Unit 12Electromagnetism

Multiple-choice Qrestions

l. Two long insulated wires carrying equal currentsare placed perpendicular to each other as shown.

.P Q.

.s R.

The points P, Q, R and S are all at equal distancesfrom the wires. At which point(s) is/are theresulting magnetic field out of the paper?A Ponly B QonlyC PandQ D RandS ( )

2. The diagram below shows a circuit with a wireconnected to a battery through the switch S. Thecompasses X andZ are placed above the wire and

the compass Y is placed below the wire.

3. The diagram shows a cross-sectional view of asimple d.c. motor. The circles represent the sides

of a rectangular coil which is placed between thepoles of a permanent magnet. The current in theleft hand side of the coil flows into the plane ofthe diagram; the current in the right hand side ofthe coil flows out of the plane of the diagram.

What is the direction of the force experienced byeach side of the coil?

Left Hand Side Right Hand SideA Towards N Towards S

B Towards N UpC Up DownD Down Up

()

4. A free-moving wire is placed in a magnetic fieldas shown. A current flows from Q to P. In whichdirection will the wire move?

S

left right

oo

C

NX

D

P

YZS

()When S is closed, which of the followingdiagrams correctly shows the orientations of thecompass needles?

A B

5. The diagram below shows an electron, e, locatedin a magnetic field.

N

There is no magnetic force on the electron whenit movesA towards the right side of the pageB towards the top of the page

C into the page

D out ofthe page ( )

SDC

I

-l

S

()

Danya

l Edu

catio

n

6. The diagram below shows a beam of electronsabout to enter a magnetic field. The direction ofthe field is into the page.

9. The diagram shows a brass rod supported on twocopper rails which are connected to a battery.The north pole of a magnet is placed as shownbeneath the rails. In which of the directions A, B,C, D does the brass rod experience a force?

brass rod

rails

xxxxxx8l%"fl"' *#

x

X magneticfield

I into page

x

x

x

I

x

x

x

x

x

x

x

x

xx x

What will be the direction of the deflection, if any,

as the beam passes through the magnetic field?A Towards the top of the page

B Towards the bottom of the page

C Out of the paperD No deflection ( )

7. Under the influence of the magnetic f,eld in thediagram below, the stream of electrons is

-.stream of electrons

A deflected towards the north pole of the magnetB deflected towards the south pole of the magnetC deflected into the plane

D deflected out ofthe plane ofthe paper ( )

8. The circle in the diagram outlines the point wherea beam of electrons moving 'into the paper'enters a magnetic field. What will happen to theparticles?

o

10. The diagram below shows two iron rodssuspended side by side inside a coil by two elasticcords. When the circuit is closed at K, what willhappen to the rods?

elastic cords

iron rods

coil

K

A The rods repel each other.B The rods attract each other.C The rods move upwards.D The rods move downwards.

I 1. Two long parallel wires PQ and RS are connectedto batteries as shown in the diagram. PQ can

move freely in any direction but RS is fixed.

PRfree to

fixedmove

QS

When both switches are closed, what happens towire PQ?

A PQ moves towards RS.

B PQ moves away from RS.

C PQ moves in a direction into the plane of the

paper.

D PQ moves in a direction out of the plane ofthe paper. ( )

\BC

\()

N

S

\/A Deflect to the leftB Deflect to the rightC Deflect upwardsD Deflect downwards ()

s

()

Danya

l Edu

catio

n

12. A rectangular coil is placed within a magneticfield as shown. Current flows along the sides PQ,

QR, RS and SP of the coil as represented by theAITOWS.

o

direction ofmagnetic field----+

S

Which combination shows the directions offorces acting on the sides ofthe coil?

PQ QR RS SPA Upwards Inwards Downwards InwardsB Upwards No force Downwards No forceC Downwards No force Upwards No forceD Downwards Inwards Upwards Upwards

()

13. Ahorizontal wire carries a current in the directionshown in the diagram below between the

magnetic poles N and S of two magnets. What isthe direction of the force?

When the switch is closed, P

-.

A shoots out of the coilB moves to and froC moves towards QD moves away from Q ( )

15. The diagram below shows part of a d.c motor.

N a

Q current into paper

@ cunent out of paper

As seen from the front, the coil willA rotate clockwiseB rotateanti-clockwiseC experience no forceD experience no torque ( )

16. Wire X is in a magnetic field and carries an

electric current.

wtre

from d.c. supply

Which one of the following changes will doublethe catapult force on wire X?A The diameter of wire X is halved.B The strength of the magnetic fleld is halved.

C The strength of the magnetic field isdoubled.

D An alternating current is used instead of a

direct current. ( )

R

SP

s current

A FromNtoSB FromStoNC In the direction ofthe currentD Vertically upward ()

14. Two nails P and Q are placed inside a coil, as

shown in the diagram below. P is free to movebut Q is fixed. The coil is joined to a battery and

a switch.

coil

I_switch Dan

yal E

duca

tion

Short-structured Qrestions

Figure (a) below show a simple DC motor. The ends of the single loop coil ABCD are soldered tocopper pieces X and Y which make contact with springy metal strips P and Q. A cell is connectedacross P and Q. Figure (b) shows an end view of the arrangement.

AxisT .-?,, D

D

Figure (a) Figure (b)

(a) Show with an arrow on Figure (a) the direction of the current in the loop ABCD.

(b) Why does side AB experience a force?

t1l

tzl

S

(c) Draw an irrrow on the side AB to show the direction of this force.

(d) What is the direction of rotation of the coil?

tll

tll

(e) Explain why the coil still rotates in the same direction after half a revolution. I2l

tt4

Danya

l Edu

catio

n

2. A metal rod AB is placed on two smooth horizontal metal rails fixed on the bench as shown belowThe set of crosses represents a uniform magnetic field pointing into the paper.

rail

xxxxxxxxx

E

B

(a) What happens when switch S is closed? Explain briefly. 12)

S xxxx

xxxxxxxxxxx-I-

(b) Explain the effects on the motion of the rod AB if(i) the magnetic field strength is increased; tll

(ii) the separation between the rails is decreased. tll

(c) Suggest one way to reverse the direction of the motion of the rod AB. tll

3 . A beam of electrons is projected between the poles of an electromagnet and produces a bright spot on

a special screen. The apparatus is enclosed in a vacuum and arranged as shown in the diagram below.

Beam ofelectrons

ron core

X

115

Danya

l Edu

catio

n

(a) When the current is switched on,(i) what magnetic pole is produce at the end marked X; tll

(ii) which direction will the spot on the screen be deflected? t1l

(b) What two changes might be observed if a beam of positive charges were used for the experimentinstead ofa beam ofelectrons? t2)

4. The diagrams show two current-carrying wires placed between the poles of magnets,

S

Magnet

N

Magnet

(a) In each case, indicate on the diagrams the direction of the force on the wire. t2)

(b) The diagram below shows a simple electric motor with a coil WXYZ free to spin about a shaftPQ.

,o

N

Wire

N S

P,Commutator

rl6

Danya

l Edu

catio

n

(i) By looking at the diagram and using the conclusion you reached in part (a), mark on the

figure:.(A) the direction of current in the coil;(B) the direction of the forces on the coil;(C) the direction of the rotation of the coil. tZ)

(ii) Describe how the brushes and the commutator allow the coil to keep spinning. t3l

(c) State two ways that can help increase the turning effect of the motor, t2)

5. The diagram shows a conductorAB placed in a magnetic field(marked by crosses) which is pointing perpendicularly into the

page. A current flows down the conductor.

(a) State which way the rod moves when the switch is closed.

Give a reason for your answer. tll

(b) What causes the force which acts on the conductor causing it to move?

x

x

x

x

A

B

xxx

x

x

x

x

x

x

x

ttl

(c) State the effects on the movement of the rod when(i) the current is increased, tll

(ii) the current is reversed. tll

tt7

Danya

l Edu

catio

n