Download Lecture 14: Magnetism

Phy2005
Applied Physics II
Spring 2017
Announcements:
• Exam 1 answers, solutions posted on tests page
• Exam 1 grades will be posted soon on Canvas
• Exam 1 mean was 67%
AC vs. DC power
•The big advantage of AC current is that it is relatively easy to change
the voltage, using a device called a transformer.
Power companies use very high voltages to transmit power over
long distances.
•Say that you have a power plant that can produce 1 million watts of
power. One way to transmit that power would be to send 1 million
amps at 1 volt. Another way to transmit it would be to send 1 amp
at 1 million volts. Sending 1 amp requires only a thin wire, and not
much of the power is lost to heat during transmission. Sending
1 million amps would require a huge wire.
• So power companies use very high voltages for transmission
(such as 1 million volts), then drop it back down to lower voltages
for distribution (such as 1,000 volts), and finally down to 120 volts
inside the house for safety.
AC vs. DC power
Thomas Edison
Nikolai Tesla
Tesla championed alternating current, while Edison insisted that
it was too dangerous. The public debate between the two became
acrimonious, reflecting the high stakes. The only casualties in this
"war of currents" were the animals Edison publicly electrocuted
with Tesla's high voltage system to prove his point. The early
victims were dogs and cats, but Edison eventually electrocuted
an elephant named Topsy.
MAGNETISM
One of the oldest subjects in Physics: ancient Greeks (near the
city of Magnesia) and Chinese realized certain strange stones
(lodestones) attracted iron.
Compass invented in China 2nd century BC
Around 1600, William Gilbert proposed that the Earth itself is
a gigantic magnet.
For a long time, people knew only one source of magnetism (from
iron). In 1821, a Danish physicist, Oersted noticed that an
electrical wire carrying current made the nearby compass
reorient.  First clue of inter-relation between electricity and
Magnetism.
Ampere, Faraday established the nature of electricity and
magnetism (from their experimental observations).
N and S poles always come as a pair.
(N.B. charges do not have to come as pair.)
S
S
S
S
N
N
N
N
N
N
force-at-a-distance, non-contact force
There should be a type of field in space that generates magnetic force!!
Magnetic Field
N pole of compass needle aligns in the B-field direction.
B-field comes out of N-pole and enters into S-pole.
B-field comes out of N-pole and enters into S-pole.
Reminder: electric charges and fields
Permanent magnets are not the only source of B-field.
Electric current generates B-field.
I
X
B-field direction from current: “right-hand rule”.
B-field is stronger near the wire.
B
A wire placed in a perpendicular B-field experiences a sideways force!
B
I
F
FB = ILB
Magnetic field
[B] = [F/IL]
= Ns/Cm = Tesla
*1 Tesla = 104 gauss
Length of the section in B-field
force on a wire with I in B-field
force on a charge q in E-field
direction depends on I direction
direction depends on charge
FB = ILB
FE = qE
Ex A wire is running perpendicular to the direction of a
uniform magnetic field of 1.2 T. The wire experiences a force
coming out of the screen when it carries a1.2 A current .
What is the direction of the current?
B = 1.2 T
1.2 A
Ex A square wire loop is placed in a uniform magnetic field
(coming out of the screen) and current flows through the wire
as shown in the figure. Indicate the direction of magnetic force
acting on each side of the loop.
.
.
.
.
.
.
.
.
.
.
.
.
B
a
I
FB = ILB┴
FB = IL(Bsina)
B┴= Bsin(a)
B
a
I
Message: only component of magnetic
field perpendicular to I counts!
Ex The earth magnetic field in Colorado has a value of 0.59 G
and is directed at an angle of 67 below the horizontal. Find the
force on a 5 m-wire carrying 20 A current from E to W.
F
X
67 degree
S
N
S
I
N
F = ILB
= (20 A)(5 m)(5.9x10-5 T)
= 5.9 x 10-3 N
Note field and wire are perpendicular, so no cos(angle)!
Q1 A wire is placed in a uniform magnetic field of 4 kG
as shown in the figure. 1 A of current flows into the port A and
comes out of the port B. What is the magnetic and direction
force acting on the 1 m-long vertical portion of the wire?
B = 4 kG
1.
2.
3.
4.
5.
0.4 N to the right
0.4 N to the left
4 N into the screen
4 N out of the screen
0
A
B
ACADEMIC HONESTY
Each student is expected to hold himself/herself to a high standard
of academic honesty. Under the UF academic honesty policy.
Violations of this policy will be dealt with severely. There will be
no warnings or exceptions.
Q2 A wire is placed perpendicular to the direction of magnetic
field as shown in the figure. The current flows into the page
through the wire. Which arrow represents the correct magnetic
force direction on the wire?
B
1.
2.
3.
4.
A
B
C
D
B
D
X
C
A
Q3 What is the force acting on the 1 m-long wire placed in a
uniform magnetic field B when current I flows through it as
shown in the figure?
1.
2.
3.
4.
5.
BI
0
BItan(30)
BIcos(30)
0.5B
B
I
30