1. Magnetism and Induction
NCEA AS 3.6
Text Chapters: 15,16,17

2. Electromagnetism
Fields are formed around current carrying wires

3. Solenoids
Fields are formed in solenoids or coils.

4. 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.

5. Induction
If the wire is connected to a circuit then current will flow. The direction of induced current is determined by a right hand rule.

6. 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)

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

8. 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
Opposing Force F=BIL
Direction of movement
Induced current I

9. Induction 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)

10. Induction
Induction can also occur if it is the magnetic field that is moved, rather than the wire.

11. Magnetic Flux
The magnetic field in a circuit is measured as magnetic flux Φ
Φ= BxA
B = mag field strength
A = area perpendicular to field
The unit for flux is the Weber Wb

12. Magnetic Flux
A useful analogy is using a net to catch whitebait in a stream….
If you don’t hold the net straight up and down, you don’t catch many whitebait!!

13. Faraday’s Law According to Faraday’s Law, V=BvL
The area of the loop in the field is zero….. L
Speed v

14. Faraday’s Law Some time t later, A has changed by ΔA = (vt x L)
ΔΦ = B x ΔA
ΔΦ = B x v x L x t
But V=BvL
So ΔΦ = V x t L A v

15. The negative sign is a reminder of Lenz’s Law
Faraday’s Law (again)
Another way to look at Faraday’s Law is that the induced voltage in a circuit is determined by the rate of change of flux
The negative sign is a reminder of Lenz’s Law

16. Generators
Rather than sliding a loop through a field, it is easier to spin it.
This is how a generator works

17. Generators
When the coil is horizontal, the induced current is maximum, as the coil is cutting across the field lines at right angles as it moves.

18. Generators
When the coil is vertical, the induced current is zero, as the coil is moving parallel to the magnetic field lines

19. Generators
If we start timing from when the coil is vertical, then at t=0, Φ= BxA N S B A

20. Generators
If the coil rotates with speed ω, then after time t the coil will have turned through angle θ = ωt
The flux will now be Φ= Bcosωt x A N S A B θ

21. Generators
Faraday’s Law says:
For a coil of N turns:

22. Generators
The formula for alternating generator voltage is often written as:
Where Vmax=BANω
This produces a voltage-time graph that looks like a sine curve
NB. Similarities to SHM!!

23. Generators
To generate A.C, slip rings are used…

24. Generators
To generate D.C, split rings are used.