ALL magnets have two poles

NORTH seeking pole

SOUTH seeking pole

Breaking a magnet produces two magnets!

N S

N S

NN SS

Opposite poles attract and like poles repel

Magnetic materials

Iron (steel), Cobalt and Nickel

Magnetic induction

When a magnetic material is close to a magnet, it becomes a magnet itself

We say it has induced magnetism

NS

NSmagnet

Soft Magnetism

Pure iron is a soft magnetic material

It is easy to magnetise but loses its magnetism easily

NS

before after

Iron nail

SN

NS

Not a magnet

N

Hard Magnetism

Steel is a hard magnetic material

It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)

NS

before after

Steel paper clip

NNS

It’s a magnet!

N

S

S N

Magnetic fields

Magnets (and electric currents) produce magnetic fields around them.

In the magnetic field, another magnet or magnetic material will experience a magnetic force.

Magnetic field lines

We can represent the magnetic field around a magnet using field lines.

Magnetic field lines

The closer the field lines are, the stronger the magnetic force felt

The arrows show the direction a compass needle would point at that point in the field.

Mind-mapMr Porter will put these slides on a loop and you are going to mind-map

them

(Colours, drawings,

single words)

Plotting magnetic fields

1. Two magnets, opposite poles facing each other 6 cm apart

2. Two magnets, like poles facing each other 6 cm apart

3. Two magnets along side each other, 5 cm apart, opposite poles opposite each other.

Field around a bar magnet

Two bar magnets

Strong uniform field

Two bar magnets

no field!

Earth’s Magnetic Field

N

S

Remember the North of a compass needle points to the geographic north pole (i.e. the geographic North pole is a magnetic south pole!)

Moving charges (currents)

Moving charges (electric currents) also produce a magnetic field

http://www.sciencebuddies.org

Conventional current – electrons flow in the opposite direction

Magnetic field around a straight wire

Stronger field closer to wire

You can use the right hand screw rule to decide which direction the field goes

To make an electromagnet strongerThe 3 C’s

• More Coils

• More Current

• Use an iron Core

Field around a coil

A side view

Field around a solenoid (coil)

The Motor Effect

When a current is placed in a magnetic field it will experience a force (provided the current is not parallel to the field). This is called the motor effect.

Can you copy this sentence into your

books please.

The Motor Effect

The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.

Centre finger = Conventional Current (+ to -)

First finger = Field direction (N to S)

Thumb = Motion

Can you copy this please?

WITH DIAGRAM!

Sample question

In this example, which way will the wire be pushed? (red is north on the magnets)

Sample question

In this example, which way will the wire be pushed? (red is north on the magnets)

Current

Field

Building a loudspeaker

These also use the motor effect

Coil in here

magnet

D.C.Motor

Commutator ensures that every half rotation the current

direction reverses in the coilbrushes

D.C.Motor

Electromagnetic induction

If a magnet is moved inside a coil an electric current is induced (produced)

Electromagnetic induction

A electric current is induced because the magnetic field around the coil is changing.

Generator/dynamo

A generator works in this way by rotating a coil in a magnetic field (or rotating a magnet in a coil)

Motor = generator

If electric energy enters a motor it is changed into kinetic energy, but if kinetic energy is inputted (the motor is turned) electric energy is produced!

Transformers

A transformer consists of 3 main parts

Transformers

A changing current in the primary coil produces a changing magnetic field in the core. This changing magnetic field induces a changing current in the secondary coil.

Changing voltages

The ratio of the number of turns on the coils equals the ratio of the voltages

Vp/Vs = Np/Ns

# of turns = Ns# of turns = Np

Changing voltages

A transformer where the voltage rises is called a step-up transformer

Changing voltages

A transformer where the voltage falls is called a step-down transformer

No energy for nothing

Power in = power out

VpIp = VsIs

a.c. only

Because transformers rely on induction (changing fields), they only work with alternating current (a.c.)

Why do we need transformers?

Current flowing through wires causes them to get hot and energy is lost.

Why do we need transformers?

Keeping the current low means electricity can be transported long distances without losing too much energy.

Why do we need transformers?

However, in order to send a large quantity of energy, high voltages are needed.

Transmission of Electricity

Step-up

Step-down