物理交流電 DSE、CE 選擇題（第 2 頁）

11.

The figure shows a model power line. An a.c. supply and two ideal transformers are used to operate a lamp of rating '6 V, 24 W'. The total resistance of the cables $AB$ and $CD$ is $10\ \Omega$ . If the lamp operates at its rated value, find the power loss in the cables.

a.c. supply, 1:20 step-up transformer, cables AB and CD, 20:1 step-down transformer and a lamp rated 6 V 24 W

12.

Ideal transformer with ammeter in primary, voltmeter and bulb P across the secondary, and bulb Q in series with switch S

Two bulbs $P$ and $Q$ are connected to an ideal transformer as shown. Which of the following statements is/are true when switch $S$ is closed ?

(1)The brightness of bulb

P

decreases.

(2)The reading of the ammeter increases.

(3)The reading of the voltmeter remains unchanged.

13.

Which of the following is/are the advantage(s) of using high voltages for long distance transmission of electricity ?

(1)The transmission speed can be increased.

(2)Some heavy industries operate at high voltages.

(3)The energy loss in the transmission cable can be reduced.

14.

A '6 V, 6 W' light bulb is to be operated at its rated value. Three circuits are set up as shown below.

Circuit (1): 12 V battery with 6 ohm resistor in parallel with the bulb; circuit (2): 12 V battery with two 6 ohm resistors and the bulb; circuit (3): 12 V a.c. supply with a transformer of 200 turns primary and 100 turns secondary

The power supplies all have negligible internal resistance. In which of the above circuits is the bulb working at its rated value ?

15.

A $4$ V a.c. supply is stepped up to $20$ V by a transformer. If the current in the primary coil is $1$ A and the power loss of the transformer is $0.8$ W, find the current in the secondary coil.

16.

Which of the following devices converts mechanical energy into electrical energy when it works ?

17.

200 V supply on a 3600-turn primary; secondary tapped with two 90-turn sections giving points P, Q and R

The primary coil of a transformer has $3600$ turns and is connected to $200$ V a.c. supply. The secondary coil has $180$ turns, which can be tapped at different points as shown above. A '40 W, 10 V' light bulb is connected to the transformer so that it works at its rated value. Which of the following statements is/are correct ?

(1)The bulb should be connected to points

P

and

Q

.

(2)The current through the bulb is

4

A.

(3)If the efficiency of the transformer is

80\%

, the current in the primary coil is

0.25

A.

18.

An electrical power of $100$ kW is transmitted at $20$ kV through cables of total resistance of $4\ \Omega$ . Find the voltage drop and power loss in the cables.

	Voltage drop	Power loss
	$20$ V	$20$ W
	$20$ V	$100$ W
	$100$ V	$20$ W
	$100$ V	$100$ W

19.

200 V a.c. supply, transformer and a light bulb marked 40 W 10 V

In the circuit shown, the rating of the light bulb is '40 W, 10 V'. The efficiency of the transformer is $80\%$ . If the bulb works at its rated value, find the current in the primary coil.

20.

200 V a.c. supply, transformer and four light bulbs in parallel on the secondary coil

In the circuit shown, the rating of each light bulb is '20 W, 10 V'. The current in the primary coil is $0.5$ A. If all the bulbs work at their rated values, find the turns ratio and the efficiency of the transformer.

	Turns ratio	Efficiency
	$5:1$ step down	$80\%$
	$20:1$ step down	$20\%$
	$20:1$ step down	$40\%$
	$20:1$ step down	$80\%$