1. Magnetism and Electromagnetism

2. Learning Objectives
Magnetism and electromagnetism b) Magnetism understand that magnets repel and attract other magnets and attract magnetic substances describe the properties of magnetically hard and soft materials understand the term ‘magnetic field line’ understand that magnetism is induced in some materials when they are placed in a magnetic field describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets describe how to use two permanent magnets to produce a uniform magnetic field pattern.
c) Electromagnetism understand that an electric current in a conductor produces a magnetic field around it describe the construction of electromagnets sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current

3. Magnets and magnetic materials
Magnets attract objects made of magnetic materials Magnetic materials include the elements iron, nickel, cobalt, alloys containing some of these such as steel and some of their compounds.
iron
nickel
cobalt
stainless steel

4. Hard and soft magnetic materials
Permanent magnets are made of magnetically HARD materials such as steel. These materials retain their magnetisation once magnetised.
Magnetically SOFT materials, such as iron, lose their magnetisation easily. They suitable for temporary magnets such as electromagnets.

5. Iron filing are attracted mostly to the poles of a magnet
Magnetic poles
Magnetic poles are the parts of a magnet that exert the greatest force. Magnetic poles occur in pairs usually called north (N) and south (S)
Iron filing are attracted mostly to the poles of a magnet

6. Why poles are called north and south
A magnet suspended so that it can rotate freely horizontally will eventually settle down with one pole facing north and the other south. This is pole is therefore called the ‘north seeking pole’, usually shortened to just ‘north pole’. The magnet has been orientated by the Earth’s magnetic field. A compass is an application of this effect.

7. The law of magnets
Like poles repel unlike poles attract

8. Magnetic fields
A magnetic field is a volume of space where magnetic force is exerted. All magnets are surrounded by magnetic fields. The shape of a magnetic field can be shown by iron filings or plotting compasses.

9. Magnetic field around a bar magnet
magnetic field line
Arrows on the field lines show the direction of the force on a free to move north pole
The stronger the magnetic field the denser the magnetic field lines.

10. Magnetic fields between two bar magnets

11. Producing a uniform magnetic field
A uniform magnetic field exerts a constant force over a region. Such a field will consist of parallel equally spaced magnetic field lines. This type of field can almost be found between a north and south magnetic pole.

12. The Earth’s magnetic field
The earth’s magnetic field is similar in shape to that around a bar magnet. It is thought to be caused by electric currents flowing through the molten outer core of the Earth. At the present the field pattern is like that with a magnetic SOUTH pole situated somewhere below northern Greenland

13. Induced magnetism
Magnetism can be induced in a magnetic material if it is placed within a magnetic field. If the material is magnetically hard it will retain its magnetism once removed from the field. Certain rocks in the Earth’s crust such as lodestone have been magnetised in this way by the Earth’s magnetic field.
iron bar
N S

14. Choose appropriate words to fill in the gaps below:
Magnetic materials are either hard or ______. Hard magnetic materials such as ______ retain their magnetisation once magnetised.
A magnetic _____ is a region where the magnetic force is greatest. Magnetic poles always occur in ______. Like poles _______, unlike attract.
A magnetic ______ is a region where magnetic force is exerted. The ________ of the magnetic field around a bar magnet is from north to south.
soft
steel
pole
pairs
repel
field
direction
WORD SELECTION:
pole
repel
steel
pairs
field
direction
soft 14

15. No current, compass points to north Current, compass deflected
Electromagnetism
No current, compass points to north
Current, compass deflected
In 1820 Hans Ørsted noticed that a wire carrying an electric current caused a compass needle to deflect. 15

16. Magnetic field patterns around wires 1. Straight wire
The magnetic field consists of concentric circles centred on the wire.
The magnetic field is strongest near the wire.
This is shown by the field lines being closest together near to the wire.
The strength of the field increases if the electric current is increased. 16

17. The right-hand grip rule (for fields)
Grip the wire with the RIGHT hand.
The thumb is placed in the direction of the electric current.
The fingers show the direction of the circular magnetic field. 17

18. Complete the diagrams below:
Add field arrows
Add current direction
Add current direction
Add field arrows
Electric current out of the page
Electric current into the page 18

19. 2. Flat circular coil
Plan view 19

20. 3. Solenoid A solenoid is a coil of wire carrying an electric current.
The magnetic field is similar in shape to that around a bar magnet.
The strength of the field increases with:
the electric current
the number of turns in the coil N S 20

21. The right-hand grip rule (for poles)
Grip the coil with the RIGHT hand.
The fingers are placed in the direction that the eclectic current flows around the coil.
The thumb points towards the north pole end of the coil. N S 21

22. Complete the diagrams below:
1. Locate north
2. Locate south
3. Add current direction S N N
4. Add coils N 22

23. Electromagnets
An electromagnet consists of a current carrying coil wrapped around an iron core. 23

24. Uses of electromagnets
1. Scrap yard crane
The iron core of the electromagnet is a SOFT magnetic material.
When current flows the iron becomes strongly magnetised and so picks up the scrap iron and steel.
When the current is turned off the iron loses its magnetisation and so releases the scrap. 24

25. 2. The electric bell
When the push switch is closed current flows around the circuit turning on the electromagnet.
The soft iron armature is pulled towards the electromagnet and the hammer hits the gong.
This causes the contact switch to open cutting off the electric current.
The spring now pulls the armature back again closing the contact switch.
Current now flows again and the hammer hits the gong again.
push switch
spring
electromagnet
contact switch
soft iron armature
hammer
gong 25

26. Label the diagram of the electric bell below:2 5 3
Contact switch 7 6 8 4 1 26

28. Domestic circuit breakers
Current normally flows between terminals A and B through the contact and the electromagnet. When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the soft iron armature towards the electromagnet. As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, and stopping current flow. A B 2 1
Domestic circuit breakers
The reset button can be pushed to bring the contact back to its original position to reconnect the circuit

29. Generator
A coil of wire turns in a magnetic field. The flux in the coil is constantly changing, generating an emf in the coil.

30. Transformers
A transformer is a device used to change the voltage in a circuit. AC currents must be used.
120 V in your house
75,000 V in the power lines
p = primary
s = secondary

31. Choose appropriate words to fill in the gaps below:
A wire carrying an electric ______ produces a magnetic field. This field increases in ________ if the current is increased.
A ________ is a coil of wire carrying an electric current. The field produced increases in strength if the number of _____ in the coil is increased or if _____ is placed inside the coil.
An ____________ consists of a coil of a solenoid wrapped around an iron core. Iron is a ______ magnetic material that loses its magnetisation once the current in the coil is switched off.
current
strength
solenoid
turns
iron
electromagnet
soft
WORD SELECTION:
solenoid
iron
strength
turns
electromagnet
current
soft 31

32. Online Simulations
Bar magnet field - Fendt Faraday Electromagnetic Lab - PhET - Play with a bar magnet and coils to learn about Faraday's law. Move a bar magnet near one or two coils to make a light bulb glow. View the magnetic field lines. A meter shows the direction and magnitude of the current. View the magnetic field lines or use a meter to show the direction and magnitude of the current. You can also play with electromagnets, generators and transformers! Field around a straight conductor - Fendt Magnetic field around a straight wire / coil / solenoid - NTNU Relay demonstration - Freezeway.com Electric Bell demonstration - Freezeway.com BBC KS3 Bitesize Revision: Bar Magnets Magnetic Fields Electromagnets Using Electromagnets - includes an applet showing how an electric bell works

33. Magnetism and Electromagnetism Notes questions from pages 179 to 186
State what happens when different types of magnetic poles are placed near to each other. (see page 180)
(a) What is the difference between magnetically soft and hard materials? (b) Give examples and uses of each type. (see page 180)
(a) Draw the magnetic field patterns between and around magnets shown on page 181.(b) Explain what the magnetic field lines show about the magnetic fields.
Explain how a uniform magnetic field can be produced. (see page 181)
Draw the magnetic field patterns around and inside; (a) a straight conducting wire; (b) a flat coil; (c) a solenoid. (see pages 182 and 183)
Draw a labelled diagram showing the construction of an electromagnet. (see page 183)
Draw a diagram and explain the operation of an electric bell. (see page 183)
Draw a diagram and explain the operation of a circuit breaker. (see page 184)
Draw a diagram and explain the operation of a relay. (see page 185)
Answer the questions on page 186.
Verify that you can do all of the items listed in the end of chapter checklist on page 186.

34. Magnetism and Electromagnetism Notes questions from pages 179 to 186
(a) Draw the magnetic field patterns between and around magnets shown on page 181.(b) Explain what the magnetic field lines show about the magnetic fields.
Explain how a uniform magnetic field can be produced. (see page 181)
Draw the magnetic field patterns around a straight conducting wire (see page 182) 34