Electromagnetic Induction

and Faradays LawRipon High School

AP Physics 2012-2013

Stuff we’ll discuss in this unit: • How to induce and emf using a changing magnetic field

• Faradays Law of Induction

• Lenz’s Law

• What happens to a conductor moving through a uniform magnetic field

General Information/Questions

• What we know about electricity and magnetism:• An electric current produces a magnetic field• A magnetic field exerts a force on an electric current or

moving electric charge

• QUESTION: If an electric current produces a magnetic field, is it possible that a magnetic field can produce an electric current?

Induced Electromotive Force• Faraday’s Experiment

• Battery• Switch• Iron Ring• Wire• Galvanometer

Induced Electromotive Force• Faraday discovered through his experiment that a

constant current could not induce a current on the other side of the iron ring.

• He did, however, notice that a current was induced on the other side whenever he opened or closed the switch.

• CONCLUSION: • A constant magnetic field does not produce a current in a conductor• A changing magnetic field does produce a current in a conductor

• Induced Current: a current that is induced in a conductor due to a changing magnetic field

Faraday’s Law of Induction

• Further experiments concerning the magnitude of the induced emf led to the following conclusions:• The more rapidly the magnetic field changes, the

greater the induced emf in a loop of wire• Other things the induced emf in a loop of wire

depends upon• The loop’s area• The loop’s angle to the magnetic field

Faraday’s Law of Induction• Magnetic Flux:

• The measure of quantity of magnetism, taking into account the strength and extent of the magnetic field

• Units: Weber [Wb] = [T·m2] Tesla·meter2

• Represented by the Greek letter Phi: Φ• Think of magnetic flux as proportional to the

amount of magnetic field lines passing through a certain area enclosed by the loop

Faraday’s Law of Induction

Faraday’s Law of Induction:

temf B

• ΔΦ – change in flux

• Δt – change in time

Faraday’s Law of Induction

• If the circuit contains a number of loops closely wrapped together, the emfs induced in each wire simply add together (think of a bunch of batteries in series)

tNemf B

• N – number of loops

• ΔΦ – change in flux

• Δt – change in time

Lenz’s Law

• The minus sign prevalent in Faraday’s Law reminds us the direction the induced emfs act, summed up by Lenz’s Law:

A current produced by an induced emf moves in a direction so that its magnetic field opposes the original

change in flux

Lenz’s Law• TWO MAGNETIC FIELDS EXIST!

• The changing magnetic field or flux that induces the current

• The magnetic field produced by the induced current

• Important Rule: The second magnetic field opposes the change in the first

Induced emf in a Moving Conductor• Another way to induce an emf in a conductor is to

move it through a magnetic field.

• For our purposes, you must assume:• The conductor has some length• The conductor has some velocity • The magnetic field is uniform• All three are perpendicular to one another

emf Induced in a Moving Conductor• In order to calculate the induced emf in a conductor,

the following equation applies:

Blvemf • B – magnetic field

• l – length of conductor

• v – velocity of conductor