物理交流電 DSE、CE 選擇題

1.

220 V mains supply, ideal transformer, lamps L1 and L2 in series on the secondary with a switch S across L1

The figure above shows an ideal transformer. The primary coil of the transformer is connected to the $220$ V a.c. mains and two lamps $L_1$ and $L_2$ are connected in series to the secondary coil. Initially, $S$ is closed. Which of the following statements is correct when $S$ is opened ?

2.

Electrical power is transmitted from a power station to local substations by an alternating current and high voltage. It is because

(1)the voltage of an alternating current can be stepped up or down using transformers easily.

(2)by using high voltages, electrical power can be transmitted faster along the cables.

(3)by using high voltages, there is less power loss in the transmission cables.

3.

Metal core with coil A connected to an a.c. supply through a switch and coil B connected to an a.c. voltmeter

The figure shows a metal core with two coils. When the switch is closed, the a.c. voltmeter shows a reading. Which of the following combinations will give the largest voltmeter reading ?

material of the metal core	no. of turns of coil $A$	no. of turns of coil $B$
copper	$500$	$1000$
copper	$1000$	$500$
iron	$500$	$1000$
iron	$1000$	$500$

4.

220 V mains with 5 A fuse, transformer and several identical 40 W bulbs in parallel

In the circuit above, the primary coil of a transformer is connected to the $220$ V mains supply with a $5$ A fuse. The efficiency of the transformer is $90\%$ . What is the maximum number of identical $40$ W light bulbs, operating at their rated values, that can be connected in parallel across the secondary coil without blowing the fuse ?

5.

Specification :
Input	a.c. $220$ V
Output	a.c. $12$ V, $100$ mA

The above figure shows a transformer. Assume the transformer is ideal, what is the current drawn from the mains supply if the transformer delivers currents at the rated value ?

6.

Specification :
Voltage input	$220$ V
Voltage outputs	$6$ V & $12$ V

The above figure shows a portable transformer. It outputs different voltage by varying the turns ratio between the primary and the secondary coils. Assume that there is no power loss in the transformer and the resistance of the load remains unchanged, when the output changes from $6$ V to $12$ V, which of the following statements describing the transformer is/are correct ?

(1)The number of turns of the primary coil should be doubled and the number of turns of the secondary coil remains unchanged.

(2)The input current should be doubled.

(3)The output power should be

4

times as before.

7.

Ideal transformer with ammeter in the primary, bulb X in parallel with bulb Y and switch S

In the above circuit, the transformer is ideal (efficiency $= 100\%$ ). At the beginning, the switch $S$ is closed and the light bulbs $X$ and $Y$ are operated at their rated values. What would happen to the brightness of the bulb $X$ and the ammeter reading if $S$ is now opened ?

	Brightness of $X$	Ammeter reading
	increases	increases
	increases	decreases
	remains unchanged	decreases
	remains unchanged	remains unchanged

8.

The photograph shows an adaptor for charging up the battery inside a mobile phone. Which of the following statements about the adaptor are correct ?

(1)It is used to step down the voltage from the mains to a value for the mobile phone.

(2)It can convert the voltage from a.c. to d.c.

(3)It is black so that it can radiate heat more efficiently than those in other colours.

Photograph of a mobile phone charger adaptor with cable and phone

9.

Which of the following statements about long distance power transmission at high alternating voltages are correct ?

(1)Alternating voltages can be stepped up or down efficiently by transformers.

(2)For a given transmitted power, the current will be reduced if a high voltage is adopted.

(3)The power loss in the transmission cables will be reduced if a high voltage is adopted.

10.

12 V a.c. supply connected to a transformer whose secondary coil is tapped at equal intervals

A $12$ V a.c. supply is connected to a transformer with turns ratio $1:1$ . The secondary coil is tapped at equal intervals as shown above. Two '6 V, 0.5 W' light bulbs are connected to the secondary coil of the transformer. In which of the following cases will the two bulbs work at their rated values ?

Three ways (1), (2), (3) of connecting the two bulbs to the tapped secondary coil