1. Magnetism and Magnetic fields
Describe forces between magnets
Give an account of induced magnetism
Distinguish between magnetic and non-magnetic materials
Describe methods of magnetisation (stroking with a magnet, use of dc in a coil, hammering in a magnetic field..)
Draw magnetic field lines around a bar magnet.
Describe an experiment to identify the pattern of magnetic field lines including direction
Distinghis between the magnetic properties of soft iron and steel
Permanent magnets and electromagnets
Explain that magnetic forces are due to interactions between magnetic fields
Describe methods of demagnetisation (hammering, heating, use of ac in a coil).

2. Opposite poles attract and like poles repel

3. Breaking a magnet produces two magnets!

4. Iron (steel), Cobalt and Nickel
Magnetic materials
Iron (steel), Cobalt and Nickel

5. M
When a magnetic material is close to a magnet, it becomes a magnet itself
We say it has induced magnetism
magnet S N N S

6. The domains in the material become aligned.
Magnetic induction
The domains in the material become aligned.
The induced pole nearest the magnet is the opposite of the pole at the end of the magnet.

7. Soft Magnetism Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily.
before
after N S S N N S N
Not a magnet
Iron nail

8. Hard Magnetism Steel is a hard magnetic material
It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)
before
after S N S N N S N N
It’s a magnet! S
Steel paper clip

9. You can strike the magnet with another magnet. Or
Magnetizing a magnet
You can strike the magnet with another magnet. Or
You can place the magnet in a solenoid and use DC current to magnetise it
You can hammer the magnet in a magnetic field

10. Demagnetizing a magnet
You can heat the magnet, so it's atoms vibrate faster and are thrown out of line Or
You can hit it with a hammer so the domains are thrown out of line.
You can put the magnet in a solenoid running on an AC current

11. Magnetic fields
On the mini-whiteboards write down as many words as you can about what you think a magnetic field is.

12. Magnets (and electric currents) produce magnetic fields around them.
The magnetic field is the region where another magnet or magnetic material will experience a magnetic force. We cannot “see” the field but we can feel its effect.

13. Plotting magnetic fields

14. Plotting magnetic fields

15. Plotting magnetic fields
Using the bar magnets and the plotting compass you are going to plot the magnetic field of:
One magnet
Reverse the single magnet and see if there are any changes.
Two magnets, opposite poles facing each other 6 cm apart.
Two magnets, like poles facing each other 6 cm apart.
Two magnets along side each other, 5 cm apart, opposite poles opposite each other.

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

17. Two bar magnets: like poles
Strong uniform field

18. Two bar magnets: unlike poles
no field!

19. Magnetic field lines: two magnets

20. Magnetic field lines
Now using your results, write down the main characteristics of the magnetic field.

21. Magnetic field lines They show the shape of the magnetic field.
They show the direction of the magnetic field. They “travel” from the north pole to the south pole.
They show the strength of the magnetic field (they are closer together where the field is stronger).
They do not cross.

22. Earth’s Magnetic Field
Where are the north and south magnetic poles of the Earth?

23. Earth’s Magnetic Field
Remember the North of a compass needle points to the geographic north pole (i.e. the geographic North pole is a magnetic south pole!)

24. Revision quiz 1. What are the magnetic poles called?
2. Draw the shape of a magnetic field around a bar magnet.
3. Which way do the arrows in a magnetic field point?
4. Do the lines of magnetic field cross each other?
5. Draw the shape of a magnetic field between poles which repel.
6. What is the difference between a temporary and a permanent magnet?

25. The shape of the magnetic field depends on the shape of the conductor.
Electromagnetism
When a current flows through a conductor, it produces a magnetic field.
The shape of the magnetic field depends on the shape of the conductor.

26. Magnetic field for a straight wire
Is circular, at right angles to the wire.
Right hand grip rule
What will happen if we reverse the direction of the current? Plot it in your book!
Ext: This magnetic field is quite weak. How could you increase it?

27. Magnetic field for a single coil
Using the Right Hand Grip Rule we can predict that a coil of wire will produce a magnetic field similar to that of a small bar magnet.

28. Magnetic field for a solenoid
A strong field can be made by coiling the wire around a piece of soft iron. We call this a solenoid.
You can also use the right hand grip rule to calculate the direction of the magnetic field. How would you do it?

29. Magnetic field for a solenoid
The shape of the magnetic field is the same as a bar magnet
The soft iron inside the coil makes the magnetic field stronger because it becomes a magnet itself when the current is flowing.
Why do we use soft iron and not steel?

30. Magnetic field for a solenoid
If you are looking at the solenoid from one end and the current is flowing anti-clockwise that end is the North Pole, Looking from the other direction the current would flow clockwise at the South Pole.

31. Soft core
A strong field can be made by coiling the wire around a piece of soft iron. The electromagnet can be switched on and off by turning the electricity on and off.
Steel forms a permanent magnet. If steel was used inside the coil, it would continue as a magnet after the electricity was switched off. It would not be useful as an electromagnet.

32. Revision quiz Draw the shape of the magnetic field around a solenoid.
What does the right hand grip rule tell us?
How do we call the conductor inside the solenoid?