物理 1985 年 DSE、CE 選擇題（第 3 頁）

21.

An immersion heater $A$ takes $10$ minutes to boil a kettle of water while another heater $B$ takes $40$ minutes to boil the same kettle of water under the same voltage supply. If the resistance of heater $A$ is $100\ \Omega$ , what is the resistance of heater $B$ ?

22.

A student, who wishes to find the resistance of a light bulb when connected to a certain battery, incorrectly connects a practical voltmeter and a practical ammeter as shown. What would be the most probable outcome of his error ?

Battery, ammeter in parallel with a branch containing a voltmeter in series with a light bulb

23.

Battery with ammeter A1, three identical bulbs B in parallel and ammeter A2

In the circuit shown, all bulbs $B$ are identical. If ammeter $A_1$ reads $1.8$ A, what does ammeter $A_2$ read ?

24.

Electric bulbs $R$ of rating " $40$ W, $200$ V" and electric bulbs $S$ of rating " $60$ W, $200$ V". Which of the following combinations would produce the maximum brightness if connected to the same voltage ?

Four circuits A to D: A two R bulbs in parallel, B two S bulbs in series, C two S bulbs in parallel, D an R bulb and an S bulb in parallel

25.

24 V battery, a resistor, switches K1 and K2 each in series with a 6 ohm resistor, and a voltmeter across the parallel combination

In the circuit, when $K_1$ is closed the voltmeter reads $12$ V. What would the reading be if both $K_1$ and $K_2$ are closed ?

26.

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 ?

27.

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 $100\%$ )

28.

Copper rod AB on two parallel wires in a magnetic field into the paper, pulled by force F to the right

A copper rod $AB$ is free to move on two parallel conducting wires. It is pulled by a force $F$ and moves in the direction shown in the figure. The magnetic field points into the paper. Which of the following statements is/are true ?

(1)A current will flow from

C

to

D

through

P

.

(2)A current will flow from

B

to

A

through

Q

.

(3)An induced voltage will be set up across

AB

.

29.

During radioactive decay, ${}^{230}_{90}\mathrm{X}$ becomes ${}^{226}_{90}\mathrm{Y}$ . Which of the following statements would be correct ?

(1)The change would involve

\alpha

decay only.

(2)One

\alpha

particle and two

\beta

particles would be emitted.

(3)

X

and

Y

are two isotopes of the same element.

30.

Which of the following will increase the resistance of a metal wire ?

(1)increasing the length of the wire

(2)increasing the cross-sectional area of the wire

(3)increasing the temperature of the wire