lec24 induction

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PHYS120003.06

General Physics

Prof. Wei-Tao LiuPhysics department

Fall 2012

2012/12/28


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Lecture 24: Outline

Induction (Chapter 22) Induced emf and magnetic flux (Chapter 23.1)

Lenzs law (Chapter 22.2)

Motional emf (Chapter 22.3) Eddy current (Chapter 22.4)

Electric generator (Chapter 22.5)

Transformers (Chapter 22.7)

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Induction

the production of an electromotive force (emf)

hence an electric current across a conductor

moving through a magnetic

field.

Emfs must be induced

by a changing magnetic

field.

The change in magnetic

field can either be its

direction or magnitude.

Current only

flows when

the magnetic

bar is inserted

or retracted.


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Induction (contd)

Current only flows when the

switch is switched on or off.

Current only flows

when the metal

wheel spins.

Current only flows when

the magnetic field

strength changes.


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Magnetic flux

Definition: product between an

area and the component of themagnetic fieldperpendicular

to it.

With one magnetic field line denoting the field intensity

of 1 Tesla/m

2

, the flux equals the numberof magneticfield lines enclosed by the area.

The flux contains information of both the magnitude ( )

and direction () of the magnetic field.

=

cos


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Lenzs Law

The emf in a conductor induced by a changing

magnetic field:

Again, the emf is

induced only by a

changing magnetic

field.

The emf is

proportional to the

rate at which the

magneticfluxvaries.

=


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Lenzs Law(contd)

The - sign: the

induced emfalways conteracts

with the change

of magnetic field

A conductor

always tries to

maintain the

magnetic flux in it.

A negative feed-back process

a manifestation of

the conservation of energy.


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Lenzs Law(contd)

Example 22.1, pg. 566

Determine the magnetic flux in the loop of interest.

Determine how the magnetic flux changes.

Determine the direction of the induced magnetic field

that opposes the previous change.

Determine the induced emf responsible for the induced

field.


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Motional emf

Emf induced due

to the motionof the rod is:

=

=

= A force of =

is caused in the

opposite direction

of = = conservation

of energy!

Example 22.2,

pg. 568


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Motional emf

For an open loop, the emf can still be induced

without a flowing current. The emf is induced

in a conductor by

cutting the field

lines. E.g., a tethered

Satellite in the Earth

magnetic field.

20 km tether


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Eddy currents

The current loop caused by the motional emf.

Magnetic damping

Metal detections


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Electric generators

Electric generators induce an emf by rotating coils

in magnetic fields. Other forms of

energy, such as the

hydropower, are

used to rotate thecoil, which then

transforms the input

energy to electric

energy (the emf).

Overall, no energyis created from nothing, the total energy is still

conserved.


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Electric generators (contd)

AC generators

= 2 = 2sin = 2sin

=

= sin

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Electric generators (contd)

DC generators


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Transformers

The output

voltage fromthe secondary

coil:

=

=

With 100%

efficiency: = =

Examples 22.5, 22.6, pgs. 574-575

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HW #9 (Not due)

Induction:

Conceptual questions: 22.1, 22.3, 22.4, 22.8,

22.11

Problems: 22.1, 22.7, 22.17, 22.26, 22.39

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