物理磁場 DSE、CE 選擇題（第 2 頁）

11.

Simple motor with magnet, coil, rheostat and battery box

The figure shows a simple motor. Which of the following changes can increase the turning effect of the coil ?

(1)using a stronger magnet

(2)reducing the resistance of the rheostat

(3)using a coil with a smaller number of turns

12.

David wants to design a battery-powered toy car. Which of the following circuits should he use ?

Four circuits A to D with a motor and switch: A battery, B a.c. source, C battery with the switch in parallel with the motor, D a.c. source with the switch in parallel with the motor

13.

Wires P and Q carrying currents into the paper and a point R equidistant from both

Two parallel straight wires $P$ and $Q$ carry equal currents flowing into the paper. A compass is placed at a point $R$ where $PR = QR$ . In which of the following diagrams is the north pole of the compass needle pointing in the correct direction ? The effect of the Earth's magnetic field may be ignored.

Options A to D: compass needle pointing left, right, up, down

14.

Three parallel wires P, Q and R carrying equal currents out of the paper, R equidistant from P and Q

$P$ , $Q$ and $R$ are three parallel straight wires carrying equal currents flowing out of the paper. $R$ is equidistant from $P$ and $Q$ . What is the direction of the resultant force acting on $R$ by $P$ and $Q$ ?

Options A to D: arrow to the right, arrow to the left, arrow upwards, arrow downwards

15.

Electromagnet: a coil on a soft iron core connected to a 12 V battery and a variable resistor

The figure shows an electromagnet which is used to pick up iron objects. Which of the following can increase the strength of the electromagnet ?

(1)increasing the number of turns in the coil

(2)reducing the resistance of the variable resistor

(3)replacing the battery with a

12

V a.c. power supply

16.

Circuit with a 12 V supply, a 10 ohm resistor and a motor M in series

A motor is connected in series with a $10\ \Omega$ resistor and a $12$ V power supply as shown. If the current in the circuit is $0.5$ A, find the power consumed by the motor.

17.

Cross-sections of wires X and Y (current out of the paper) and Z (current into the paper), with a direction key: upwards, downwards, left, right

The above diagram shows the cross-section of three parallel straight wires $X$ , $Y$ and $Z$ . $X$ and $Y$ carry currents flowing out of the paper and $Z$ carries a current flowing into the paper. What is the direction of the resultant force acting on $Y$ ?

18.

Two long insulated wires crossing at right angles with points P, Q, R and S around the crossing

Two long insulated wires carrying equal currents are placed perpendicular to each other on a table as shown in the figure. The points $P$ , $Q$ , $R$ and $S$ are all of equal distances from the wires. At which point(s) is the resulting magnetic field pointing out of the paper ?

19.

Which of the following involve(s) the application of electromagnets ?

(1)An electric bell

(2)A telephone receiver

(3)A moving-coil loudspeaker

20.

Electromagnet: a coil on a soft iron core connected to a battery and a variable resistor

The above diagram shows a simple electromagnet. Which of the following can increase the strength of the electromagnet ?

(1)Decreasing the resistance of the variable resistor.

(2)Replacing the soft iron core with one made of steel.

(3)Replacing the battery with a

50

Hz a.c. source.