ideal step-down transformer.
The number of turns in the primary coil is 1800 and that in the secondary coil is 90.
State Faraday’s law of induction.
Explain, using Faraday’s law of induction, how the transformer steps down the voltage.
The input voltage is 240 V. Calculate the output voltage.
Outline how energy losses are reduced in the core of a practical transformer.
Step-up transformers are used in power stations to increase the voltage at which the electricity is transmitted. Explain why this is done.
Markscheme
the size of the induced emf
is proportional/equal to the rate of change of flux linkage
The word ‘induced’ is required here.
Allow correctly defined symbols from a correct equation. ‘Induced’ is required for MP1.
varying voltage/current in primary coil produces a varying magnetic field
this produces a change in flux linkage / change in magnetic field in the secondary coil
a «varying» emf is induced/produced/generated in the secondary coil
voltage is stepped down as there are more turns on the primary than the secondary
Comparison of number of turns is required for MP4.
output voltage \( = \frac{{90 \times 240}}{{1800}}\)
= 12 «V»
laminated core reduces eddy currents
less thermal energy is transferred to the surroundings
for a certain power to be transmitted, large V means low I
less thermal energy loss as P = I2R / joule heating
