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.

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Magnetism and Induction

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Magnetism and induction

Magnetism and Induction

NCEA AS 3.6

Text Chapters: 15,16,17


Electromagnetism

Electromagnetism

  • Fields are formed around current carrying wires


Solenoids

Solenoids

  • Fields are formed in solenoids or coils.


Induction

Induction

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


Induction1

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

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)


Induction2

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


Induction3

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


Induction4

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)


Induction5

Induction

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


Magnetic flux

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 flux1

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!!


Faraday s law

Speed v

L

Faraday’s Law

  • According to Faraday’s Law, V=BvL

  • The area of the loop in the field is zero…..


Faraday s law1

v

A

L

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


Faraday s law again

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

Generators

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

  • This is how a generator works


Generators1

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.


Generators2

Generators

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


Generators3

B

A

N

S

Generators

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


Generators4

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


Generators5

Generators

  • Faraday’s Law says:

  • For a coil of N turns:


Generators6

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!!


Generators7

Generators

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


Generators8

Generators

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


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