1. MAGNETISM AND ELECTROMAGNETISM
SECTION 6:
MAGNETISM AND ELECTROMAGNETISM
Chapter 21
Magnetism and Electromagnetism

2. Magnets attract magnetic materials but not non-magnetic materials.
Magnetism is a non-contact force (acts at a distance).
Magnetic materials:
iron
steel
nickel
cobalt
The region around a magnet where a magnetic force can be felt is called the magnetic field.
The magnetic field is strongest at the poles of a magnet.

3. Unlike poles attract Like poles repel north-seeking pole
north pole
south-seeking pole or
south pole
Like poles repel
Unlike poles attract

4. N S S N N S N S S N N S S N S N REPULSION ATTRACTION REPULSION
Magnetic field around a magnet has a specific shape. S N S N
ATTRACTION

5. A magnetic field around a bar magnet has a shape and direction.
The magnetic field is represented using magnetic field lines (lines of force, flux lines) that show the shape, direction and strength of the field.

6. Investigate the shape of the magnetic field between two bar magnets.

7. N S UNIFORM MAGNETIC FIELD.
Magnetic field lines are parallel and equally spaced apart.

8. Made from magnetically hard materials, such as steel.
PERMANENT MAGNETS
TEMPORARY MAGNETS
Made from magnetically hard materials, such as steel.
Made from magnetically soft materials, such as iron.
Retain their magnetism once they have been magnetised.
They are easy to magnetise but lose their magnetism easily.
Magnetism can be induced in some materials when they are placed in a magnetic field.

9. Electromagnetism
A wire carrying a current has a magnetic field around it.
The magnetic field around a current carrying wire has a circular shape.

10. The right hand grip rule
Use the Right-hand grip rule to remember the direction of the current (conventional current +  -)

11. The magnetic field around a flat coil

12. You can use your right hand to determine the direction of the magnetic field of a coil If you curl your fingers up so they are pointing in the direction in which current flows around the coil, your thumb will be pointing towards the north end of the coil.

14. Solenoids
The magnetic field around a solenoid has the same shape as the field around a bar magnet.
To increase the strength of the magnetic field around a current-carrying wire we can:
 increase the current in the wire
 wrap the wire into a coil or solenoid
The field inside the solenoid is very strong and uniform.
It can be used to magnetise objects.

15. Polarity of a coil

16. The strength of the magnetic field around a solenoid can be increased by:
Increasing the current.
Increasing the number of turns on the solenoid.
Using a magnetically soft core such as iron.
Bringing the poles together.

17. Using electromagnets
Advantages of using electromagnets: Can be switched on or off, strength can be controlled.
Disadvantages: must be supplied with energy continuously.

18. The electric bell

19. Circuit breaker

20. An electromagnetic door lock
There is an iron bar keeping the door locked.
If the security officer agrees to let you in, he presses a switch which allows a current to flow through the solenoid.
What happens to the iron bar?

21. The electromagnetic relay