Physics 212 Lecture 13, Slide 1

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Physics 212 Lecture 13
Torques
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If Luke Skywalker and Princess Leia's mother died
immediately after giving birth to them, how does
Leia have memories of her mother? What would
happen if you made the jump to light speed and
passed through an asteroid from a destroyed
planet, such as Alderan? How can blasters be so
inaccurate if they are concentrated photons of
coherent light? How much light would be needed in
the form of a laser blast to damage a Tie
Fighter? What is the magnitude of the magnetic
field generated by Luke's use of the Force to
lift his X-Wing from the swamp on Dagobah? How
powerful of a laser would be needed in order to
blow up Alderan? How strong of what type of field
would be required in order to bend light back on
itself to create a Lightsaber? Why do Lightsabers
not pass through each other if they are made of
light? Would the Force be a magnetic, electric,
gravitational, or some other type of field? What
is the potential difference (Voltage) of the
Emperor's Force Lightning? How many G's are
experienced by the Millennium Falcon when it
jumps to light speed? What would be the advantage
of shaping a Wookie laser weapon in the shape of
a crossbow? What is the momentum of a blaster
shot? How many blaster shots could one get from
an AA battery? How many batteries at what voltage
would be needed to power a blaster? A Lightsaber?
An X-Wing? An Imperial Star Destroyer? What type
of field is used to protect the spacecraft? Why
are the pilots of the X-Wings not killed from
solar radiation? What force is needed in order to
accelerate the Millennium Falcon to light speed
in 7 seconds? How is the giant worm capable of
living in the asteroid? Why is Mace Windu's
Lightsaber purple instead of blue or green? Would
a purple Lightsaber be stronger since purple is a
higher wavelength than red, blue, and green? What
about Plo Koon's yellow Lightsaber? Can the color
be altered by changing the voltage sent to the
Lightsaber, since Plo Koon's is blue later in the
series? What causes the photon bombs used by
Y-Wings to have a parabolic arch when fired? Do
they have enough mass to be affected by the
gravitational field of the ships and surrounding
planets? If a ship in light speed where to pass
through a black hole, what would happen? Would
they be stopped by the black hole or carry enough
momentum to pass through it? Would they gain a
slingshot effect if they were far enough out and
accelerate way past the speed of light as they
approached the black hole? How can a Jawa fire a
directed EMP without affecting their electronics?
What force is needed in order to make a Force
jump (30 feet)? How can ships communicate if
they are moving past the speed of light? Why are
all holograms blue? Can they be in full color
like TV? If the Old Republic and the Empire
armies are made of clones, why are certain
soldiers chosen as sharpshooters and others as
engineers? If a ship empties its garbage into
space, where does it retrieve resources? How can
the X-Wing pilots hear the sounds of other
fighters near them? How can Tatooine have two
stars that it orbits? How can a city planet such
as Coruscant survive? Can someone from the Star
Wars galaxy reach another galaxy or is that too
far? What can limit a ships reach? How can a
tractor beam work? What allows one to pull a ship
into an Imperial Star Destroyers hanger? Can a
well-placed, single photon torpedo actually
destroy an entire Death Star? How can a photon
torpedo make a 90 degree turn? May the force be
with you! Unless you don't post this in the
lecture
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Finding the directions of the different
quantities using the right hand rule and their
dependence on the angle of orientation
Just keep doing what you're doing now. It works
well. I like the solved problems.
Im having trouble understanding this last set of
questions regarding magnetic torque.
potential energies and torques
How to build a motor using this principle! We are
engineers!
This pre-lecture seemed to really make sense.
That being said, I have probably jinxed myself.
Potential energy and work....I always mess up the
signs!!!! Who cares about positives and
negatives. Isn't it better to always be positive
anyway? I'm sure that mentality will get you
further in life than being negative all the time.
Therefore, physics is obviously bad since it
makes us think negatively...see the correlation?
How unfair it is that i never had the chance to
see led zeppelin live...
The French TGV (Train à Grande Vitesse) is the
fastest conventional train in the world, using
powered metal wheels riding on metal rails. In
April 2007, the TGV broke its own 1990 record
with a new speed of 574.8 km/h (357.18 mph) under
test conditions with a shortened train (two power
cars and three passenger cars).
Lets discuss the nutritional value of corn. Did
you know that while corn is full of yummy
vitamins necessary to life, it is deficient in B
vitamins? It is actually not possible to survive
on a staple crop of corn alone, much in the same
way it is not possible to survive on rabbit meat
alone (except in the case of rabbits, their meat
is 90 lean and is therefore deficient in fat).
Hair gel. Barbershops sell hair gel. Barbershop
quartets dress in stripes. Stars and Stripes.
John Phillip Sousa wrote songs. Sousaphones are
like big tubas. Tubas collect spit. Spit contains
urea. Urea is an important biological compound.
Biology is the study of living things. Living
things can be big or small. Big things have lots
of mass. Mass is proportional to energy. Emc2.
Einstein had hair that sometimes stuck up. To
make hair stick up, you use hair gel.
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How confident are you in your understanding of
the concepts presented in the prelecture? A  I
am confused by all of it. B  I understand a
little but I am confused by most of it. C  I
understand some parts and I am confused by other
parts. D  I understand most of itE  I
understand everything F I didn't view the whole
prelecture so I can't comment.
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Key Concepts

• Forces Torques on loops of current due to a
  magnetic field.
• The magnetic dipole moment.

Todays Plan

• Review of cross product
• Forces Torques
• Magnetic dipole moment
• Example problem

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Last Time
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CD
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• What is the force on section d-a of the loop?
• Zero
• Out of the page
• Into the page

CD
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y
x

• Given the forces that are acting on it, which way
  does the loop want to rotate?
• (assume the z axis is out of the page)
• Around the x axis
• Around the y axis
• Around the z axis

CD
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F
R
CD
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Magnetic Dipole Moment
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m makes torque easy!
The torque always wants to line m up with B !
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B
In this case m is out of the page (using right
hand rule)
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Magnetic Field can do Work on Current
From Physics 211
From Physics 212
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CD
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known. How much does the potential energy of the
system change as the coil moves from its initial
position to its final position.
z
z
.
B
B
30
y
y
a
I
x
final
initial

• Conceptual Analysis
• A current loop may experience a torque in a
  constant magnetic field
• t m X B
• We can associate a potential energy with the
  orientation of loop
• U - m B

• Strategic Analysis
• Find m
• Calculate the change in potential energy from
  initial to final

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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
y
a
I
x
final
initial
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
y
a
I
x
final
initial
z
B
y
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
y
a
I
x
final
initial
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
y
a
I
x
final
initial
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
y
a
I
x
final
initial
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Calculation
A square loop of side a lies in the x-z plane
with current I as shown. The loop can rotate
about x axis without friction. A uniform field B
points along the z axis. Assume a, I, and B are
known.
z
z
.
B
B
30
y
a
I
x
final
initial

• What is the potential energy of the final state?

(A) (B)
(C)