10a. Magnetic Induction

8/4/2019 10a. Magnetic Induction

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MAGNETIC|

INDUCTION


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Magnetic induction

INTRODUCTION

During the 1830s, several pioneeringexperiments with magnetically inducedemf were carried out by Michael

Faraday (in England) and JosephHenry (in the US).

They discovered that changing magnetic fieldinduces current.

TERMINOLOGY INDUCE to produce (an electric current, electric

charge, or magnetic change) without direct

contact.


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Magnetic Flux Flux is the number of field

lines passing through anarea.

fF cosBAABm ==rr

== S nSm dABdAnB r

F

UNITS: 1Wb(Weber)=1Tm2

If the surface is a plane

with area A, and B isconstant in magnitude anddirection over the surfaceand makes an angle f

fF cosNBAm=


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Gauss law for magnetism

fF cosBAABm

==rr

0,

=== dABdAnB nnetmr

F

REMARKS Electric monopoles exists, but magnetic monopoles do

not.

The fundamental unit of magnetism is the magneticdipole.

Magnetic field lines are always continuous, they neverhave end points.


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INDUCTION EXPERIMENTS

Induced emf

If the magnetic flux through an area bounded by a circuit ischanged by any means, an emf equal in magnitude to the rateof change of the flux is induced in the circuit.

We usually detect the emf by observing a current in the circuit.


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Induced emf and Faradays law The common element in all induction effect is changing

magnetic fluxthrough a circuit.

The magnetic flux can be changed in many ways:

The current producing the magnetic field may beincreased or decreased.

The area of the circuit in a fixed magnetic field maybe increased or decreased.

Changing the orientation of the circuit with respect tothe magnetic field.

Magnets may be moved toward or away from thecircuit

AdBBdAABddm

rrrrrr

+==F


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FARADAYS LAW OF INDUCTION The induced emf in a circuit equals the negative time

rate of change of magnetic flux through the circuit.

dt

dm

F

e -=

dt

d

Nm

F

e -=

DIRECTION OF INDUCED EMF

Define a positive direction for the vector are A.

The directions of A and the B determine the sign of themagnetic flux Fmand its rate of change dFm/dt.

Curl the fingers of your right hand around A with yourright thumb in the direction of A. An emf or current in the

circuit that has the same direction as your curled fingersis positive.

Induced emf and Faradays law


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Electromagnetic induction

E does NOT depend on the resistance of the loop

Iinduced depends on the resistance of the loop

Iinduced

=e

R


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Electromagnetic induction

The () sign is related to the polarity ofe

related to right-hand rule and Lenzs law

To know the polarity of e, it is important to know whetherFB is increasing or decreasing


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Lenzs Law is a convenient alternative method fordetermining the sign or direction of an induced current oremf or the direction of the associated non-electrostaticfield.

Lenzs Law states that:

The direction of any magnetic induction effect is suchas to oppose the cause producing it.

Heinrich Friedrich Emil Lenz (1804-1865)

Russian physicist

1833: formulated Lenzs law

Lenzs law

The induced current tends to keep the original magnetic flux through the loop fro


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Lenzs lawa simple principle

If FBincreasese must create an induced magneticfield to decreaseFB !!!

If FBdecreasese must create an induced magneticfield to increaseFB !!!

Right-hand rule must be utilized.


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Test run on Lenzs lawSSS

NNN

SSS

NNN

NNN

SSS

NNN

SSS

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10a. Magnetic Induction