# Faraday s Law (PowerPoint)

Document Sample

```					              Group Work
1. What are the directions of the forces on
the opposite charges moving with velocity
v through magnetic field B?

B        −       +

v
Moving conductors

through magnetic fields
CPS Question
What is the direction of
the Lorentz force on the   B
charges moving along
this wire?
A.         D.

B.         E.

C.         F.                  I
Current in a Field Feels a Force

B

F
I
Loop in a Field Has Torque

B

I      I
Torque Turns a Motor

current I
I
force F
I
B
Motion Creates a Potential

B                –

v
+   induced potential
Rotating Loop Creates Circuit

B

V
V
Rotating Loop Creates Circuit

B

V      V
Rotation Powers a Generator

Motion

through the field         v

induces a potential   V
V
which generates a                 v
B
current
that charges the
battery
Magnetic Induction

Transforming energy with
magnetism
Objective

• Explain the electric potential created by a
changing magnetic field.
What’s the Point?
• This is how electric current is harnessed to
do mechanical work.
• It’s another way to think about the Lorentz
force. ☺
Lorentz Force Reminder
F = qv  B

Source: Griffith, The Physics of Everyday Phenomena

F is directed out of the screen.
Magnetic Flux
• Field through a surface
F = BA
• Effectively the number of field lines
enclosed by a loop
Flux Change Creates Potential
The rate of magnetic flux change
through a loop equals the electric
potential induced around the loop.
DF
V=
Dt
Magnetic Flux

a     b      c    d      e

•   Flux changes as loop turns
•   At a, c, e: Greatest flux F, least change
•   At b, d: Zero flux, fastest flux change DF/Dt
Induced current direction
Lenz’s Law
When a changing magnetic flux
induces a current, the induced
current’s magnetic field opposes the
flux change.
Lenz’s Law Example
Induced current

a         b       c   d   e

• From a to b:
– flux decreases; DF into face of loop ←
– Loop current increases
– Induced field increases out of face of loop
Lenz’s Law Explains Generator
changing field

e
e
opposing field
B

needed I
Group Work
What is the
Lorentz force on
each side?
B   Which direction is
the flux change?
What current
would oppose it?

2. If the loop inside this magnetic field
expands, in what direction will the
induced current around it be?
How a Microphone Works

microphone
geophone
Flux inside loop
becomes greater
Wave-Powered Generator

Copyright__. Originally appeared in Smithsonian magazine.
How a Loudspeaker Works

loudspeaker        induced motion
of magnet

current-
current
generated field
attracts or
repels magnet
Reading for Next Time
• Electromagnetic Waves

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 28 posted: 5/14/2011 language: English pages: 25