物理交流電 DSE、CE 選擇題（第 3 頁）

21.

Which of the following can increase the efficiency of a transformer ?

(1)Increasing the number of turns of the secondary coil.

(2)Using a laminated iron core.

(3)Using a thicker copper wires to make the coils.

22.

Fused 200 V primary circuit, 10 V secondary with identical bulbs rated 40 W 10 V in parallel

In the above circuit diagram, the transformer is $100\%$ efficient. What is the maximum number of identical light bulbs, each of rating '40 W, 10 V' that can be connected in parallel across the secondary coil without blowing the fuse ? (Assume that the fuse will blow if the current flowing through it exceeds $15$ A.)

23.

200 V a.c. supply, transformer, two lamps each marked 10 V 40 W in parallel

In the circuit shown, the efficiency of the transformer is $80\%$ . If the two lamps are to work at their rated values, what is the current in the primary coil and what kind of transformer is being used ?

	Current in primary coil	Transformer
	$0.2$ A	$20:1$ step down
	$0.4$ A	$20:1$ step down
	$0.5$ A	$20:1$ step down
	$0.4$ A	$10:1$ step down

24.

Generator, transformer P, transmission cable, transformer Q and factory in series

The figure above shows how electrical power can be transmitted to a distant factory. The transformers $P$ and $Q$ should be

	$P$	$Q$
	step up	step up
	step up	step down
	step down	step up
	step down	step down

25.

Which of the following correctly shows the major change of energy in the devices ?

	Device	From	To
(1)	an electric motor	electrical	mechanical
(2)	a loudspeaker	sound	mechanical
(3)	a transformer	electrical	electrical

26.

A transformer can be used to

(1)step-up the voltage of a given a.c. supply.

(2)increase the power of a given a.c. supply.

(3)step-up the voltage of a given battery.

27.

The efficiency of a transformer is $50\%$ and the input and output voltages are $200$ V and $12$ V respectively. If the power output is $30$ W, the input current is

28.

Block diagram: 300 V generator, transmission cables, machinery rated 200 V 10 kW

The $300$ V generator in the above diagram is supplying electrical power to operate a piece of machinery which has a rating of $10$ kW at $200$ V. The total resistance of the transmission cables is $2\ \Omega$ . What is the power loss in the transmission system ?

29.

Transformer with 200 V a.c. supply on the primary coil and a 100 ohm resistor on the secondary coil

The diagram shows a transformer with the primary coil connected to an a.c. voltage of $200$ V while the secondary coil is connected to a resistor of $100\ \Omega$ . If there are $100$ turns in the primary coil and $10$ turns in the secondary coil, what is the current passing through the resistor ? (Assume that the efficiency of the transformer is $100\%$ )

30.

Transformer with 200 V across the primary coil and 10 V across the secondary coil carrying 3 A

As shown in the diagram, a $200$ V mains supply is stepped down to $10$ V by a transformer. If the output current is $3$ A and the efficiency of the transformer is $75\%$ , what is the current in the primary coil ?