1. Electro-magnetism
NCEA AS2.6
Text Chapters:

2. Magnetism Permanent or ferro-magnetism
Earths field shape, causing by currents in core
Permanent or ferro-magnetism
Like poles repel, unlike poles attract
Magnetic field strength B measured in Tesla T

3. Magnetic Fields Magnetic fields can be represented by field lines
Closer together=stronger field
Point from N to S

4. Magnetism Electromagnetism. Caused by moving electric charges.
B=kI/d (k=2x10-7NA-2)
Demo with “wire” and charm compasses
Coils can have an iron core inserted into them and become electromagnets, very strong, able to be turned on and off.
Uses, MRI scanners, scrap metal yards, relay switches
Demo with electric bell
Right hand rules for wires and solenoids
Practice questions 10A
Problem sheet
Electromagnetism.
Caused by moving electric charges.
The earth has a magnetic field around it because of iron atoms spinning around the core.

5. Electromagnetism
Fields are formed around current carrying wires

6. Right Hand Curl Rule Thumb = points in direction of current flow
Fingers curl in direction of field lines

7. Magnetic Force Two parallel current carrying wires F=kI1I2L/d
Transmission Lines
Qu 10D No.3,4
Two parallel current carrying wires
F=kI1I2L/d
Current flowing same direction in both – wires are attracted
Current flowing opposite directions – wires repel each other

8. Solenoids
Fields are formed in solenoids or coils.

9. Right Hand Curl Rule
Fingers curl in direction of current flow around the coil
Thumb = points towards the north pole

10. Electromagnets Electromagnets have a range of uses:
Attached to cranes in scrap metal yards
Electric bells
Relay switches
More useful than permanent magnets as they are stronger and can be switched off

11. Magnetic force
TV tubes
Detection of radioactive particles
Qu 10C
Force on moving charges - a charged particle moving in a magnetic field will experience a force
F=Bqv (sinθ)
B=mag field strength
q= charge
v=velocity
(This formula applies to a small charge moving at right angles to magnetic field lines – if not right angles multiply by sinθ) +Q v F B

12. Right Hand Slap Rule For a positive charge moving in a magnetic field:
Thumb = direction of movement
Fingers = direction of magnetic field
Palm = direction of force acting on charge
Right hand rule gives direction for positive charge – negative
must be reversed

13. Force on Moving Charge
This principle can be used to deflect the electron beam in a TV set to make it scan across the screen

14. Force on Moving Charge
It can also be used to determine the mass of unknown substances in a device called a “mass spectrometer”

15. Force on Current-carrying Wire
Demo with wire in field
Diagram showing the addition of fields
Current balances to measure field strength.
Qu 10B
A wire carrying a current will experience a force when placed into a magnetic field
F=BIL (sinq)
Applies to current flowing in a wire running at right angles to magnetic field lines. (if not 90°, then multiply by sinq)
Direction (for conventional current) given by right hand rule.

16. Right Hand Slap Rule For a wire carrying current in a magnetic field:
Thumb = direction of current
Fingers = direction of field lines
Palm = direction of force on wire

17. Magnetic Force
This principle can be used in magneto-hydro propulsion units…

18. Magnetic Force
And in loud-speakers...

19. Electric Motors
Qu 10D No. 1, 2,
Handout on motors
Oht
Demo with ruler and boxes
Split rings/commutator
If magnetic force acts on opposite sides of a current carrying coil which is mounted on an axis, a torque is produced which makes the coil spin

20. Electric Motors
This idea is the basis for all devices that run by an electric motor.

21. Induction
Demo with magnet, coil and galvanometer
If a wire is moved through a magnetic field then a voltage can be induced across the ends the wire.
If the wire is connected to a circuit then current will flow. The direction of induced current is determined by a right hand rule.

22. Right Hand Slap Rule v=direction of wire movement
B=direction of magnetic field lines
F= force on a positive charge (ie direction of current flow)

23. Induction The size of this induced voltage is given by: V=BvL
(B=mag field strength, v=velocity of movement, L=length of wire in field)
This is known as Faraday’s Law

24. Induction
Prevents violation of cons of energy law
Qu 11A
How voltage is induced in terms of forces on charges
Qu 11B
The direction of the induced current is such that it creates an opposing force on the motion that is causing it.
This is known as Lenz’s Law
Induced voltage/current can be made larger if:
The mag field is stronger
The wire is longer
The movement is faster
(Solenoid has an iron core)

25. Induction
Induction is the principle behind the microphone

26. Induction
And the dynamo..

27. Generators
Hand out on generators
OHT of dynamo
If a coil is spun around inside a magnetic field then current can be induced that can be used to operate devices that run on electricity
This is called generation
Both DC and AC can be generated depending on whether slip rings or split rings are used

28. Transformers These consist of 2 coils wound close to each other.
Changing the current in one coil makes the field around it change. This changing field induces current in a nearby coil.

29. Transformers
The ratio of the windings determines how much voltage/current is induced
The voltage can be calculated using:
N=number of turns
V=Voltage
P=primary coil
S=secondary coil

30. Transformers No transformer is 100% efficient, but assuming it was:
Power in = Power out

31. Transformers 3 Types: Step up : Vs > Vp Step down : Vs < Vp
Isolating (a safety device) : Vs = Vp