Physics 100 Lecture 2

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Physics 212 Lecture 12
Today's Concept Magnetic Force on moving charges
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How about the fact that soon our planet's
magnetic field will turn on it's end and switch
polarity, if it doesn't disappear in the process
that is, like it did for Mars. This flip will
confuse animals that rely on internal compasses
for migration. Also, due to the standard
anomalies in polarity prior to the flip (see
graphic at http//www.pbs.org/wgbh/nova/magnetic/r
eversals.html) our field will weaken enough to
make cancer even more prevalent that it is
currently from increased exposure to radiation.
Also these anomalies will create multiple north
and south poles covering much of the upper
northernand lower southern hemisperes in auroras
at night. Also, the weakening of the magnetic
field may allow radiation to interfere with
satellite transmissions. Hello cable TV! And most
importantly, after the flip our compasses will
point south.
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How bad will exam 1 ruin my grade for this class?
eh, i dunno. more magnetic field examples cause i
was a bit rushed
DIRECTIONS, please. The cross product is
confusing the heck out of me,
current flow in electric wires causing forces
between the wires.
Reviewing the right hand rule and the equation
about the radius the particle travels in a
magnetic field.
I like demos. Especially dangerous ones.
You know that if you google "physics 212" the
UIUC homepage is the first thing that shows up?
Even though UC Berkeley apparently has a physics
212 homepage as well (and it is second on the
list of results), ours comes up first. Apparently
we are kind of a big deal.
Why is it that when I swallowed magnets as a
child I had to go to the hospital?
how many licks does it take to get to the center
of a tootsie pop? GO RED SOX!
I've been dazed and confused for so long it's not
true Wanted a woman, never bargained for you Lots
of people talk and few of them know Soul of woman
was created below
iwouldreallyliketodomoreexamplesofmagneticfieldsan
dmaybeworkonusingtherighthandrule.ican'treallytell
ifiamusingtherighthandrulecorrectly. i maen i
kpet gttenig it wonrg in mtah calss for smoe
raseon its qiute sad ralely. hvae you eevr try
tpynig wtiouht sacpes it is a lot hrader tahn you
wluod tinhk. i am amezad taht i can tpye tihs all
msesed up so wlel. you konw taht eevn if the wrod
is scarbmeld taht if the fisrt and lsat ltteer
are crreoct taht you can raed it wtih the
cnodiiton aslo taht the wrod has the smae nmuber
of lteters in it. tihs is turly azmanig snice i
can not sepll coecttly for the msot prat.
Did you know that the Janitor from Scrubs was
actually in "The Fugitive" as the cop who got
shot on the train?
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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

• The force on moving charges due to a magnetic
  field.
• The cross product.

Todays Plan

• Review of magnetism
• Review of cross product
• Example problem

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Magnetism
I
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Magnetic Observations

• Bar Magnets
• Compass Needles
• Magnetic Charge?

N S
N S
S N
N S
N S
cut in half
N S
N S
N S
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The earth is a big (weak) magnet
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Magnetic Observations

• Compass needle deflected by electric current

I

• Magnetic fields created by electric currents
• Magnetic fields exert forces on electric currents
  (charges in motion)

I
F
I
F
F
I
I
F
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Magnetism Moving Charges

• All observations are explained by two simple
  equations

Today
Next Week
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Cross Product Review

• Cross Product different from Dot Product
• A?B is a scalar A x B is a vector
• A?B proportional to the component of B parallel
  to A
• A x B proportional to the component of B
  perpendicular to A
• Definition of A x B
• Magnitude ABsinq
• Direction perpendicular to plane defined by A
  and B with sense given by right-hand-rule

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Remembering Directions The Right Hand Rule
y
x
F
B
v
z
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Magnetic Force
y
x
F
B
v
z
q
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Magnetic Force
I
F
F
I
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Preflight
Three points are arranged in a uniform magnetic
field. The B field points into the screen.
1) A positively charged particle is located at
point A and is stationary. The direction of the
magnetic force on the particle is
Since it has no velocity, there is no magnetic
force.
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Preflight
Three points are arranged in a uniform magnetic
field. The B field points into the screen.
1) A positively charged particle is located at
point A and is stationary. The direction of the
magnetic force on the particle is
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Motion of Charge q in Uniform B Field

• Force is perpendicular to v
• Speed does not change
• Uniform Circular Motion

• Solve for R

R
Uniform B into page
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(No Transcript)
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Preflight
The drawing below shows the top view of two
interconnected chambers. Each chamber has a
unique magnetic field. A positively charged
particle is fired into chamber 1, and observed to
follow the dashed path shown in the figure.
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Preflight
The drawing below shows the top view of two
interconnected chambers. Each chamber has a
unique magnetic field. A positively charged
particle is fired into chamber 1, and observed to
follow the dashed path shown in the figure.
It turns sharper...the stronger the field the
sharper the turns
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Calculation
exits here
A particle of charge q and mass m is accelerated
from rest by an electric field E through a
distance d and enters and exits a region
containing a constant magnetic field B at the
points shown. Assume q,m,E,d, and x0 are
known. What is B?
x0
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
d
x0
q, m
E
B
enters here
B

• Conceptual Analysis
• What do we need to know to solve this problem?

(A) Lorentz Force Law (B) E field
definition (C) V definition
(D) Conservation of Energy/Newtons
Laws (E) All of the above

• Absolutely ! We need to use the definitions of V
  and E and either conservation of energy or
  Newtons Laws to understand the motion of the
  particle before it enters the B field.
• We need to use the Lorentz Force Law (and
  Newtons Laws) to determine what happens in the
  magnetic field.

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Calculation
exits here
A particle of charge q and mass m is accelerated
from rest by an electric field E through a
distance d and enters and exits a region
containing a constant magnetic field B at the
points shown. Assume q,m,E,d, and x0 are
known. What is B?
x0
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
d
x0
q, m
E
B
enters here
B

• Strategic Analysis
• Calculate v, the velocity of the particle as it
  enters the magnetic field
• Use Lorentz Force equation to determine the path
  in the field as a function of B
• Apply the entrance-exit information to determine
  B

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exits here
x0
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
d
x0
q, m
E
B
enters here
B

• What is v0, the speed of the particle as it
  enters the magnetic field ?

(A) (B)
(C) (D)

• Why??
• Conservation of Energy
• Initial Energy U qV qEd
• Final Energy KE ½ mv02

• Newtons Laws
• a F/m qE/m
• v02 2ad

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exits here
x0
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
d
x0
q, m
E
B
enters here
B

• What is the path of the particle as it moves
  through the magnetic field?

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 X X X X
(A) (B)
(C)

• Why??
• Path is circle !
• Force is perpendicular to the velocity
• Force produces centripetal acceleration
• Particle moves with uniform circular motion

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exits here
x0
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
d
x0
q, m
E
B
enters here
B

• What is the radius of path of particle?

(A) (B) (C)

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exits here
x0
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
d
What is B in terms of v, x0, q and m?
q, m
E
B
enters here
B
(A) (B) (C)
(D)

• Why??

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Done
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Some not-so-good pictures claiming to illustrate
the RHR