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Magnetism and Induction. NCEA AS 3.6 Text Chapters: 15,16,17. Electromagnetism. Fields are formed around current carrying wires. Solenoids. Fields are formed in solenoids or coils. Induction.

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Magnetism and Induction

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## Magnetism and Induction

NCEA AS 3.6

Text Chapters: 15,16,17

### Electromagnetism

• Fields are formed around current carrying wires

### Solenoids

• Fields are formed in solenoids or coils.

### Induction

• If a wire is moved through a magnetic field then a voltage can be induced across the ends the wire.

### 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.

### 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)

### 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

Induced current I

Opposing Force F=BIL

Direction of movement

### Induction

• 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

### 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)

### Induction

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

### 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

### 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!!

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

### 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

### Generators

• Rather than sliding a loop through a field, it is easier to spin it.

• This is how a generator works

### 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.

### Generators

• 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

### Generators

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

B

θ

N

S

A

### 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

### Generators

• Faraday’s Law says:

• For a coil of N turns:

### 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!!

### Generators

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

### Generators

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