3.2 understanding the force on a current-carrying conductor in a magnetic field

7/30/2019 3.2 Understanding the Force on a Current-carrying Conductor in a Magnetic Field.

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PROGRAM DIDIK CEMERLANG AKADEMIK

SPM

3.0 ELECTROMAGNETISM

ORGANISED BY:

JABATAN PELAJARAN NEGERI PULAU PINANG

PHYSICS

MODULE 16


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TOPIC:3.0 ELECTROMAGNETISM

3.2 Understanding the force on a current-carrying conductor in a magnetic field.CONTENTS:

1.CHAPTER HIGHLIGHT (30 MINUTES)

2.ACTIVITY (50 MINUTES)3.ASSESSMENT (40 MINUTES)

4.MARKING SCHEME (ANSWER KEY)


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1. CHAPTER HIGHLIGHT (30 MINUTES)A. FORCE ON A CURRENT-CARRYING CONDUCTOR

1. A current carrying wire has a magnetic field around it. If this wire is place in another

magnetic field, the two magnetic fields may interact and produce a force on the wire.

2.When a current is passed through the loop, the loop moves upwards. A force is acting

on the wire segment inside the magnetic field.

3.When the direction of current is reversed, the loop moves downwards.

4.When the polarities of the magnet are reversed, the loop moves downwards.

5.When the current and magnetic field strength are increased, the loop will bend more.

The force acting on the wire is therefore proportional to the current and the magnetic

field strength.

6. The directions of the magnetic field, current and force acting on the loop are mutually

perpendicular.

7. The direction of the force can be determined by Flemings left hand Rule which is also

known as the Motor Rule.

B. Flemings Left hand Rule (Motor Rule)

1. By placing the forefinger, second finger and thumb of the left hand mutually at right

angles, the forefinger will point in the direction of the field, the second finger in the

direction of the current and the thumb in the direction of the force or motion.


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2. The force on the wire can be increased by

Using a larger current

Using a stronger magnetic field

Using a greater length of the wire in the field

3. Only when the wire is inside the magnetic field, there is force acting on it. If you

increase the length of the wire but the increased segment is outside the field, the force

cannot be increased.

Force on a beam of charged Particles

1. When a beam of moving particles enters a magnetic field, there is force acting on the

charged particles. They are deflected inside the magnetic field. Flemings Left Hand Rule

can be applied to determine the direction of deflection of the beam of charged particles.(a) A beam of positive charged particles. Direction of current is same as the direction of

movement of the charged particles.


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(b) A beam of negative charged particles (electrons) Current is in an opposite direction to

that of the flow of negative charges.

2. The pattern of the combined magnetic field due to a current-carrying conductor in a

magnetic field.

Magnetic Field Magnetic field due to Catapult Effect

a current-carrying conductor

Magnetic Field

Magnetic field due to a current-carrying conductor


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Catapult Effect

3. The Catapult Effect (also called the Motor Effect).

The catapult effect shows the force on a wire in a magnetic field when current flows

through the wire. If you put two magnets near to each other, their magnetic fields will

interact. The interaction means that the magnets will feel forces on them as like poles will

repel and unlike poles attract. It follows then that a wire in a field from a permanent

magnet

will feel a force when current flows through it. The magnetic field generated around the

wire by the current will interact with the field around the magnet and the two fields will

push or pull on each other. The magnetic field around a straight wire is circular.

The magnetic field between two attracting poles is straight. When the two interact,

the wire is pushed away from the field between the attracting poles at right angles (90)

both to the straight field lines and to the direction of current flow.

4. The Catapult Effect and Fleming's Left Hand Rule.

If we show the two magnetic fields from the wire and the permanent magnet, we can see

that on one side of the wire the fields have the same direction and repel the wire, on the

other side of the wire the fields have opposite directions and attract the wire. This is

called the catapult effect (or motor effect).
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You can predict which way the wire will move by using Fleming's Left Hand Rule. The

thumb, first finger and second finger of the left hand are all pointing at 90 to each other.

1. The thumb points in the direction of motion of the wire.

2. The first finger points in the direction of the field (from the permanent magnet)

3. The second finger points in the direction of the current through the wire.

This works well in theory but in practice it may be difficult to get your thumb and fingers

all pointing in the right direction.

Electric Motor

1. Electric motors have a wide variety ofuses. The catapult effect (motor effect) is used

to make a simple electric motor. The wire is pushed in the opposite direction if the

direction of the current through it is reversed. In a motor, the wire is wound around a

central block called an armature. A spindle through the armature allows it to rotate. The

current flows in opposite directions on each side of the armature, so one side is pushed

while the other is pulled. This makes the armature rotate.
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After the armature has rotated through half a turn (180), then the side of the armature

being pushed upwards in the above picture is now on the left and the side being pulled

down on the right. The armature would be trying to turn in the opposite direction. For the

armature to continue to spin in the same direction, the direction of the current flowing

through the wire must be reversed every half turn. This is achieved using a split - ring

commutator.


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Figure 2

Fingers and thumb are spread out so that thumb is at right angles to index finger which is

at right angles to both the second finger and the thumb.

The thumb represents thrust or motion direction. The first finger represents the magnetic field direction from North to South. The second finger represents the current direction in the usual way assumed from

positive to negative.

Now have a look at the electromagnetic effect in action. Remember that the magnetic

field is going from the north at the top to the south at the bottom. Notice that nothing

happens until the switch is closed and current flows in the direction shown by the arrows.

We use the convention that current flows from positive to negative because it has been so

useful historically. You'll have to twist your hand around a bit to make it match but it will

reinforce the idea for you.

Fleming's left-hand rule

The direction of the current, the direction of the magnetic field and the direction of

movement are at right angles to each other. The way to remember how the wire moves is

by using


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Fleming's left-hand rule

Fleming's left-hand rule can be used to work out the direction of the force when a current

flows in a magnetic field.

Fleming's left-hand rule. Hold your left hand as shown in the picture in Fig.2. Follow

these three steps to learn how to predict which way the wire will move:

1. Rotate your hand so the First finger is in the direction of the magnetic Field.2. Point your seCond finger in the direction of the Current.3. Your thuMb is now pointing in the direction of the Movement.

Let's put this into practice. For example, when the magnetic field points from left to right,

point the first finger of your left hand to the right. The current flows out of the screen, so

point your second finger towards you, out of the screen. Now your thumb is pointing

upwards. This is the direction of motion, and agrees with what you found in the

experiment.

The force exerted on the wire depends on two factors:

the strength of the magnetic field the current flowing through the wire

A larger magnetic field produces a larger force, and so does a larger current.

This is how Flemings Left Hand Rule works. Fingers and thumb are spread out so that

thumb is at right angles to index finger which is at right angles to both the second finger

and the thumb.


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The thumb represents thrust ormotion direction. The first finger represents the magnetic

field direction from North to South. The second finger represents the current direction in

the usual way assumed from positive to negative. Now have a look at the electromagnetic

effect in action. Remember that the magnetic field is going from the north at the top to

the south at the bottom. Notice that nothing happens until the switch is closed and

current flows in the direction shown by the arrows. We use the convention that current

flows from positive to negative because it has been so useful historically. You'll have to

twist your hand around a bit to make it match but it will reinforce the idea for you.

Motor

2. Turning Effect on a current-carrying coil

In the figure below, a rectangular coil ABCD is placed in a uniform magnetic field. The

coil can rotate freely about the horizontal axis YY


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When the switch is turned off, the current flows from point A to point D through the coil.

Apply Flemings Left hand Rule on the wire segments AB and CD. It is found that there

is a downward force acting on AB and upward force acting on CD. Thus the coil turns in

clockwise direction. This turning effect can be increased by

(i) increasing the current

(ii) increasing the magnetic field strength

(iii) increasing the number of turns of the coil

(iv) inserting a soft iron core within the coil to concentrate the magnetic field lines.

3. AC and DC

"DC" stands for direct current in which the current flows in the one direction constantly

throughout the circuit. One side of the power source is nominated as the positive (+) pole

and the other as the negative (-) pole. An automotive battery or dry cell gives DC. In

"AC" or alternating current the current periodically reverses its direction along the

conductor, i.e. one fraction of a second the right-hand terminal is "negative", the next

fraction of a second it is "positive". In 50 Hertz AC current, such as is commonly used in

Australia, this change from + to - to + occurs as a cyclic variation 50 times a second, the

current thus changing direction of flow 100 times a second. See Fig. 5.


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4. Magnetic Propulsion within a Motor

The basic principle of all motors can easily be shown using two electromagnets and a

permanent magnet. Current is passed through coil no. 1 in such a direction that a north

pole is established and through coil no. 2 in such a direction that a south pole is

established. A permanent magnet with a north and South Pole is the moving part of this

simple motor. In Figure 5-the north pole of the permanent magnet is opposite the north

pole of the electromagnet. Similarly, the south poles are opposite each other. Like

magnetic poles repel each other, causing the movable permanent magnet to begin to turn.

After it turns part way around, the force of attraction between the unlike poles becomes

strong enough to keep the permanent magnet rotating. The rotating magnet continues to

turn until the unlike poles are lined up. At this point the rotor would normally stop

because of the attraction between the unlike poles. (Figure 5-b)

Figure 5


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If, however, the direction of currents in the electromagnetic coils was suddenly reversed,

thereby reversing the polarity of the two coils, then the poles would again be opposites

and repel each other. (Figure 5-c). The movable permanent magnet would then continue

to rotate. If the current direction in the electromagnetic coils was changed every time the

magnet turned 180 degrees or halfway around, then the magnet would continue to rotate.

This simple device is a motor in its simplest form. An actual motor is more complex than

the simple device shown above, but the principle is the same.

5. Direct-Current Motors - DC motors are divided into three classes, designated

according to the method of connecting the armature and the field windings as shunt-series

and compound wound. There are different kinds of D.C. motors, but they all work on the

same principles. To understand what goes on inside a motor, here is an example (click theillustration below for a full size image).

Simple Motor

When a permanent magnet is positioned around a loop of wire that is hooked up to a D.C.

power source, we have the basics of a D.C. motor. In order to make the loop of wire spin,

we have to connect a battery or DC power supply between its ends, and support it so it

can spin about its axis. To allow the rotor to turn without twisting the wires, the ends of

the wire loop are connected to a set of contacts called the commutator, which rubs against

a set of conductors called the brushes. The brushes make electrical contact with the

commutator as it spins, and are connected to the positive and negative leads of the power

source, allowing electricity to flow through the loop. The electricity flowing through the
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loop creates a magnetic field that interacts with the magnetic field of the permanent

magnet to make the loop spin.

6.AC Current

How the current is reversed in the coil so as to change the coils polarity, you ask. Well, as

you probably know, the difference between DC and AC is that with DC the current flows

in only one direction while with AC the direction of current flow changes periodically. In

the case of common AC that is used throughout most of the United States, the current

flow changes direction 120 times every second. This current is referred to as "60 cycle

AC" or "60 Hertz AC" in honor of Mr. Hertz who first conceived the AC current concept.

Another characteristic of current flow is that it can vary in quantity. We can have a 5

amp, 10 amp or 100 amp flow for instance. With pure DC, this means that the current

flow is actually 5, 10, or 100 amps on a continuous basis. We can visualize this on a

simple time-current graph by a straight line as shown in Figure 6.

Figure 6 - Visualization of DC

But with AC it is different. As you can well imagine, it would be rather difficult for the

current to be flowing at say 100 amps in a positive direction one moment and then at the

next moment be flowing at an equal intensity in the negative direction. Instead, as the

current is getting ready to change directions, it first tapers off until it reaches zero flow

and then gradually builds up in the other direction. See Figure 7. Note that the maximumcurrent flow (the peaks of the line) in each direction is more than the specified value (100

amps in this case). Therefore, the specified value is given as an average. It is actually

called a "root mean square" value, but don't worry about remembering this because it is

of no importance to us at this time. What is important in our study of motors, is to realize


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that the strength of the magnetic field produced by an AC electro-magnetic coil increases

and decreases with the increase and decrease of this alternating current flow.

Figure 7 - Visualization of AC.

7. Basic AC Motor Operation

An AC motor has two basic electrical parts: a "stator" and a "rotor" as shown in Figure 8.

The stator is in the stationary electrical component. It consists of a group of individual

electro-magnets arranged in such a way that they form a hollow cylinder, with one pole

of each magnet facing toward the center of the group. The term, "stator" is derived from

the word stationary. The stator then is the stationary part of the motor. The rotor is the

rotating electrical component. It also consists of a group of electro-magnets arranged

around a cylinder, with the poles facing toward the stator poles. The rotor, obviously, is

located inside the stator and is mounted on the motor's shaft. The term "rotor" is derived

from the word rotating. The rotor then is the rotating part of the motor. The objective of

these motor components is to make the rotor rotate which in turn will rotate the motor

shaft. This rotation will occur because of the previously discussed magnetic phenomenon

that unlike magnetic poles attract each other and like poles repel. If we progressively

change the polarity of the stator poles in such a way that their combined magnetic field

rotates, then the rotor will follow and rotate with the magnetic field of the stator.


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Figure 8 - Basic electrical components of an AC motor.

This "rotating magnetic fields of the stator can be better understood by examining Figure

9. As shown, the stator has six magnetic poles and the rotor has two poles. At time 1,

stator poles A-1 and C-2 are north poles and the opposite poles, A-2 and C-1, are south

poles. The S-pole of the rotor is attracted by the two N-poles of the stator and the N-pole

of the rotor is attracted by the two south poles of the stator. At time 2, the polarity of the

stator poles is changed so that now C-2 and B-1 and N-poles and C-1 and B-2 are S-

poles. The rotor then is forced to rotate 60 degrees to line up with the stator poles as

shown. At time 3, B-1 and A-2 are N. At time 4, A-2 and C-1 are N. As each change is

made, the poles of the rotor are attracted by the opposite poles on the stator. Thus, as the

magnetic field of the stator rotates, the rotor is forced to rotate with it.

Figure 9 - The rotating magnetic field of an AC motor.

One way to produce a rotating magnetic field in the stator of an AC motor is to use a

three-phase power supply for the stator coils. What, you may ask, is three-phase power?

The answer to that question can be better understood if we first examine single-phase


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power. Figure 7 is the visualization of single-phase power. The associated AC generator

is producing just one flow of electrical current whose direction and intensity varies as

indicated by the single solid line on the graph. From time 0 to time 3, current is flowing

in the conductor in the positive direction. From time 3 to time 6, current is flowing in the

negative. At any one time, the current is only flowing in one direction. But some

generators produce three separate current flows (phases) all superimposed on the same

circuit. This is referred to as three-phase power. At any one instant, however, the

direction and intensity of each separate current flow are not the same as the other phases.

This is illustrated in Figure 10. The three separate phases (current flows) are labeled A, B

and C. At time 1, phase A is at zero amps, phase B is near its maximum amperage and

flowing in the positive direction, and phase C is near to its maximum amperage but

flowing in the negative direction. At time 2, the amperage of phase A is increasing and

flow is positive, the amperage of phase B is decreasing and its flow is still negative, and

phase C has dropped to zero amps. A complete cycle (from zero to maximum in one

direction, to zero and to maximum in the other direction, and back to zero) takes one

complete revolution of the generator. Therefore, a complete cycle is said to have 360

electrical degrees. In examining Figure 10, we see that each phase is displaced 120

degrees from the other two phases. Therefore, we say they are 120 degrees out of phase.

Figure 10 - The pattern of the separate phases of three-phase power.

To produce a rotating magnetic field in the stator of a three-phase AC motor, all that

needs to be done is wind the stator coils properly and connect the power supply leads

correctly. The connection for a 6 pole stator is shown in Figure 11. Each phase of the


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three-phase power supply is connected to opposite poles and the associated coils are

wound in the same direction. As you will recall from Figure 4, the polarity of the poles of

an electro-magnet are determined by the direction of the current flow through the coil.

Therefore, if two opposite stator electro-magnets are wound in the same direction, the

polarity of the facing poles must be opposite. Therefore, when pole A1 is N, pole A2 is S.

When pole B1 is N, B2 is S and so forth.

Figure 11 - Method of connecting three-phase power to a six-pole stator.

Figure 12 shows how the rotating magnetic field is produced. At time1, the current flow

in the phase "A" poles is positive and pole A-1 is N. The current flow in the phase "C"

poles is negative, making C-2 an N-pole and C-1 is S. There is no current flow in phase

"B", so these poles are not magnetized. At time 2, the phases have shifted 60 degrees,

making poles C-2 and B-1 both N and C-1 and B-2 both S. Thus, as the phases shift their

current flow, the resultant N and S poles move clockwise around the stator, producing a

rotating magnetic field. The rotor acts like a bar magnet, being pulled along by the

rotating magnetic field.

Figure 12 - How three-phase power produces a rotating magnetic field.


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Up to this point not much has been said about the rotor. In the previous examples, it has

been assumed the rotor poles were wound with coils, just as the stator poles, and supplied

with DC to create fixed polarity poles. This, by the way, is exactly how a synchronous

AC motor works. However, most AC motors being used today are not synchronous

motors. Instead, so-called "induction" motors are the workhorses of industry. So how is

an induction motor different? The big difference is the manner in which current is

supplied to the rotor. This is no external power supply. As you might imagine from the

motor's name, an induction technique is used instead. Induction is another characteristic

of magnetism. It is a natural phenomena which occurs when a conductor (aluminum bars

in the case of a rotor, see Figure 13) is moved through an existing magnetic field or when

a magnetic field is moved past a conductor. In either case, the relative motion of the two

causes an electric current to flow in the conductor. This is referred to as "induced" current

flow. In other words, in an induction motor the current flow in the rotor is not caused by

any direct connection of the conductors to a voltage source, but rather by the influence of

the rotor conductors cutting across the lines of flux produced by the stator magnetic

fields. The induced current which is produced in the rotor results in a magnetic field

around the rotor conductors as shown in Figure 14. This magnetic field around each rotor

conductor will cause each rotor conductor to act like the permanent magnet in the Figure

9 example. As the magnetic field of the stator rotates, due to the effect of the three-phase

AC power supply, the induced magnetic field of the rotor will be attracted and will follow

the rotation. The rotor is connected to the motor shaft, so the shaft will rotate and drive

the connection load. That's how a motor works! Simple, was it not?

Figure 13 - Construction of an AC induction motor's rotor.


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Figure 14 - How voltage is induced in the rotor, resulting in current flow in the rotor

conductors.

8. DC Machines, Principles of Operation: Generator

In a generator, moving a conductor through a stationary magnetic field generates

voltage. If a coil is rotated through a magnetic field as shown in Figure 4, an alternating

voltage will be produced. To make this voltage available to a stationary external circuit,

two slip rings and brushes must be provided. For the external circuit to produce DC

voltage, it is necessary to reverse the polarity of the external leads at the same time the

voltage in the coil is reversed. This is accomplished by segmenting a slip ring to form

what is called a commutator. An elementary two segment commutator is illustrated in

Figure 5. This single coil, two pieces of commutator will yield a unidirectional butpulsating voltage as shown in Figure 6. However, when a large number of commutator

segments or bars are used, the resulting voltage will be more uniform as shown in Figure

7.

Figure 4.

Brushes and slip rings provide AC

voltage


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Figure 5.

Brushes and Commutator provides

DC voltage

Figure 6. Unidirectional, Pulsating Voltage

Figure 7.

2.ACTIVITY (50 MINUTES)1. A current flows in wire hanging between the poles of a magnet. The wire starts to

move in the direction shown.


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Which diagrams shows the position and the polarity of the magnet?

Candidates need to apply Fleming Left hand Rule to the diagrams one by one to find out

the correct answer D.

2. A long flexible wire is wrapped round two wooden pegs. A large current is passed in

the direction shown. Which two pairs do the lengths of wire attract each other?

Wires with currents flowing in the same direction attract each other. First pair J and K.

Second pair is L and M.

3. A coil, carrying a current is arranged within a magnetic field. The coil experiences

forces that can make the coil move. How does the coil move?


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Candidates should trace the direction of current in the wire inside the magnetic field and

then apply Flemings Left Hand Rule to determine the direction of movement of the coil.

4. Why is a commutator used in a d.c. motor?

Without commutator, a d.c. motor cannot turn continuously in one direction. The function

of the commutator is to reverse the direction of current through the coil every half-turn so

that the motor can continue to turn in one direction.

5. Figure below shows a rectangular current-carrying coil mounted on a freely pivoted

shaft between the poles of a permanent magnet. The connections to a battery and the

direction of the current in each side of the coil as shown, the sides of the coil are labeled

J, K and M.

Draw arrows to show the directions of the forces, if any acting on the sides J, K, L and

M. State what will happen to the coil as a result of these forces acting on it.


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The forces acting on the coil produce a net anticlockwise moment about the shaft when

viewed from the front. Hence the coil will rotate in an anticlockwise direction. Two

arrows should have the same length as they represent two equal forces. There is no force

acting on the sides K and M as the direction of current is parallel to the magnetic field.

There is no continuous rotation as slip rings are used instead of commutators. The coil

will oscillate.

6. The following is a diagram of a d.c. motor.

State the direction of movement of side AB and side CD when the current is in thedirection shown. Explain the reason for your choices of direction. When the coil ABCD

is vertical, the brushes line up with the gaps in the spilt-ring commutator. The coil rotates

past the vertical position. Explain what happens to the current in the coil and to the forces

on the sides AB and CD of the coil.

Side AB: downwards. Side CD: upwards. The direction of force acting on each side of

the coil is given by Flemings Left hand Rule and is perpendicular to both the direction of

current flowing through the wire and the direction of magnetic field. The direction of the

coil is reversed. Each force reverses its direction so that the coil continues to rotate in the

same direction. At first, current is flowing from A to B and C to D. When the coil is at

the vertical position, the spilt-ring commutator is not in contact with the carbon brush and

therefore no current flows. Because of inertia, the coil will continue to rotate and the


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commutator touches the carbon brushes again. However, the current is then flowing from

B to A and D to C. The function of the split-ring commutator is to ensure that the motor

rotates in one direction.

3. ASSESSMENT (40 MINUTES)1. Each of the diagrams below is a cross-section through two parallel current-carrying

conductors. Which diagram correctly shows the magnetic field pattern formed by the

currents in the two conductors?

2. A wire hangs between the poles of a magnet. When there is a current in the wire, in

which direction does the wire move?


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3. The diagrams below shows a two pole single-coil electric motor.

The split-ring commutator reverses the current in the coil as it rotates. How many times is

the current reversed if the coil is rotated once?

A 1 B 2 C 3 D 4

4. The diagram below shows a coil in a magnetic field.

When the coil is part of a d.c. motor, what must be connected directly to X and Y?

A d.c. supply

B slip ring

C soft-iron core

D split-ring commutator

5. A coil carrying a current, is arranged within a magnetic field. The coil experiences

forces that can make the coil move.


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How does the coil move?

A from X to Y

B out of the paper

C along the magnetic field

D turns about the axis XY

6. (a) Figure below shows a solenoid which is connected to a battery so that the current in

the solenoid is in the direction shown.

(i) Draw the pattern of the magnetic field due to the current, both inside andoutside the solenoid.


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(ii) Draw arrows at P and Q to show the direction of the magnetic field at each ofthese points.

(iii) A beam of electrons is now directed at P such that the beam passes into thediagram. Draw an arrow at P to show the direction of the force exerted on the

electrons by the magnetic foeld due to the current in the solenoid L and the

arrow with the letter R.

(b) Figure below shows a rigid rectangular coil mounted on the axle XY which is

perpendicular to the axis of the solenoid. The coil is connected through slip rings and

brushes to a battery in a circuit which includes a aswitch

(i) Explain why the coils begins to rotate when the switch is closed and the direction of

the current in the rectangular coil is as shown.

(ii) Continuous rotation of the coil does not take place. Explain why this is so.

7. Figure below shows a light aluminium rod resting between the poles of a magnet. A

current is passed through the rod from two brass strips connected to a power supply.


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Draw the direction of the current in the rod when the switch is closed. State which way

the rod moves when the switch is closed. Give a reason for your answer. State the effect

on the movement of the rod when the current is increased and the current is reversed.

4. MARKING SCHEME (ANSWER KEY)1. C Use Right Hand grip rule to establish the direction of the magnetic field produced by

the current in each wire. The thumb of the right hand represents current in the wire and

the curled fingers give the general direction of the field lines.

2. A Use Flemings Left hand Rule to determine the direction of the force on conductor.

The first finger of the left hand represents the direction of the magnetic field, the second

finger represents the direction of the current and the thumb gives the direction of the

force.

3. B The current in the coil reverses for every half rotation of the coil. Since one rotation

consists of two half-rotations, the current is reversed twice.

4. D Fact

5. D Use Flemings Left hand Rule to determine the force acting on the coil.

6.


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(b) (i) A wire will experience a force once the current flows though it in the presence of

the magnetic field. By Flemings Left hand Rule, the force is downwards and

perpendicular to the wire further away from the solenoid and upwards for the wire nearer

to the solenoid. Thus, the coil will experience a turning effect and it begins to rotate.

(ii) The turning moment of the coil decreases to 0 as the coil reaches the vertical position

since the distance between the line of actions of the two above-mentioned forces

decreases to 0. Also as the coil rotates clockwise past the vertical position, the turning

moment becomes anti clockwise. The coil will then oscillates about XY with the

oscillations dying away.

7.

The rod moves into the magnet. By Flemings Left Hand Rule, the direction of the

magnetic force acting on the rod is perpendicular to both the direction of the magneticfield and the direction of the current in the rod. The force is increased and therefore the

rod moves faster. The rod moves in opposite direction. Conventional current flows

externally from the positive terminal to the negative terminal of the battery. Only the

segment of the wire which is inside the magnetic field will experience a magnetic force.

When only current or polarity of magnetic field is reversed, the direction of force is


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reversed. If both current and polarity of magnetic field are reversed at the same time, the

direction of force remains the same.

3.2 understanding the force on a current-carrying conductor in a magnetic field

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