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Module 4, Lecture 7: Introduction to Magnetism
Magnetic Field and Force
• Magnetic north and south poles’ behavior is similar to
electric charges.
A moving charge or an electric current produces an
electric field and a ________ _________ in the
surrounding space.
• The magnetic field exerts a ______ on any other moving
charge or current that is in the field.
•
• For magnets, like poles _____ and opposite poles _______.
•
A permanent magnet will attract an un-magnetized metal
(like iron) with either the ________ or _________ pole.

B
• In contrast to electric charges, magnetic poles always come
in _________ and cannot be isolated.
– Magnetic field

 
F  q (v  B )

B
Breaking a magnet in two
yields two magnets, not two
isolated poles.
Force F is ____________
to both v and B
units of _________ T
1 T = 1x104 G (Gauss)
1 T = N /(A·m)
EF 152
Lecture 4-7
1
Examples
3̂
5̂
6
m/s.
  0 qv  rˆ
B
4 r 2
1
c2 
A beam of protons (q =1.6 x 10-19 C) moves at
3.0 x 105 m/s through a uniform magnetic field
of 2.0 T. The velocity of each proton lies in the
xz-plane at an angle of 30° to the +z-axis. Find
the force on a proton.
Lecture 4-7
2
A moving charge will generate
a magnetic field relative to
the velocity of the charge.
4 ̂ mT
What is the magnetic force on the particle?
EF 152
Lecture 4-7
The Magnetic Field due to a Moving Charge
A particle with charge q = –2 µC has a velocity of 7 ̂
The magnetic field is:
EF 152
 0 0
0
 10 7 T  m / A
4
3
EF 152
Lecture 4-7
4
Magnetic field - straight current-carrying wire
B
Field around two wires carrying
current in opposite directions
0 I
2 r
Btot 
A long, straight 120 V wire powers a 1500
W hair dryer. At what distance is the
magnetic field from the conductor equal
to the earth’s magnetic field in Knoxville,
about 0.51x10-4 T?
EF 152
Lecture 4-7
0 I r
2 R 2
B
0 I
2r
xd
d
d
x
Two wires carry 12.5 A in the opposite direction and are 3 mm apart.
At what distance is the magnetic field from the conductor equal to the
earth’s magnetic field in Knoxville, about 0.51x10-4 T?
5
rR
 x 2  d 2 
2d = distance between wires
x = distance from midpoint
Field of a long cylindrical conductor
B
 0 Id
EF 152
Lecture 4-7
6
Simple DC Motor
rR
R = radius of cylindrical conductor
Magnetic field outside cylindrical
conductor is same as for a straight wire
EF 152
Lecture 4-7
7
EF 152
Lecture 4-7
8