Transcript Induced emf

Objectives

Apply the laws of magnetism and induced
emf.
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Faraday’s Law and Flux

The magnitude of induced emf (voltage) is
proportional to the rate of change of the magnetic flux
passing through the loop of area A. So what is
flux…..
 Magnetic
flux equals product of magnetic field
perpendicular to coil’s face and area of coil
 Flux
is Measured in T-m2 or webers where 1Wb = 1 T-m2.
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Magnetic flux – when B is
perpendicular to the face of the coil.
  BA
The change in magnetic flux is equal to the magnetic field, B and
the area, A. If either of these changes, the flux changes.
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Faraday’s Law

Factors affecting magnitude of emf (voltage):
 The
rate of change of B or rate of change of magnetic
flux.
 Time; more rapidly magnetic field changes, greater
induced emf
 Proportional to number of wire loops.

So what does this look like mathematically?
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Faraday’s Law of Induction –
 B
  N
t
•emf, ε, is equal to the negative of the number of loops of wire, N,
multiplied by the change in magnetic flux, ΔΦ divided by the
change in time Δt.
•The negative tells us the direction the emf acts and is Lenz’s
Law…..
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Lenz’s Law

Direction of induced emf gives rise to a current that
opposes the original change in flux.





In real words… a changing magnetic flux induces an emf and
that emf then creates a current that flows in such a way that it’s
magnetic field is in opposition to the original changing flux.
So if the flux increases the induced current will be in a direction
such that it’s magnetic field will be opposite in direction to the
increasing flux.
So if the flux decreases the induced current will be in a direction
such that it’s magnetic field will be in the same direction to the
decreasing flux.
This is natures way of trying to keep the flux constant.
Review Example 21-3
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Practice problem

1.
2.
What is the direction of the induced
current as the coil is reduced as shown?
Magnetic field is the dots coming out of page like the tips of arrows.
Red circle is the coil of wire.
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Practice problem answer

What is the direction of the induced current as
the coil is reduced as shown?
1. Is the flux increasing or decreasing? Decreasing because A of loop decreasing.
2. If flux is decreasing then induced current will move in a direction to create a
magnetic field in the same direction as the decreasing flux. (out of the page)
3. To get a magnetic field coming out of the page the current must be
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counterclockwise in the red wire.
Motional EMF

Another way to induce an emf.
 Produced
when flux from external magnetic
field flows through loop of changing crosssectional area.
 Induced current obeys Lenz’s law.
 See Equation 21-3 on page 627.
Motional EMF = -NBperpvl
 Review example 21-4.

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Flux and Electric Field

A changing magnetic flux produces an Electric
field. (we will talk about these in detail in Ch1617)
F qvB
E 
 vB
q
q
E =F/q (definition on electric field rom Ch16)
F = qvB (from Ch20 and don’t forget right hand rule
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Transformers
How does an electrical transformer work?
 AC only due to need for changing flux.

Vs N S

VP N P
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Motors and Generators
Research motors, generators, and
transformers.
 What do these things have to do with
magnetism and electricity?

 Generators
transform mechanical energy into
Electric energy.
 Motors transform electric energy into
mechanical energy.
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