Chapter 21

Electromagnetism

Magnetic Effect of a Current

Electric current produce a even if the wire itself is not made of magnetic material.

magnetic field

Magnetic Effect of a CurrentMagnetic field pattern due to a straight wire. The direction of the magnetic field can be

determined by the Right-hand rule.

If you imagine that the conductor is held in the right hand, with the thumb pointing in the direction of the current flow, then the fingers curl the direction of the magnetic field

Magnetic Effect of a CurrentMagnetic field pattern due to a straight wire. The direction of the magnetic field can be

determined by the Right-hand rule.

current

Current flowing out of paper

Magnetic Effect of a CurrentMagnetic field pattern due to a straight wire. The direction of the magnetic field can be

determined by the Right-hand rule.

current

Current flowing into the paper

Magnetic Effect of a CurrentMagnetic field pattern due to a flat coil

Magnetic Effect of a CurrentMagnetic field pattern due to a flat coil To increase the strength of the magnetic field

at the centre of the flat coil.

- Increase the current in the coil.- Increase the number of turns of

the flat coil.

Magnetic Effect of a CurrentMagnetic field pattern of a solenoid

Magnetic Effect of a CurrentMagnetic field pattern of a solenoid To increase the strength of the magnetic field

of a solenoid.

- Increase the current in the solenoid.- Increase the number of turns

per unit length of the solenoid.- Using a soft-iron core within the

solenoid.

Quiz Describe an experiment that will allow you

to observe the magnetic field patterns produced by a current-carrying straight wire.

Applications of a Magnetic Effect of Current A ______________ is used to protect the

appliance from excessive current flow.circuit breaker

Applications of a Magnetic Effect of Current When a larger _______ flows through the

circuit breaker, the magnetic field produced by the solenoid becomes ___________.

The solenoid becomes a ______________ It is now able to ___________ the latch and the spring decompresses. The spring pushes the safety bar out of the interrupt point and ___________ the circuit.

current

stronger

strong electromagnet.

attract

switches off

Applications of a Magnetic Effect of Current When the reset button is pushed, the spring

compresses and the safety bar returns to _______ the interrupt point. The spring stays compressed due to the soft iron ___________ and current can flow through the circuit breaker again.

close

latch

Applications of a Magnetic Effect of Current A circuit breaker is not suitable for use with

high voltage electricity because the electric current can _______ across the small opening in the interrupt point when the potential difference is sufficiently ___________.

flow

high

Quiz State the effect of a larger current on the

magnetic force between the solenoid and the soft iron latch in a circuit breaker.

The magnetic force will be stronger.

Force on a Current-Carrying Conductor in a Magnetic Field

Investigation of the force on a current-carrying conductor in a magnetic field (Lorentz Force)

(Textbook, Page 422)

Force on a Current-Carrying Conductor in a Magnetic Field Apparatus

Stiff wire, string permanent magnets, 9V d.c. power supply.

Force on a Current-Carrying Conductor in a Magnetic Field Procedures:

Bend a stiff wire ABCD into the shape of a swing as shown in the figure.

Force on a Current-Carrying Conductor in a Magnetic Field Procedures:

Place the magnet over the wire BC as shown.

Force on a Current-Carrying Conductor in a Magnetic Field Procedures:

Switch on the current. Observe the direction in which the wire is swung.

Force on a Current-Carrying Conductor in a Magnetic Field Procedures:

Reverse the direction of the current by switching the polarity of the dry cell. In which direction is the swing flung now?

Force on a Current-Carrying Conductor in a Magnetic Field Observations:

With the current flowing in the direction A --> B --> C --> D, the wire is observed to swing outwards from the magnet.

If the current is reversed, the swing of the wire will be reversed (inwards)

Force on a Current-Carrying Conductor in a Magnetic Field In conclusion to the experiment, we can

say that A acts on the

when placed in a The acts at to

both the direction and the direction of the .

force current-carryingwire magnetic field.

force right anglecurren

t magnetic field

Force on a Current-Carrying Conductor in a Magnetic Field

In conclusion to the experiment, we can say that

When the direction of current (or magnetic field) is reversed, the

direction of the force on the wireis also reversed.

Fleming’s Left-hand Rule The direction of the force can be

deduced by using this rule.

thuMb - direction ofMotion of the wire.

Motion of force

Fleming’s Left-hand Rule The direction of the force can be

deduced by using this rule.

Forefinger - direction ofmagnetic Field

Motion of force

(N-S direction)

Magnetic Field

Fleming’s Left-hand Rule The direction of the force can be

deduced by using this rule.

SeCond finger - direction of Current

Motion of force

(Conventional current)

Magnetic Field

Current

Force on a Current-Carrying Conductor in a Magnetic Field To explain the force exerted on the wire, we

need to consider the combined magnetic fields due to the current flowing through the straight wire and the magnet.

S

N

22.1 Force on a Current-Carrying Conductor in a Magnetic Field

The wire moves because the magnetic fieldof the permanent magnets reacts with themagnetic field of the current in the wire.

S

N

22.1 Force on a Current-Carrying Conductor in a Magnetic Field

The two fields acting in the same direction combine to give a field, but the two field opposing each other combine to give a field.

stronger

weaker

stronger field

weaker field

22.1 Force on a Current-Carrying Conductor in a Magnetic Field

Hence the fields on both sides produces a that exerts on the wire.

unbalancedforce

stronger field

weaker field

Force on a Moving Charge in a Magnetic Field Fleming’s left-hand rule can be applied to all

moving charges.

Note that the conventional current (flow of positive charges) travels in an direction to that of the electron flow.

opposite

Force Magnetic Field

Current (conventional current)

Simple Circuit Breaker

Simple Circuit Breaker• When the live wire carries the usual

operating current the electromagnet is not strong enough to separate the contacts.

Simple Circuit Breaker• If something goes wrong with the

appliance and a large current flowsthe electromagnet will exert a strong magnetic force to separate the contacts and break the circuit.The spring then keeps the contacts apart.

Simple Circuit Breaker• After the fault is repaired, the contacts can

then be pushed back togetherby lifting a switch on the outside of the circuit breaker.