Transcript Motors and Generators_ppt_RevW10

Physics 106 Lesson #21
Magnetism:
Motors and Generators
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review: The Magnetic Force on a Wire
•The force on a wire
carrying current is
perpendicular to both
the wire and the
magnetic field
•The magnitude of the
force is the product of
the current, the wire
length and the
component of the field
perpendicular to the wire
•The force on a wire
carrying current parallel
to the magnetic field is
therefore zero!
B
Fingers point in the
direction of the current
flowing in the wire
F
Right Hand
IL
F  ILB for B to the wire
Motors: Exploiting the Magnetic
Force on Current-Carrying Wires
• A current is established in a loop by a
battery.
• When the current-carrying loop is placed in a
magnetic field the field exerts a torque on the
loop causing it to rotate.
Generating Electric Power:
Faraday’s Discovery
 Changing magnetic fields
induce voltages in loops and
therefore drive currents!
Michael Faraday 1791-1867
An Experiment: A Changing
Magnetic Flux Induces a Voltage
• Faraday demonstrated that a
voltage was induced if the
magnetic flux through a loop
changed with time
• Qualitatively, magnetic flux
is the number of field lines
passing through the loop
• Move a magnet near a loop
or a loop near a magnet and
you can generate electricity!
• Faraday Induction turned on
the lights for all mankind!
Demonstration
Faraday Induction:
Basic Observations
1. Constant magnetic fluxes induce no voltage.
2. The magnitude of the induced voltage is
proportional to how fast the magnetic flux is
changing with time.
3. When there are many loops, the total induced
voltage increases proportional to the number
of loops.
4. The induced voltage is also called an
“Electromotive Force” or EMF
Quantifying Induced Voltage
(EMF) With Magnetic Flux
N Loops
• Define Magnetic Flux Φ as the product
of the area of the loop and the
component of the magnetic field
perpendicular to the loop.
• Φ is quantitatively equivalent to the
number of field lines passing through
the loop.
B
  B A

EMF   N
t
How to Generate EMF
N Loops
To change the magnetic flux
with time you can:
1) Change the magnetic field with time.
2) Change the loop area with time.
3) Change the angle between the loop
and the field with time (spin the loop).
4) Any combination of 1 – 3.
The Alternating Current (ac) Generator
Rotating a coil of
wire in a magnetic
field at a constant
rate produces
alternating current
that varies with time
as a sine function