Example: Magnetic Force Directions from Right Hand Rule

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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?
Example Magnetic Force Vectors Calculate the
magnetic force acting on a particle with charge q
moving with velocity
in a magnetic field

• a) A motionless proton?
• b) A proton moving towards the front of the
  room?
• An electron moving towards the front of the room?
• 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?

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ConcepTest 11 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?
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
p
e
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ConcepTest 12 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.)
Discussion Magnetic Forces Work 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?
B-field Paths
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Example Velocity Selector An electron travels
with velocity through a region of uniform
magnetic field directed into the page as
shown. The electron is moving in a straight
line. How do you know there must be another
field present? If the other field is a uniform
electric field, what is its direction?
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
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What is the magnitude of this electric
field? 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?)
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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?
Think about This if magnetic forces do no work,
how can the wire start from rest and begin to
move?
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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.
x x x x x x x x x x x
x x x x x x x x x x
top bottom
I
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?
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.
Think about This Would anything be different if
the charge carriers were negative instead of
positive?
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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
c) What is the direction of the net torque
acting on the entire loop? (Take center of loop
as reference point.)
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?
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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 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.
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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
30o
What is the magnitude of the torque acting on the
loop?
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.