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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

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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