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Chapter 27
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Magnetic Fields and Forces (cont.)
Force and torque on a current loop
•
•
The net force on a current loop in a uniform magnetic field is
zero. But the net torque is not, in general, equal to zero.
Figure 27.31 below shows the forces and how to calculate the
torque.
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Q27.11
A circular loop of wire carries a constant current. If the loop is
placed in a region of uniform magnetic field, the net magnetic
force on the loop is
A. perpendicular to the plane of the loop, in a direction given by
a right-hand rule.
B. perpendicular to the plane of the loop, in a direction given by
a left-hand rule.
C. in the same plane as the loop.
D. zero.
E. The answer depends on the magnitude and direction of the
current and on the magnitude and direction of the magnetic field.
© 2012 Pearson Education, Inc.
A27.11
A circular loop of wire carries a constant current. If the loop is
placed in a region of uniform magnetic field, the net magnetic
force on the loop is
A. perpendicular to the plane of the loop, in a direction given by
a right-hand rule.
B. perpendicular to the plane of the loop, in a direction given by
a left-hand rule.
C. in the same plane as the loop.
D. zero.
E. The answer depends on the magnitude and direction of the
current and on the magnitude and direction of the magnetic field.
© 2012 Pearson Education, Inc.
Q27.12
A circular loop of wire carries a constant current. If the loop is
placed in a region of uniform magnetic field, the net magnetic
torque on the loop
A. tends to orient the loop so that its plane is perpendicular to the
direction of the magnetic field.
B. tends to orient the loop so that its plane is edge-on to the
direction of the magnetic field.
C. tends to make the loop rotate around its axis.
D. is zero.
E. The answer depends on the magnitude and direction of the
current and on the magnitude and direction of the magnetic field.
© 2012 Pearson Education, Inc.
A27.12
A circular loop of wire carries a constant current. If the loop is
placed in a region of uniform magnetic field, the net magnetic
torque on the loop
A. tends to orient the loop so that its plane is perpendicular to the
direction of the magnetic field.
B. tends to orient the loop so that its plane is edge-on to the
direction of the magnetic field.
C. tends to make the loop rotate around its axis.
D. is zero.
E. The answer depends on the magnitude and direction of the
current and on the magnitude and direction of the magnetic field.
© 2012 Pearson Education, Inc.
The Hall Effect
•
•
•
Using the Hall effect we can determine that the charge carriers in conductors
are negatively charged.
For some semiconductors “holes” are the particles!
This is related to our earlier “mystery”.
© 2012 Pearson Education, Inc.
Chapter 28
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© 2012 Pearson Education, Inc.
Sources of Magnetic Field
The magnetic field of a moving charge
•
•
A moving charge generates a
magnetic field that depends on
the velocity of the charge.
Figure 28.1 shows the direction
of the field.
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Q28.1
A positive point charge is moving directly toward point P. The
magnetic field that the point charge produces at point P
A. points from the charge toward point P.
B. points from point P toward the charge.
C. is perpendicular to the line from the point charge to point P.
D. is zero.
E. The answer depends on the speed of the point charge.
© 2012 Pearson Education, Inc.
A28.1
A positive point charge is moving directly toward point P. The
magnetic field that the point charge produces at point P
A. points from the charge toward point P.
B. points from point P toward the charge.
C. is perpendicular to the line from the point charge to point P.
D. is zero.
E. The answer depends on the speed of the point charge.
© 2012 Pearson Education, Inc.
Q28.2
Two positive point charges move side by side in the
same direction with the same velocity.
What is the direction of the magnetic force that the
upper point charge exerts on the lower one?
+q
v
+q
v
A. toward the upper point charge (the force is attractive)
B. away from the upper point charge (the force is repulsive)
C. in the direction of the velocity
D. opposite to the direction of the velocity
E. none of the above
© 2012 Pearson Education, Inc.
A28.2
Two positive point charges move side by side in the
same direction with the same velocity.
What is the direction of the magnetic force that the
upper point charge exerts on the lower one?
+q
v
+q
v
A. toward the upper point charge (the force is attractive)
B. away from the upper point charge (the force is repulsive)
C. in the direction of the velocity
D. opposite to the direction of the velocity
E. none of the above
© 2012 Pearson Education, Inc.
Magnetic field of a current element
• The total magnetic field of
several moving charges is
the vector sum of each field.
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Force between parallel conductors
•
•
The force per unit length on each
conductor is F/L = 0IIL/2πr.
(See Figure 28.9 at the right.)
The conductors attract each other if
the currents are in the same
direction and repel if they are in
opposite directions.
© 2012 Pearson Education, Inc.
Q28.3
y
A long straight wire lies along the y-axis and
carries current in the positive y-direction.
A positive point charge moves along the xaxis in the positive x-direction. The magnetic
force that the wire exerts on the point charge
is in
A. the positive x-direction.
v
O
+q
B. the negative x-direction.
C. the positive y-direction.
D. the negative y-direction.
E. none of the above
© 2012 Pearson Education, Inc.
I
x
A28.3
y
A long straight wire lies along the y-axis and
carries current in the positive y-direction.
A positive point charge moves along the xaxis in the positive x-direction. The magnetic
force that the wire exerts on the point charge
is in
A. the positive x-direction.
v
O
+q
B. the negative x-direction.
C. the positive y-direction.
D. the negative y-direction.
E. none of the above
© 2012 Pearson Education, Inc.
I
x
Q28.4
Two long, straight wires are
oriented perpendicular to the xyplane. They carry currents of equal
magnitude I in opposite directions
as shown. At point P, the magnetic
field due to these currents is in
A. the positive x-direction.
B. the negative x-direction.
C. the positive y-direction.
D. the negative y-direction.
E. none of the above
© 2012 Pearson Education, Inc.
A28.4
Two long, straight wires are
oriented perpendicular to the xyplane. They carry currents of equal
magnitude I in opposite directions
as shown. At point P, the magnetic
field due to these currents is in
A. the positive x-direction.
B. the negative x-direction.
C. the positive y-direction.
D. the negative y-direction.
E. none of the above
© 2012 Pearson Education, Inc.