Electromagnetic

Induction

Physics

Mrs. Coyle

Part I

• Faraday’s Law of Induction

The Flying Ring

• http://teachertube.com/viewVideo.php?video_id=125587

How can current be induced in a wire?

• Michael Faraday

• English professor

• 1791- 1867

• When the magnetic field that is “felt” by the wire is changed, a current is induced in the wire.

http://itp.nyu.edu/~ndl5/electricity/images/faraday.gif

Note

• When a wire is moved in a constant magnetic field, the wire “feels” a changed magnetic field and current is induced.

Electromagnetic Induction in a Generator

• http://www.teachersdomain.org/asset/ate10_int_generating/

Faraday's Law of Induction

V= - / t

When the magnetic flux, is changed, a current with a voltage V, is induced in a wire loop.

Magnetic Flux,

• The number of magnetic (flux) field lines which pass through a given cross-

sectional area A.

Which wire loop has the largest magnetic flux?

Which wire , 1,2 or 3 has the largest magnetic flux?

Note

• Voltage is also called electromotive force (emf), E.

How can current be induced in a loop that is in a magnetic field?

• By changing the magnetic flux over time.

The magnetic flux can be changed by:

1. Changing the orientation of the wire loop in which the current is to be induced (movement).

2. Changing the strength of the magnetic field (change current of wire that causes the field).

3. Changing the area of the coil. 

1. Movement of the wire (Motional EMF)

V = - B Lv sinL: length of the wire

angle between v and B

L

2.Electromagnetic Induction

by changing the current causing the B-field (thus changing the B-field).

• http://higheredbcs.wiley.com/legacy/college/halliday/0471320005/simulations6e/index.htm?newwindow=true

Question

• In the “flying ring” demo, how was the magnetic field “felt” by the copper ring changed?

• Answer: AC current

3.When B is constant and A (area) is

changing.

http://www.walter-fendt.de/ph14e/generator_e.htm

Generator

Generators at Hoover Dam

http://nrgfuture.org/Hoover_Dam_generators.jpg

3 minute video on Hoover Dam

• http://www.teachersdomain.org/resource/phy03.sci.phys.energy.hooverelec/

Part II

• Lenz’s Law

What is the direction of the induced current?

• Lenz’s Law: The induced current has a magnetic field that opposes the original change in flux.

• Indicated by minus (–) sign in Faraday’s equation.

In which direction is the current induced in the loop?

Answer: Clockwise, because as the bar moves to the right, more field lines are in the loop. The current should act in the (opposite) direction to weaken the magnetic flux (field).

Example: Change in Area

• The new smaller area contains less field lines (in the area), so the current will act in direction to strengthen the magnetic field, so the current will be induced clockwise.

What is the direction of the induced current?

Before After

Applet –Change in Area

• http://dev.physicslab.org/asp/applets/javaphysmath/java/indcur/default.asp

Question During which intervals will voltage be

induced and how is the current direction in the intervals related?

http://www.walter-fendt.de/ph14e/generator_e.htm

Generator