21 Electromagnetism.pdf

Chapter 21:

Chapter 21.1 :

Magnetic Effect of a

Current

Learning Outcomes: At the end of the lesson, you should be able to…

•Draw the magnetic field pattern of currents in wires and solenoids

•State the effect of changing magnitude or direction of current on the magnetic field

21.1 : Magnetic Effect of a Current

• Danish Professor.

• Demonstrating

heating effect of a

current.

• Noticed

compass

needle

nearby

deflected.

• Discovery

by

accident

?!?!

Who discovered it?

21.1 : Magnetic Effect of a Current

When the circuit is closed,

•a compass A placed above the wire XY would point to the East.

•Another compass B is placed below

the wire would point to the West.

Conclusion:

•A current-carrying conductor produces a magnetic field around it.

When a wire is placed vertically through a small

hole in a horizontal cardboard, the resulting

magnetic field lines form

concentric circles

around the wire.

Magnetic Field PATTERN

AROUND A STRAIGHT WIRE

To determine the direction of

the magnetic field, we use the

the

right-hand grip rule

• The

thumb

represents

the

direction of the

current

.

Magnetic Field PATTERN

AROUND A STRAIGHT WIRE

When the current is in the

opposite

direction, the

direction

of the magnetic

field is also

reversed

.

*Points to note (when drawing

magnetic fields) :

•

Concentric

shape.

•

Arrow tips

to represent the

direction

of the field.

• Field lines near the wires are

closer

. Hence the magnetic field

is

stronger

.

Plotting the magnetic field lines around a current-carrying straight wire.

Magnetic Field PATTERN

AROUND A STRAIGHT WIRE

The magnetic field of a long, straight

current-carrying wire is

stronger

when

1.

closer

to the wire, or

Example 1

A current flows in a long straight wire in the direction shown in the figure below. Draw, in the diagram, the pattern and direction of the magnetic field produced.

A

B

C

D

Example 2

MAGNETIC FIELD PATTERN

AROUND A FLAT COIL

• A flat coil is obtained when you wind a straight

wire.

• The magnetic field at the

centre

of the coil is

MAGNETIC FIELD PATTERN

AROUND A FLAT COIL

Reason:

•The magnetic fields due to both currents are in

the

same

direction.

Magnetic Field PATTERN

AROUND A STRAIGHT WIRE

Magnetic field pattern around

a flat coil

Magnetic field pattern due to current in a flat coil.

There are two ways to

increase

the

magnetic field

strength at the

centre

of the flat coil:

1.

Increase

the

current

.

Magnetic field pattern

of a solenoid

• The magnetic field pattern of a solenoid

resembles that of a

bar magnet

, hence it has

two

poles

and

can

be

used

as

an

The Textbook Method

To determine the

N-pole

, use the Right-hand grip rule:

• Grip the solenoid with the right hand and with the

fingers

pointing in the

direction of the current

flow.

• The end of the solenoid where the

thumb

points is

the

N-pole

.

Note :

The magnetic field inside the solenoid is

still quite weak in nature.

A shortcut method :

Magnetic field of a solenoid

The magnetic field strength in a solenoid can be

increased by:

1.

increasing

the

current

,

2.

increasing

the

number of turns

per unit

length of the solenoid, or

3. placing a

soft iron core

within the solenoid.

A It is attracted by the coil. B It is repelled by the coil. C It is not affected.

Example 3

What happens to the magnet if the switch is closed?

A

B

C

D

2 A 2 A

3 A _{2 A}

cardboard tubing

Example 4

Uses of electromagnets

Uses of electromagnets

** Magnetic Levitation Train (Maglev)

1. Circuit breaker

2. Magnetic relay (optional)

3. Electric Bell (optional)

4. Magnetic Resonance Imaging (MRI)

Mag

netic

Lev

itation Trains (Maglev)

Uses of electromagnets

The highest recorded speed of a maglev train is 581 km/h (361 mph), achieved in Japan

by the CJR's MLX01

1. Circuit breaker

Connected to the

live wire.

Connected to the

neutral wire.

• When there is a sudden surge in

current, the solenoid becomes a

strong electromagnet.

• The solenoid is thus able to

attract the soft iron latch, hence

releasing the spring which pushes the safety bar outwards.

• The interrupt point is now open

and the circuit is switched off.

2. Magnetic relay

• A magnetic relay is a device used to control the switch of another circuit which requires a higher current or voltage.

• The input circuit supplies a small current to the electromagnet.

• When the current is switched on, the electromagnet attracts one end of the pivoted iron armature and raises the other, closing the contacts in the second circuit.

3. Electric bell

An electric bell. • When the bell button is pressed, the

circuit is closed and current flows. The electromagnet is magnetised and attracts the soft iron armature, causing the hammer to strike the gong.

• As the hammer moves towards the gong, the circuit is broken and the electromagnet loses its magnetism. The springy metal strip pulls back the armature, connecting the contact at S and closes the circuit. The cycle is repeated.

4. Magnetic Resonance Imaging (MRI)

• Medical imaging that provides views of tissues in the body.

• Consists of a scanner containing superconducting solenoids which produces very strong magnetic fields.

• Causes atoms in the body to emit radio waves.

• The emitted radio waves are then picked up by detectors and processed by computers.

4. Magnetic Resonance Imaging (MRI)

Uses of electromagnets

Large electromagnets are used for lifting heavy iron

objects.

(T/F)

bell X bell Y

Example 5

Evaluate if the given statement is true and give a reason for your answer.

Bell X will not work properly because the contact is not connected to the electrical circuit and thus would not turn on and off the current to switch on and off the electromagnetism at the solenoid.

bell X

Example 5

Evaluate if the given statement is true and give a reason for your answer.

21.1: Magnetic Effect of a Current

Key Ideas

1. A current-carrying conductor produces a magnetic field around it. 2. A straight current-carrying conductor produces circular magnetic

field lines around it.

3. A current-carrying solenoid has a magnetic field pattern similar to that of a bar magnet.

4. The magnetic field strength of a current-carrying conductor can be increased by increasing the magnitude of the current or by increasing the number of turns of the solenoid. Reversing the direction of the current will reverse the direction of the magnetic field.

21.2 :

Force on

Current-carrying Conductors

Chapter 21.2 :

Force on Current-carrying

Conductors

Learning Outcomes: At the end of the lesson, you should be able to…

Describe experiments that show the force on a current-carrying conductor or a beam of charged particles in a magnetic field

Describe the effect of reversing the direction of the current or magnetic field on the force

Chapter 21.2 :

Force on Current-carrying Conductors

The motor effect

• When a

current

carrying

wire

is placed in an

external

magnetic

field

.

The

wire

experiences a

force

.

• The force acting on the current-carrying wire placed in a magnetic field is perpendicular

to both the direction of the

current and the direction of the magnetic field.

• The direction of the force is also reversed when the direction of the current or magnetic field is reversed.

Expt 21.2 – Conclusions.

Chapter 21.2 :

Fleming’s left-hand rule

To predict the direction of the force

,

•

Point the thumb, forefinger and second fingers at

right angles

to one another.

•

Point

the

forefinger

in

the direction

of the

magnetic field

(N to S direction) and

•

the

second finger

in the direction of the

current

.

21.2: Force on a

current-carrying conductor

• A current-carrying conductor experiences a force

in a magnetic field (non-parallel).

• The direction of force on the current-carrying

conductor depends on the relative directions of

current and magnetic field.

• The magnitude of the force is

maximum

when

the magnetic field and current directions are

21.2 : Force on Current-carrying

Conductors

Why does a current-carrying conductor experience a force when placed in a magnetic field?

Separate magnetic fields of a current flowing through a wire and of two magnetic poles

Why does a current-carrying conductor experience a force when placed in a magnetic field?

Combined magnetic field when the wire is placed between the poles of the

magnet Superimposed field patterns

of (a) and (b)

=

Explanation For This Phenomenon

From the diagram, we can see

that :

There is a

stronger field

on

one side of the wire

at A

, since

all the magnetic

field lines

are

in the

same direction

.

Combined magnetic field when the wire is placed between the poles of the magnet.

At B

, the combined

field is

weaker

due

to

opposing

magnetic

field lines

.

The

unbalanced

fields

on

both sides produces a

force

then

acts on the wire

from

the

stronger

field

to

the

weaker field

.

Example 6

The figure below shows a wire placed between two magnetic poles.

(a) If the current in the wire flows from A to B, in which direction does a force act on the wire?

(a)

By using Fleming’s Left-Hand Rule, we find that

the force acts vertically downward on the wire

AB.

(b) If the current flows from B to A, the force

reverses in direction and acts vertically upward.

The Figure below shows a current-carrying wire placed between the poles of a magnet.

(a) Mark on the diagram the direction of the force acting on the wire.

(b) What would happen to the motion of the wire if the pole of the magnets were reversed?

S

N

I

F

Answer:

(a) Force is downwards.

(b) When the magnetic field is reversed, the

motion of the wire will be reversed (upwards).

S

N

I

F

Force on a current-carrying

conductor

S N

Force on a current-carrying

conductor

N S

Fleming’s left hand rule

N S

F

B

+

Force on a Moving Charge in a

Magnetic Field

• When a beam charged particle enter a magnetic field region, the beam is deflected in a circular path.

+

Force on a Moving Charge in a

Magnetic Field

• Following the

conventional current

direction, we take

current to be:

• in the

same

direction of the beam of

positive

charge

(e.g. protons).

• in the

opposite

direction to that of the beam of

Magnetic field into paper

+

Magnetic field into paper

Magnetic field out of paper

+

Force on a Moving Charge in a

Magnetic Field

Force on a Moving Charge in a

Magnetic Field

Magnetic field out of paper

-Key Ideas

1. When a current-carrying conductor is

placed in a magnetic field, the conductor

experiences a force.

2. The

direction

of

the

force can

be

determined by

Fleming’s left-hand rule.

Forces between two parallel

current-carrying wires

When two current-carrying wires are placed parallel

to each other, we would expect a force to act on

each wire.

When the current in each conductor flows in the

Forces between two parallel

current-carrying wires

When the current in each conductor flows in the

21.3 :

Force on

Current-RECTANGULAR COIL IN A

MAGNETIC FIELD

21.3 : Force on Current-RECTANGULAR

COIL IN A MAGNETIC FIELD

Learning Outcomes: At the end of the lesson, you should be able to…

•Explain how a current-carrying coil in a magnetic field experiences a turning effect, and how the turning effect can be increased.

•Discuss how this turning effect is used in a simple electric motor.

• When a wire coil is placed between the poles of a strong magnet and a current is passed through the coil, the coil will experience a turning effect.

• This turning effect on a current-carrying coil in a magnetic field has a very important application : the direct current (d.c) motor.

• The purpose of a d.c motor is to convert electrical energy into mechanical energy.

• It consists of the following:

1. Rectangular coil connected in series to a battery and rheostat.

2. Permanent magnets 3. Split-ring commutator 4. Two carbon brushes.

The D.C MoTOR

axle

P

X Y

Q

Single coil

of wire

Rheostat

Commutator

Carbon brush

Permanent

• The diagram above shows the structure of a simple d.c. motor. When the circuit is closed, current will be flowing in the (a) coil. The coil spins on an axle in which the direction of the spin can be determined by Fleming’s left-hand rule. The magnetic field in the d.c. motor is provided by the (b) magnet (remember magnetic filed always point from North to South). The ends of the coil is

connected to a split-ring known as a

(c) commutator (XY). Each half of the copper ring is connected to one end of the coil and rubs against two

(d) carbon brush (P and Q). The (e) rheostat controls the size of current in the coil.

axle

P

X Y

Q

Single coil

of wire

Rheostat

Commutator

Carbon brush

Permanent

magnet

When circuit is closed,

• current will flow from battery to QY, through the coil and back to battery through XP.

• Right-hand side of the coil experiences a downward

force.

• Left-hand side of the coil experiences an equal upward

force.

• This couple makes the coil rotates clockwise until it reaches a vertical position.

• At this point, the current is cut off because neither X nor Y is in contact with P and Q; and there is no turning effect acting on the coil.

• However, momentum of the coil allows it to be carried slightly beyond this vertical position.

• The carbon brushes and commutator are in contact again but X

and Y reverses position.

• The coil continues to rotate in the same direction.

• The current in coil reverses each time the coil passes the vertical direction.

• To increase the turning effect of the coil in the d.c motor, we can

1. Insert a soft iron core or cylinder into the coil to concentrate the magnetic field lines.

2. Increase the number of turns

in the coil.

3. Increase the current in the coil.