1 of 33

What is a force field?• A diagram of force lines...

... which show the strength & direction of a force• What does the Earth’s gravitational field look like?

phet: magnet and compass

© Boardworks Ltd 20063 of 33

weakest field further away from poles

What is the shape and direction of the lines of force in the magnetic field around a bar magnet?

N S

Shape of a magnetic field

Where is the magnetic field strongest?

strongest field

at poles

strongest field

at poles

© Boardworks Ltd 20064 of 33

Magnetic Field Lines

• Show the strength and direction of the magnetic force

• Field lines are shown by arrows and always move from North to South

• Field lines close together show a stronger magnetic force

• Field lines don’t cross – watch out for this!

© Boardworks Ltd 20065 of 33

What Causes the Earth’s Magnetic Field?

• Caused by moving electric charges (electromagnetism)

• The earth has a magnetic field around it because of charged iron atoms spinning around the core.

© Boardworks Ltd 20066 of 33

Magnetic Field Strength

• Magnetic field strength B is measured in Tesla T

• Quiz: what is the strength of:

1) The Earth’s magnetic field on the equator? 31 µT

2) The Earth's magnetic field in NZ? 58 µT

3) A typical fridge magnet? 5 mT

4) Medical magnetic resonance imaging? 1.5 – 3 T

5) The strongest magnetic field obtained in the Lab? 91.4 T

(in Dresden, in 2011, lasted for several milliseconds)

Note that a 730 T magnetic field was produced in a lab in Tokyo,

but it destroyed all the equipment, but not the lab itself...

6) A neutron star? 106 – 108 T

© Boardworks Ltd 20067 of 33

Bring the north poles of two bar magnets together.

What happens to the magnets?

Viewing magnetic fields: N poles together

N SS N

Next, bring the two north poles as close to each other as possible and place a piece of paper on top of the magnets.

Carefully use your compass to plot the field lines onto the paper.

© Boardworks Ltd 20068 of 33

NNS S

Magnetic field pattern: N poles together

What do you notice about the pattern of the lines of force in the region between the two north poles?

Sketch the magnetic field – include the direction

NNS S

© Boardworks Ltd 200610 of 33

Bring the north and south poles of two bar magnets together.

What happens to the magnets?

Viewing magnetic fields: N and S poles together

S N S N

Next, put the north and south poles close to each other, without letting them touch, and place a piece of paper on top.

Carefully use your compass to plot the field lines and draw the pattern created by the magnets

© Boardworks Ltd 200611 of 33

NN

How does this pattern compare with the pattern between the two north poles?

Magnetic field pattern: N and S poles together

What do you notice about the pattern of the lines of force in the region between the north and south poles?

S S

Sketch the magnetic field – include the direction

NNS S

spot the mistake?