Page 477 - Physics Coursebook 2015 (A level)

Chapter 29: Alternating currents
8 The rectified output from a transformer is connected to a resistor R of resistance 1000 Ω. Graph A in Figure 29.24 shows the variation with time t of the p.d. V across the resistor. Graph B shows the variation of V when a capacitor is placed across R to smooth the output.
V / V 1.0
0.8 0.6 0.4 0.2
0 0
Figure 29.24
0.01
0.02
0.03
V /V 1.0
0.8 0.6 0.4 0.2
0 0
0.01 0.02 0.03
Graph B
t / s For End-of-chapter Question 8.
0.04 t / s
Graph A
a Explain how the rectification is achieved. Draw a circuit diagram to show the components involved. [6]
b Explain the action of the capacitor in smoothing the output. [3]
c Using graph B between t = 0.005 and t = 0.015 s, determine:
i the time during which the capacitor is charging [1]
ii the mean value of the p.d. across R [1]
iii the average power dissipated in R. [2]
9 A transformer is used to produce an output of peak value 12 V from the 230 V r.m.s. mains supply.
a The primary coil of the transformer contains 2000 turns. Calculate the number of turns in the
secondary coil. [3]
b The output from the secondary coil is half-wave rectified, and connected to a resistor R and
capacitor C in parallel.
Sketch graphs on the same axes to show the variation with time of:
i the output p.d. of the secondary coil [1]
ii the p.d. across R. [2]
c State and explain what happens to the p.d. across R when another capacitor of equal value is placed
in parallel with C. [3]
10 Electrical energy is supplied by a high-voltage power line which has a total resistance of 4.0 Ω. At the input to the line, the root-mean-square (r.m.s.) voltage has a value of 400 kV and the input power is 500 MW.
a i Explain what is meant by root-mean-square voltage. [2]
ii Calculate the minimum voltage that the insulators which support the line must withstand without
breakdown. [2]
b i Calculate the value of the r.m.s. current in the power line. [2] ii Calculate the power loss on the line. [2] iii Explain why it is an advantage to transmit the power at a high voltage. [2]
c Power at 400 kV is converted to power at 124 kV by a step-down transformer. Describe the basic
principle of a step-down transformer. [2]
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