物理家居用電 DSE、CE 選擇題（第 3 頁）

21.

Three identical lamps $L_1$ , $L_2$ and $L_3$ of ratings ' $6$ V, $12$ W' are connected to a $6$ V battery as shown in the figure. Which of the following statements is correct ?

Circuit with a 6 V battery, lamps L1 and L2 in parallel, in series with lamp L3

22.

Photograph of the inside of a plug with the live and neutral wires interchanged

If the live and neutral wires of an electric kettle are mistakenly interchanged inside the plug as shown above, which of the following will happen ?

23.

Which of the following diagrams shows the correct connection of the fuse and switch of an electric kettle to the mains supply ? ( $L$ : live, $N$ : neutral, $E$ : earth)

Four diagrams A to D showing different positions of a fuse and switch in the live, neutral and earth wires of an electric kettle

24.

Two electric heaters $X$ and $Y$ are of ratings ' $110$ V, $40$ W' and ' $110$ V, $80$ W' respectively. Which of the below deductions about the two heaters is/are correct ?

(1)The operating resistance of

X

is twice that of

Y

.

(2)

X

will consume a power of

80

W when it is connected to a

220

V mains supply.

(3)Both heaters work at their rated values when they are connected in series to a

220

V mains supply.

25.

Electric iron with heating element, contact points X and Y, switch, fuse, metal case and earth connection

The above diagram shows the main parts of an electric iron. In which of the following situations will the fuse blow when the switch is closed ?

(1)The insulation at contact point

X

is worn out so that the wire touches the metal case.

(2)The insulation at contact point

Y

is worn out so that the wire touches the metal case.

(3)The heating element is broken.

26.

Electrical appliance connected to a power supply through connecting wires of total resistance R

An electric appliance is connected to a power supply of voltage $V$ by long connecting wires of total resistance $R$ as shown in the circuit. It is found that the current passing through the appliance is only $\frac{1}{2}I_0$ , where $I_0$ is the current required for the appliance to work at its rated value. Which of the following changes could increase the current through the appliance to $I_0$ ?

	Voltage of power supply	Total resistance of connecting wires
	increases to $2V$	remains unchanged
	increases to $2V$	reduces to $R/2$
	remains unchanged	increases to $2R$
	remains unchanged	reduces to $R/2$

27.

The kilowatt-hour is a unit of

28.

Which of the following statements about the earth wire in an electric iron is/are correct ?

(1)The earth wire should be connected to the metal body of the iron.

(2)If the iron is working properly, no current pass through the earth wire.

(3)In case the neutral wire is broken, the earth wire provides a spare wire for the return path to the mains socket.

29.

Iron connected to a 200 V a.c. supply with the switch S and fuse in the neutral wire

The switch $S$ and the fuse of an iron are incorrectly fitted in the neutral wire as shown above. Which of the following statements is correct ?

30.

Which of the following values is equivalent to one kilowatt hour ?