Transcript Electricity
Electricity
Part 3: Magnetic fields, Faraday’s
Law, Electrical Generation
If a positive and a negative charge are sitting
next to each other, which of the following is true?
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A. The charges will
attract each other
B. The charges will
repel each other
C. The charges will
neither attract or
repel
D. None of the above
A material in which charges
move freely is a
A. Insulator
B. Conductor
C. Semiconductor
D. Convector
E. Radiator
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A solar car runs on a 100 V battery pack. If
the motor draws 10 A of current, how much
power is the battery supplying?
A. 10 W
B. 100 W
C. 1000 W
D. 10000 W
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A solar car runs on a 100 V battery pack. If
the motor draws 10 A of current, what is the
resistance of the motor?
A. 1 W
B. 10 W
C. 100 W
D. 1000 W
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Magnetic Fields
Similar to electric
fields, but some
differences.
Instead of charges,
poles called North
and South
Similar to +/- charges
Different in that N/S
poles always come as
a pair.
Like poles repel and unlike poles attract
each other.
Similar to like charges repel, unlike charges
attract
Magnetic field lines are always closed
loops
Notation: We usually use the symbol B to
represent magnetic fields
Hans Oersted
Until 1820, Electricity
and Magnetism were
considered to be
separate phenomena.
Oersted discovered
that a current carrying
wire (moving charges)
deflected a compass
needle, i.e. currents
create magnetic fields
Magnetic Force on a Moving
Charge
If charge is not
moving there is no
force.
If charge is
moving, Force is
to both B and v.
If v is || to B there
is no force.
Magnitude of the force
is F = qvB
v is the part of the
velocity to the B
field.
F = qvBsinq
F = qv × B
Use Right Hand Rule
to find the Direction of
the force.
Magnetic Force
A proton moves at 5.5 x 106 m/s at 60º to a
magnetic field of 2.5 T. What are the size and
direction of the force?
Force on proton moving at 5.5 x
106 m/s at 60º to 2.5 T B-field?
A. 1.7 x 1015 N
B. 8.8 x 10-19 N
C. 8.8 x 109 N
D. 1.9 x 10-12 N
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B-field straight down, proton moving from
left to right RHR gives direction of force as
A. Out of page
B. Into page
C. To right
D. Upward
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Magnetic Force
A proton moves at 5.5 x 106 m/s at 60º to a
magnetic field of 2.5 T. What are the size and
direction of the force?
F = qvBsinq
(1.602 x 10-19 C)(1.0 x 105 m/s)(5.5 x 10-5 T)(.866)
F = 1.9 x 10-12 N
Magnetic Force on Wires
Since a current in a
wire is moving
charges, they also
experience magnetic
forces.
If the wire is B then
the magnitude of the
force is
F ILB
L length of wire
Units of B
By definition
N
1 Tesla 1 T 1
Am
Application: Motors
Current flow in each
side of the wire loop
produces a force in
opposite directions
Causes loop to rotate.
Motional Potential
When a wire moves in a Bfield the electrons in the
wire are moving in the Bfield.
Results in a magnetic force
on the charge of F = qvB
In diagram, electrons will try
to move down, this leaves +
charges behind and creates
an E-field along the wire.
Process continues until the electric force
and the magnetic force balance each
other.
qE = qvB
E = vB
The voltage difference along the wire is
V = Ed = vBd
Example: Tether Experiment
Estimation of tether voltage
Orbital velocity of the space shuttle is
7600m/s (17,000 mph)
B = 510-5 T
d = 5000 m
What is the voltage generated along the
wire?
Voltage across tether
A. 4250 V
B. 19000 V
C. 1900 V
D. 425 V
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Estimation of tether voltage
Orbital velocity of the space shuttle is v =
7600m/s (17,000 mph)
B = 510-5 T
d = 5000 m
Voltage = vBd = 1900 V
It really works, but because B is so small,
you either have to go really fast or have a
really long wire.
Magnetic Flux
Flux is a measure of
how much magnetic
field passes through a
surface
= BA
Actually only want the
part of B that is
perpendicular to the
area.
More generally
= BAcosq
Faraday’s Law
You can induce a voltage in a loop of
wire by changing the magnetic flux
through the loop.
V
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Faraday’s Law
You can induce a voltage in a loop of
wire by changing the magnetic flux
through the loop.
Three way to change the flux
1. Change A (usually not practical.)
2. Change B (important for a lot of uses)
3. Change q (This is how we usually do it
for power generation.)
Generators
Basically a “backwards” motor.
Instead of running current through the
loop to get the shaft to rotate, rotate the
shaft to get electrical current.
This is what is done in essentially all
power plants. You run a heat
engine/water wheel/wind mill to turn the
shaft.