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= - D F / Dt When the magnetic flux, F, is changed, a current with a voltage V, is induced in a wire loop.

Magnetic Flux, F 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?

Magnetic Flux, F: The number of magnetic (flux) field lines which pass through a given cross-sectional area A Units: F webers B Tesla A area m2 angle formed between B and the normal to the loop (area vector A) The area vector A is perpendicular to the surface A and has a magnitude equal to the area A.

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: Changing the orientation of the wire loop in which the current is to be induced (movement). Changing the strength of the magnetic field (change current of wire that causes the field). Changing the area of the coil. 

1. Movement of the wire (Motional EMF) V = - B Lv sinq L: length of the wire q: angle between v and B

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.

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

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 What is the direction of the induced current? Before After 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.

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?

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