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Magnetism and InductionPowerPoint Presentation

Magnetism and Induction

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Magnetism and Induction

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Magnetism and Induction

NCEA AS 3.6

Text Chapters: 15,16,17

- Fields are formed around current carrying wires

- Fields are formed in solenoids or coils.

- 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.

- v=direction of wire movement
- B=direction of magnetic field lines
- F= force on a positive charge (ie direction of current flow)

- 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

Induced current I

Opposing Force F=BIL

Direction of movement

- 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)

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

- 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

- 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!!

Speed v

L

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

v

A

L

- 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

- 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

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

- 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.

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

B

A

N

S

- If we start timing from when the coil is vertical, then at t=0, Φ= BxA

B

θ

N

S

A

- 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

- Faraday’s Law says:
- For a coil of N turns:

- 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!!

- To generate A.C, slip rings are used…

- To generate D.C, split rings are used.