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GCSE Physics

Magnetism and Electromagnetism

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Lesson 3 – Fleming’s LHRAims:

•To know that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field.

•To recall that a force is exerted on a current-carrying wire in a magnetic field, and, how this effect is applied in loudspeakers.

•To predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field (Flemming’s LHR).

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The Big Picture•In our first lesson we looked at materials that were either not magnetic (aluminium), magnetic (iron), or magnets (magnetised iron). We also studied the attraction and repulsion of two magnets.

•In our next lesson we saw that an electric current in a conductor creates a magnetic field around that conductor.

•Today we study the interactions between electromagnets and permanent magnets.

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Repel and attract

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Magnetic force

•When a current passes through a piece of foil a magnetic field is created around the strip.

•The Permanent horseshoe magnet repels the foil upwards.

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Reverse the current

•If the direction of the electric current is reversed the magnetic field acts in the opposite direction.

•The permanent horseshoe magnet attracts the foil downwards.

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Flemming’s apparatus

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Flemming’s apparatus

•The d.c. electric current passes through one iron bar, across the moveable axle and back through the other iron bar.

•The electric current passing through the axle creates a magnetic field.

•The permanent magnet will either attract of repel the axle causing it to move.

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Fleming’s Left Hand Rule

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Get your hands ready

•We are now going to look at the interaction of the current in a wire with a magnetic field.

•We know that the two will either attract or repel when a current is present.

•Fleming’s Left Hand Rule (LHR) will let us work out the direction of force on a wire near a magnet.

•This will allow us to understand how a motor and electric speaker work.

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LHR

•Grab a pen and get ready to write on your hand.

•The second finger of your left hand represents the direction of the electric current, draw a ‘C’ on this finger.

•The first finger of your left hand represents the direction of the magnetic field, draw a ‘F’ on this finger.

•Your left thumb represents the direction of thrust or force on the wire, draw a ‘Th’ on your thumb.

•Now point these three so that they are at right angles to each other!

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direction of force

(thumb)

direction of magnetic field

(first finger)

direction of current

(second finger)

To use Fleming’s left-hand rule, hold the thumb and the first two fingers of your left hand at right angles to each other.

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We can use the ‘Left Hand Rule’ to see which way the wire moves

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LHR example

Which way will the wire move?

Direction of the Force on a Wire = ?

Force on a Wire

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Moving particles•Remember that an electric current represents a flow of positive particles from positive to negative.

•If a proton was moving through a magnetic field you could use Flemming’s left hand rule to work out how its motion would be changed.

•For negative particles, such as an electron, you can still use the left hand rule but the direction of force/thrust will need to be reversed.

•Any particle moving along a magnetic field line will not be affected by the field.

Aurora

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Loudspeakers

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Varying an a.c. through the wire makes the loudspeaker cone vibrate back and forth. The vibrations cause sounds.

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Good vibrations

•The a.c. current creates an oscillating magnetic field.

•The cone is pushed in and pulled out.

•The cone creates a vibration in the air that we can hear.

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Learn bullet points

•Loudspeakers use a permanent magnet.

•Loudspeakers use a coil of wire.

•A current is present in the wire.

•The current creates magnetism.

•The wire is attracted or repelled by the permanent magnet.

•The speaker moves in and out.

•An alternating current can create a vibration (sound wave).

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Relax

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Do not worry

The magnetic field points upwards from North to South.

The dot tells us that the current is coming out of

the screen.

Arrange your hand to fit the left hand rule, your thumb should now be pointing right to left.

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Take it easy

The magnetic field points downwards from North to South.

The cross tells us that the current is going into

the screen.

Arrange your hand to fit the left hand rule, your thumb should now be pointing right to left.

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Relax

·

×

··

××

The electrical supply is a.c.

If the direction of current changes the direction of force also changes.

This makes the speaker move in the opposite direction.

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Varying an a.c. through the wire makes the loudspeaker cone vibrate back and forth. The vibrations cause sounds.

Torque on a Current Loop

Torque, cont. • (a) Maximum torque occurs when the normal

to the plane of the loop is perpendicular to the magnetic field, while in (b) the torque is zero when the normal is parallel to the field.

How to increase the turning effect on the coil?

• Increase the current

• Increase a stronger magnet

• Increasing the number of turns on the coil

• Increasing the area of the coil in the field.

Construction of Galvanometer

AB

F F

A

B

F

Fω AB

F F

ω Due to inertia

F

F

ω

A

B

How will this current loop move?

Vibrate about horizontal plane continuously.

How can a current loop rotate continuously in one direction?

Commutator

Rings and brushes

•The electric current passes from the contact to the ring – they ‘brush’ each other.

•Current is present but the ring can move.

•You can sometimes see sparks from motors such as in a drill from the ring and brushes.

Changing the current but not the B-field

•The ring has two small splits opposite one another.

•To start with the current flows from red to blue.

•In the middle there is no current and the motor turns due to its own momentum.

•The current then flows blue to red, but the coil can keep on turning.

Motors convert electrical energy

into kinetic energy.

Practical motors

• Several sets of coils are used, each set at a different angle and with its own pair of commutator

• Each coils contain hundreds of turns of wire and are wound on a core call an armature.

Practical motors

• The pole pieces are curved to create a radial magnetic field

Practical motors

• Several sets of coils are used, each set at a different angle and with its own pair of commutator.

• Each coils contain hundreds of turns of wire and are wound on a core call an armature.

• The pole pieces are curved to create a radial magnetic field.

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All motion is relative

This is really important!

All motion is relative, in this lesson we discussed what would happen to the charged particle or current.

In general it does not matter if the magnetic field or particle is moving – it is their relative movement that is really important!

Do not forget this!

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Summary – Flemming’s LHR•When a proton, electron or current carrying wire are in a magnetic field they will experience a force.

•Flemming’s left hand rule can be used to find the direction of the force. Thumb = Force/thrust. First finger = direction of magnetic field. Second finger = direction of electric current.

•In a speaker the magnetic field from the current in the wire either attracts or repels the permanent magnets, the speaker moves in and out creating a sound wave.