1. Magnetism and the Motor Effect
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Magnetism and the Motor Effect
2016 EdExcel GCSE Physics
Topic 12
W Richards
The Weald School

2. Permanent vs Induced Magnetism
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Permanent magnets
Induced magnets
Always have a magnetic field around them
Can attract or repel other magnets
Become magnetic only when in a magnetic field
Can only attract magnetic metals, not repel them
Loses its magnetism quickly

3. The main magnetic metals are iron, steel, nickel and cobalt
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Q. In this experiment a magnet is held towards coins made in different years. Some of the coins were attracted, some weren’t. What does this prove?
Conclusion 1 – some coins are made of magnetic metals, some aren’t.
Conclusion 2 – the magnetic metals can only be attracted by a magnetic field, not repelled.
The main magnetic metals are iron, steel, nickel and cobalt

4. Magnetic field around a bar magnet
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Q. What would these fields look like if the field was weaker? N S
The force is the strongest at each pole

5. Magnetic fields in more detail
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In this experiment the magnets only start attracting each other when they are within each other’s magnetic fields.
Definition of “field” – “the region in which a force will act”
The direction of the field is drawn to show what the field will do to a north pole at that point. This is why the arrows point AWAY from a north pole. N S

6. Attracting and Repelling
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What would the magnetic field lines look like here?
Magnetic attraction and repulsion are examples of non-contact forces.

7. The Earth’s Magnetic Field
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Q. How does a compass work?

8. Plotting a Magnetic Field
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How could you use a compass to plot a magnetic field? N S

9. Magnetic Field around a current-carrying wire
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Magnetic Field around a current-carrying wire
Notice that the field gets weaker as you get further away (or if the current is reduced)
“Right hand corkscrew” or “right hand grip” rule

10. Making an electromagnet
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11. Electromagnets Experiment
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Number of coils
Number of paper clips picked up 5 10 15 20 25
Voltage on powerpack/V
Number of paper clips picked up 1 2 3 4 5

12. Making an electromagnet
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Task: To make your own electromagnet
Method: How will you do this?
Diagram of apparatus:
Conclusion:
Did it work?
What could you do to make it a better electromagnet? Name two different things you could do:

13. Why did the nail become magnetised?
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Compare the magnetic field around a bar magnet and around a coil of wire (also called a “solenoid”): N S
Magnet:
The field inside here is strong and uniform
Coil of wire:
The field on the outside is weaker as the fields from individual loops cancel each other out

14. How could you reverse the direction of the magnetic field?
Electromagnets
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The strength of an electromagnet can be increased by doing three things:
Increasing the voltage
Increasing the number of coils
Inserting an iron core
How could you reverse the direction of the magnetic field?

15. The Motor Effect (HT only)
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What will happen to this wire?
How can you make it move with more force/faster?
How can you make it move in a different direction?

16. Force on a current-carrying wire in a magnetic field
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Force on a current-carrying wire in a magnetic field N
F = Force
B = Magnetic field
I = Current S
Notice that the magnet also experiences an equal and opposite force pushing on it.
Q. Where will this wire go?

17. Magnetic force on a wire
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Clearly, the size of the force on this wire (and on the magnet) depends on these things:
The strength of the magnetic field
The current in the wire
The length of the wire (in the field)
These things are related by the simple formula…
F = BIL
B is called “magnetic flux density” and is measured in Teslas. To use this equation, make sure length is in metres and current in Amps!

18. Example questions
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1) A 2m long wire is placed in a magnetic field of strength 5T. If the current through the wire is 0.5A how much force will act on the wire? 5N
0.2m
2) This wire experiences a force of 0.5N on it. What it the value of the magnetic field strength and which direction would the wire move in? S N
0.1A
25N, into the page

19. Using Motors
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The following devices are just some examples of devices that use electric motors:

20. Electric Motors (HT only)
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Electric Motors (HT only)

21. Electric Motors
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The “split ring commutator” changes the current every half rotation