Download Notes & ConcepTests

Example: Magnetic Force Directions
from Right Hand Rule
A uniform magnetic field in this room
points from the floor towards the ceiling.
What is the direction of the magnetic force
acting on the particles in the various
situations below?
a) A motionless proton?
b) A proton moving towards the front of
the room?
c) An electron moving towards the front
of the room?
d) A neutron moving towards the front
of the room?
d) A proton moving straight up toward
the ceiling?
e) An electron moving to the right?
f) A proton moving diagonally from the
back right to the front left?
Example: Magnetic Force & Vectors
Calculate the magnetic force acting on a
particle with charge q moving with

velocity v  vx iˆ  vy ˆj  vz kˆ in a

magnetic field B  By ˆj  Bz kˆ
ConcepTest #11:
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An electron and a proton move with the
same velocity. They enter a region that has
a uniform magnetic field directed into the
page as shown. Which of the following
sketches best represents their paths?
1.
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ConcepTest #12:
Discussion: Magnetic Forces & Work
A charged particle is in a region of
space with a uniform magnetic field (no
other fields are present). Which of the
following describes a possible path the
charged particle can travel? (Put up as
many cards as you think possible.)
A charged particle is in a region of space
with a uniform magnetic field (no other
fields are present). What can we say
about the change in kinetic energy of the
charged particle?
1. No motion
4. Circle
2. Straight line
5. Helix
3. Parabola
6. Square
B-field Paths
Example: Velocity Selector
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through
 a region of uniform magnetic
field B directed into the page as
shown.
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The electron is moving in a straight
line.
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An electron travels with velocity v0
How do you know there must be
another field present?
If the other field is a uniform electric
field, what is its direction?
Example: Velocity Selector
What is the magnitude of this electric field?
An electron travels with constant

velocity v0 through a region
 of
uniform magnetic field B directed
into the page (as
 shown) and uniform
electric field E pointing down (not
shown).
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What happens if you send in a particle with
charge +2e but the same velocity?
What happens if you send in an electron
with a slower speed (but same direction?)
Example: Current-carrying wire in
Magnetic Field
A conducting wire is in a uniform
magnetic field directed to the left as
shown.
a) Conventional current I flows in the wire
towards the top of the page. Conventional
current is the motion of positive charges.
What is the direction of the magnetic force?
b) The actual charge carriers in this wire
are electrons, so the electron current is
towards the bottom of the page. What is
the direction of the magnetic force?

 
dF  Id   B
  
F  IL  B
Think about This: if magnetic forces
do no work, how can the wire start
from rest and begin to move?
Example: Hall Effect
A wire is held fixed in a uniform
magnetic field pointing into the page as
shown. The wire carries conventional
current I going towards the right.
If the charge carriers were positively
charged, what would be the sign of
DVbottomtop = Vtop – Vbottom, the potential
difference between the top side and the
bottom side of the wire?
Think about This: Would anything
be different if the charge carriers
were negative instead of positive?
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What is the potential difference between the
top and bottom side of the wire? Assume
you know/can measure the distance
(thickness) of the wire.
ConcepTest #13:
A square loop of wire with sides of length
L carries a current I in a clock-wise
direction; the current loop is in a uniform
magnetic field B that is in the plane of the
page and pointing to the right, as shown.
Consider the following choices:
1. Up
4. Left
2. Down
5. Into the page
3. Right
6. Out of the page
c) What is the direction of the net torque
acting on the entire loop? (Take center of
loop as reference point.)
(back of card) No direction (zero)
a) What is the direction of the magnetic
force acting on the right hand piece of the
loop?
b) What is the direction of the net
magnetic force acting on the entire loop?
Think about This: assuming the
loop started from rest, what would
be its subsequent motion?
Current Loops, Magnetic Moments,
and Torque
A current loop can be treated using a
magnetic moment model. In this model,
replace the current loop with a little bar
magnet (the magnetic moment).
Consider a uniform magnetic field
pointing to the right as shown. A little
bar magnet is placed in the field; the
magnet is free to pivot (in three
dimensions) about its center.
What is the direction of the torque acting
on this magnetic moment?
N
S
  
  B

 || NAI
N = number of turns of wire in loop
A = area of loop
I = current in loop
Direction from RHR II: curl fingers
in direction of current, thumb points
in direction of magnetic moment.
Example: Torque on Current Loop
The plane of a current loop (N = 10,
A = 0.01 m2 , I = 1 A,) makes an angle of
30o with a uniform magnetic field of
magnitude 0.2 T directed to the right, as
shown in the figure. Assume the current
is going into the page at the lower left
hand part of the loop and out of the page
at the upper right hand part of the loop,
as indicated on the sketch
ConcepTest #14:
What is the direction of the torque acting
on this current loop? Hold up as many
cards as you need to specify the direction.
1. Up
4. Left
2. Down
5. Into the page
3. Right
6. Out of the page
(back of card) No direction
30o
What is the magnitude of the torque
acting on the loop?