Electromagnetic

Induction

Faraday’s Law

Induced Emf

A magnet entering a wire causes current to move with in the wires I = Emf / R

The induced current increases as the number of coils and length of coils increases (N*L)

The induced current increases as the velocity of the magnet increases (v)

The induced current increases as the magnetic field increases (B)

I = N*L*vB / R

Induced Emf

Which way will the current flow?

What direction is the magnetic field changing?Into the coil

The direction of current will be determined by the induced magnetic field produced by the induced current.

The induced magnetic field produced by the induced current will oppose The change in magnetic field (B)

This statement is called Lenz’s law

Induced Emf

The induced magnetic field produced by the induced current will oppose

The change in magnetic field (B)

This statement is called Lenz’s law

a) Magnet into wire• B field is increasing to the left• So the current flowing in the wire must

induce a magnetic field to the right• Current is counter clockwise

Induced Emf

The induced magnetic field produced by the induced current will oppose

The change in magnetic field (B)

This statement is called Lenz’s law

a) Magnet out of the wire• Decreasing to the right • Induced magnetic field must be to the

left• Current is clockwise

Magnitude of Induced Emf

• Emf = - Change magnetic flux

time

Magnetic flux = equals the amount of B Field line passing through a given area A

The more field lines the greater the flux The greater the area the greater the flux

Chapter 19 and 20 Review

Magnetic properties

• Like poles repel

• Opposite poles attract

• Field lines are strongest at poles

• Field goes from south to North inside a magnet

• Field goes away from North and towards south, outside magnet

• Magnets always have two poles

Magnetic properties continued

• The closer the lines are together the stronger the field

• Magnetic domains are microscopic magnetic field lines caused from the movement of electrons

• Domains line up when external magnetic field is present

• Magnetic field lines per area is called magnetic flux

Moving charges induce magnetic fields

• Current moving through a straight wire induces a magnetic field in a direction according the first right hand rule

IX B

Current moving in a loop

• When current flows around a loop a magnetic field is induced according to the second right hand rule

I B B X i

Magnetic Force FB

• A Force is exerted on a wire when a current carrying wire is placed inside a external magnetic field in a direction according to the third right hand rule

North

B

F i

South

Magnitude of force

• F = BIL B magnetic field I Current L Length of wire

• F = qvBq charge v velocity B Magnetic field

Electromagnetic Induction

• A wire moving inside a magnetic field causes charges to flow in the wire

• The EMF or voltage is induced Emf = IR

EMF electric motive force

• EMF = B x v x L B Magnetic field increases

V Motion of the wire increases

L Length of the wire increases