1. Magnetism
In this section you will learn:
What a magnistism is.
About magnetic fields and field lines
To show magnetic fields due
Bar magnet
Solenoid
Straight current carrying wire
Understand how an electromagnet works
Understand a compass and the Earth's magnetic field.
Note: Show phet símulations ón 'magnet and compass' and 'magnets and electromagnets'
Use visualiser for demonstration of magnetic fields of bar magnet, solenoid, straight wire and how an electromagnet works.

2. Magnetism
Did you know that the name "magnet" was first used by the Greeks as early as 600 B.C. for describing a mysterious stone that attracted iron and other pieces of the same material?
A magnet is an object made of certain materials which create a magnetic field.
Every magnet has at least one north pole and one south pole. By convention, we say that the magnetic field lines leave the north end of a magnet and enter the south end of a magnet.
Magnets are made from iron, nickel and cobalt and various alloys.

3. Magnetic fields Magnetic field
The area over which the effects of a magnet can be felt is called the magnetic field. Its presence is indicated by the use of field lines. They show you the way a free north pole would move in the field.
If they are bunched together, the field is strong and if they are far apart the field is weak. (contours of a map analogy)
© TPS 2010

4. Magnetism Permanent Magnets
Certain materials (iron nickel and cobalt) contain small areas of magnetic moments called domains.
There are many different types of magnets. Permanent magnets never lose their magnetism.
Most of the magnets you see around you are man-made. Since they weren't originally magnetic, they lose their magnetic characteristics over time. N S
© TPS 2010

5. Magnetism
Magnetic field due to a bar magnet
Easily shown by placing a magnet under a sheet of paper.
Iron filings a sprinkled ón the paper.
A plotting compass gives the direction of the field.
NB the field is 3 dimensional.

6. There is no net force at the neutral point.
Magnetic fields N
Neutral Point
There is no net force at the neutral point. S N
Attraction
Repulsion
What field pattern is produced when two south poles are brought together?
© TPS 2010

7. In 1820, a Danish scientist named Hans Oersted discovered that a magnetic compass could be deflected from its resting position if a wire carrying electric current were placed near the compass. This deflection of the compass only occurred when current was flowing in the wire. When current was stopped, the compass returned to its resting position.

8. Magnetic fields
Any electric current has a magnetic field associated with it.
The magnetic field pattern due to a straight
Wire is circular. It can be
Shown using a card around the wire
And sprinkling iron filings on it
The field shown here is the field due to a long straight wire.

9. Your fingers will clasp around the wire in the direction of the field.
Magnetic fields
The right hand grip rule is handy for remembering which way the field lines go.
Imagine grasping your hand around the wire so that your thumb points in the direction of the conventional current (positive to negative).
Your fingers will clasp around the wire in the direction of the field.
positive
© TPS 2010

10. In this diagram, the current is coming out of the screen, towards you.
Magnetic fields
Looking directly down on top of the wire, the field would look like this:
In this diagram, the current is coming out of the screen, towards you.
The field lines get further apart further from the wire to show that the field is getting weaker.
© TPS 2010

11. In this diagram, the current is going into the screen, away from you.
Magnetic fields
If the electric current was travelling in the other direction, the field would look like this: X
In this diagram, the current is going into the screen, away from you.
The only difference is that the field lines are travelling in the opposite direction.
© TPS 2010

12. Magnetic fields
The magnetic field due to a current carrying loop.

13. Magnetic field due to a coil.
Magnetic fields
Magnetic field due to a coil.
The field is stronger than a circular loop but the shape is similar.
© TPS 2010

14. Plan View Magnetic fields
It is possible to work out what the field would look like due to various wires. This is the field due to a flat loop of wire. + -
Plan View
This end of the coil looks like a south pole
© TPS 2010

15. Magnetic fields
The field due to a solenoid looks just the same as the field due to a bar magnet.
The front end of this solenoid looks just like a north pole of a bar magnet.
Note that the field inside the solenoid is almost uniform – the same strength all around as shown by the roughly equal field line spacing.
© TPS 2010

16. Electromagnets
The term electromagnetism is defined as the production of a magnetic field by current flowing in a conductor.
If a soft iron core is placed in a solenoid it is a powerful magnet when the current is switched on and loses its magnetism when the current is switched off.
Simply put when you run a current through a wire wrapped around say a nail it becomes a magnet.

17. Electromagnets
Uses of Electromagnets.
Electric motor
Electric bell
Scrap iron crane
Electromagnetic relays
Trip switches.
Loudspeaker.

18. Electromagnetism
Earth's Magnetic Field
The behaves as if there were a large bar magnet in is centre.
The south of this imaginary magnet is close to the north magnetic pole.
The angle between the true north and magnetic north is called magnetic declination.
Its size depends on your location.
The compass works because of the Earth's magnetic field.