What is an electromagnet?

• An electromagnet can be made by sending an

electric current through a coil of wire wound

around an iron core.

Magnetic field

• Determine the direction of magnetic field

around a currentcurrent-- carrying wire carrying wire

Plan View of the Magnetic Field

1. Straight wire,

• A magnetic field will be generated when a

current flows through a wire.

• The magnetic field forms by straight wire are

concentric circlesconcentric circles

around the wire.

Strength of the Magnetic Field

The strength of the magnetic field form by a current carrying

conductor depends on:

1. the magnitude of the current.

A stronger current will produce a stronger magnetic field A stronger current will produce a stronger magnetic field

around the wire.

2. the distance from the wire.

The strength of the field

decreases as you move

further out.

2. Coil

• Grip the wire at one side of the coil with your

right hand, with thumb pointing along the

direction of the current.

• Your other fingers will be pointing in the

direction of the field.direction of the field.

Factors affecting the strength

There are 2 ways to increase the strength of

the magnetic field:

• increase the current and• increase the current and

• increase the number of turns of the coil.

3. Solenoid

• A solenoid is a long coil made up of a numbers

of turns of wire.

Strength of the Magnetic Field

The strength of the magnetic field can be

increased by

• Increasing the current,• Increasing the current,

• Increasing the number of turns per unit length

of the solenoid,

• Using a soft-iron core within the solenoid.

Application of Electromagnet

Electric Bell

• When the bell push is pressed, a current flows in the coils of the electromagnet, causing it to be magnetized.

• The magnetized electromagnet attracts the soft-iron armature, causing the hammer to strike the gong.

• The movement of the armature breaks the contact and causes the electromagnet to lose it magnetism.

• The light spring pulls the armature back, remaking the contact and completing the circuit again.

Magnetic Relay

• A relay has at least two circuits. One circuit can be used to control

another circuit. The 1st circuit (input circuit) supplies current to the

electromagnet. The electromagnet is magnetised and attracts one

end of the iron armature.

• The armature is then closes the contacts (2nd switch) and allows

current flows in the second circuit. When the 1st switch is open

again, the current to the electromagnet is cut, the electromagnet again, the current to the electromagnet is cut, the electromagnet

loses its magnetism and the 2nd switch is opened. Thus current

stop to flow in the 2nd circuit.

Circuit Breaker• Acts as an automatic switch that breaks open a circuit when

the current becomes too large.

• In a household circuit, the current may become excessive when there is a short circuit or an overload.

• The strength of the magnetic field of the electromagnet increases suddenly.

• The soft iron armature is pulled

towards the electromagnet. towards the electromagnet.

This results in the spring pulling

apart the contacts. The circuit is

broken and the current flow stops

immediately.

• The reset button is pushed to

switch on the supply again

Telephone Earpiece

• When you speak to a friend through the telephone, your sound will be converted into electric current by the mouthpiece of the telephone.

• The current produced is

a varying current and

the frequency of thethe frequency of the

current will be the same

as the frequency of your

sound. The current will be

sent to the earpiece of the

telephone of your friend.

Force on a Current-carrying

Conductor in a Magnetic Field

Catapult Force

• When a current-carrying conductor is placed

in a magnetic field, the interaction between

the two magnetic fields will produce a force a force

on the conductoron the conductor, which called a catapult on the conductoron the conductor, which called a catapult

force.

Fleming's Left Hand Rule (Motor Rule)

• The fore finger, middle finger and the thumb

are perpendicularly to each other.

• The forefinger points along the direction of

the magnetic field, the magnetic field,

• middle finger points

in the current direction.

• the thumbthumb points along

the direction of the forceforce.

Strength of the Catapult Force

The strength of the force can be increased by:

• Increase the current

• Using a stronger magnet

Force between 2 current carrying conductor

• When 2 current carrying conductors are placed close to each other, a force will be generated between them.

• If the current in both • If the current in both conductors flow in the same direction, they will attract each other,

• whereas if the currents are in opposite direction, they will repel each other.

Turning Effect of a Current-carrying

Coil in a Magnetic Field

Turning Effect of a Current-carrying

Coil in a Magnetic Field

• If a current carrying coil is

placed in a magnetic field, a

pair of forces will be

produced on the coil. produced on the coil.

• This is due to the

interaction of the magnetic

field of the permanent

magnet and the magnetic

filed of the current

carrying coil.

Electric motor

Electric motor Electric motor

• Electrical energy � mechanical energy

• Types of motor:

1. Alternating current motor ( a.c motor)

~ work on an alternating current supply

2. Direct current motor (d. c motor)2. Direct current motor (d. c motor)

~ work on an direct current supply

Direct Current Motor

• The function of the commutator is to change the

direction of the current in the coil and hence

change the direction of the couple (the 2 forces in

opposite direction) in every half revolution.

• This is to make sure that the coil can rotate • This is to make sure that the coil can rotate

continuously.

Direct current motor ( D.C. Motor)

• Determine the direction of motion of the

conduct AB .

Horizontal position

• The direction of force/ motion can be determined by using Fleming’s Left-hand Rule.

• What will happen to the direction of the

rotation if the direction of the current flow is

reversed?

Moving coil meter

Electromagnetic Induction

Electromagnetic Induction

• When a magnet is moved into and out of the solenoid, magnetic flux is being cut by the coil.

• The cutting of magnetic flux by the wire coil induces an by the wire coil induces an e.m.f in the wire.

• When the solenoid is connected to a closed circuit, the induced current will flow through the circuit.

Faraday's Law

• Faraday's Law states that the magnitude of

the induced e.m.f is directly proportional to

the rate of change of magnetic flux through a

coil or alternatively the rate of the magnetic coil or alternatively the rate of the magnetic

flux being cut.

Lenz's Law

• Lenz's Law states that the induced current

always flows in the direction that opposes the

change in magnetic flux.

Fleming's Right Hand Rule (Generator Rule)

• Fleming's Right-Hand Rule is used to

determine the direction of the induced

current that flows from the wire when there is

relative motion with respect to the magnetic relative motion with respect to the magnetic

field

Direct Current Generator

• A simple d.c generator essentially the

converse of a d.c. motor with its battery

removed.

DC Generator - Display of the Voltage in a CRO• Initially the armature is vertical. No

cutting of magnetic flux occurs and hence

induced current does not exist.

• After rotating by 90°, the armature is in

the horizontal position. The change in

magnetic flux is maximum and hence the

maximum induced e.m.f is produced.

• At the 180° position, there is no change

in flux hence no induced current exists.in flux hence no induced current exists.

• The induced current is achieves its

maximum value again when the armature

is at 270°.

• After rotating 360°, the armature returns

to its original position.

The current in the external circuit always

flows in one direction. This uni-

directional current is known as direct

current.

A.C generator

• Generator can be modified to an a.c generator by

replacing its commutators with two (separate) slip

rings.

• The two slip rings rotate in tandem with the

armature. Carbon brushes connect the armature to armature. Carbon brushes connect the armature to

the external circuit.

AC Generator - Display of the Voltage in a CRO

• The armature is initially at the vertical position. No

magnetic flux is cut and hence no induced current exists.

• When the armature rotates, the change in magnetic flux

increases and the induced current increases until its

maximum value at the horizontal position.

• As the armature continues on its rotation, the change in

magnetic flux decreases until at the vertical position, no

induced current exists.

• Subsequently upon reaching the horizontal position again,

the induced current is maximum, but the direction of the

induced current flowing through the external circuit is

reversed.

• The direction of the induced current (which flows through

the external circuit) keeps on changing depending on the

orientation of the armature.

• This induced current is also known as alternating current.

The current is positive (+) in one direction and negative in

the other (-). The slip rings play a critical role in the

generation of alternating current.

Differences between DC and AC

generator

Differences between DC and AC

generator

D.C and A.C Current

Direct Current

• Direct current is uniform current flowing in

one fixed direction in a circuit.

• The magnitude of a direct current can be

either uniform or varying with time.either uniform or varying with time.

• Direct current (d.c) is usually supplied by acid-

based batteries or dry cells.

Alternating CurrentAlternating Current

• Alternating current is a current which changes

its direction periodically in a circuit.

• Alternating current (a.c) is generated from

alternating current generators such as alternating current generators such as

hydroelectric power generators. Its magnitude

also changes with time.

Root Mean Square Voltage

• The effective potential difference for an a.c is equal

to the potential difference of a alternating current if

both results in the same heating effect.

• The effective potential difference for a.c is known as

the root mean square voltage (r.m.s) of the a.c. and

is given y the following equation:is given y the following equation:

Direct Current and Alternating Current with Capacitor

• A direct current (d.c.) cannot flows through a capacitor.

• An alternating current (a.c.) can flows through a capacitorthrough a capacitor

Therefore :

• A direct current (d.c.) causing no effect on the moving coil loudspeaker.

• An alternating current (a.c.) can cause a moving coil loudspeaker functioning properly.