Physics 104 Exam 1

1
Physics 104 Exam 1

• Thursday, February 19 from 545 - 700 PM.
• Sections 302, 304, 305, 306, 307, 310, 313, 314,
  316, 317, 321, 325 (TAs Borman, Darwish, Guha,
  Abhyankar, Straight) meet in 125 Ag Hall
• Sections 301, 303, 311, 315, 320, 322 (TAs
  Bender, Glatzmaier, Rao) meet in B130 Van Vleck
• Sections 309, 312, 318, 319, 323, 324, 326 (TAs
  Burns, Makwana, Suryanarayanan) meet in 2103
  Chamberlin
• All alt exams are in 3320 Chamberlin
• Material from Chapters 15 -18 inclusive
• One page of notes (8.5 x 11) allowed
• 20 multiple choice questions plus test code
• Scantron will be used - bring 2 HB pencils
  calculator
• You must know your section number (301 - 330)
  fill it in on the test

2
Currents and Magnetism

• (Last lecture) Moving charge in magnetic field
  experiences Lorentz Force F qvB sin(?)
• (Chapter 17) A current is a stream of moving
  charge inside a conductor
• (Today) Forces on moving charges (current) in a
  magnetic field
• (Today) Moving charge (current) creates a
  magnetic field Amperes Law

2
3
Force on a Current

• F q v B sin(q)
• Out of the page (RHR)

q

• Current InqvA
• F q v B sin(q) (nAL)
• I L B sin(q)
• Out of page

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4
A rectangular loop of wire is carrying current as
shown. There is a uniform magnetic field parallel
to the sides ab and cd.
Preflight 1
c
d
I
Here ? 0.
b
a
What is the direction of the force on section a-b?
force is zero out of the page into the page
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5
A rectangular loop of wire is carrying current as
shown. There is a uniform magnetic field parallel
to the sides ab and cd.
Preflight 2
What is the direction of the force on section b-c
of the wire?
force is zero out of the page into the page
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6
Force on c-d is Zero!
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Preflight 3
c
d
Force on a-dout of the page.
I
b
a
Net force on loop is zero out of the
page into the page
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Preflight 4
c
d
I
b
a
Will the loop move? Yes No
But the torque is not zero!
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W
L
We have found the direction of F on each section
of the loop. How about the magnitude?

On b-c and a-d sections F IBW
Torque on loop is t (L/2) F sin(f) (L/2)F
sin(f) ILWB
sin(f).
The area of the loop is A WL
? torque is t I A B sin(f)
q90º
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Torque on Rotated Loop

• What is the torque on the loop below?
• t lt IAB
• t IAB
• t gt IAB

x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x
f 0
t 0
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Torque on Rotated Loop
normal
F
f
d
B
a
c
Magnitude t I A B sinf
b
F
Direction
The torque tries to line up the normal with B!
Even if the loop is not rectangular, as long as
it is flat t I A B sinf.
N
of turns in coil
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Electric Motor
Converts electric energy to rotational mechanical
energy. The torque is rectified by switching the
direction of current as the loop rotates.
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Torque Question
1
2
Compare the torque on loop 1 and 2 which have
identical area, and current.
Area points out of page for both! f 90 degrees
1) t1 gt t2 2) t1 t2 3) t1 lt t2
t I A B sinf
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Currents Create B fields
Amperes Law
Magnitude
Unit Tesla (T)
Permeability of free space
r distance from wire
Right-Hand Rule - 2 Thumb on I Fingers
point to position Palm gives direction of B
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Amperes circuital law
Magnitude

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v
Preflight 6
v


b)
a)
I
r
r
A long straight wire is carrying current from
left to right. Near the wire is a charge q with
velocity v
Compare magnitude of magnetic force on q in (a)
vs. (b)
a) has the larger force
b) has the larger force
c) force is the same for (a) and (b)
v and B are normal in both cases sinq 1
F has different directions
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Adding Magnetic Fields
Two long wires carry opposite current
B
E
x
x
x
What is the direction of the magnetic field
above, and midway between the two wires carrying
current?
1) Left 2) Right 3) Up 4) Down 5) Zero
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Force between wires carrying current
I towards us

Conclusion Currents in same direction attract!
Conclusion Currents in opposite direction repel!
Note this is different from the Coulomb force
between like or unlike charges.
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Electric and Magnetic Field
Electric Magnetic Source Charges Moving
Charges Act on Charges Moving
Charges Magnitude FEq F q v B
sin(q) Direction Parallel E Perpendicular
to v,B
Direction
Opposites Charges Attract Currents Repel
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What is the direction of the force on the top
wire, due to the two below?
1) Left 2) Right 3) Up 4) Down 5) Zero
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Solenoids B Direction
Preflight 8

• What is the direction of the magnetic field
  produced by the solenoid?
• into the right end of the solenoid.
• out of the right end of the solenoid.

Direction of Field inside of solenoid Thumb
direction of I, fingers point toward center,
palm gives direction of B.
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Solenoids B Magnitude
Magnitude of Field inside of solenoid Bm0 n
I
n is the number of turns of wire/meter on
solenoid. m0 4p x10-7 T m /A
Note N is the total number of turns, n N / L
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Magnetic field of a solenoid
Magnitude
B L m0 N I m0n L I
B
L
Bm0 n I
Solenoids are like parallel plate capacitors for
currents
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What is the net force between the two solenoids?
a)Attractive b) Zero b)Repulsive
Look at field lines, opposites attract. Look at
currents, same direction attract.
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Magnetic Domains
Orbits of electrons about nuclei
Intrinsic spin of electrons (more important
effect)
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