Concept 18.1: Permanent magnets exert
forces on one another that are different than
electrostatic forces.
18. Magnetic forces
This cannot be an electrostatic force, since:
1. The bar magnets are neutral conductors.
2. The bar magnet does not affect a charged
object.
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Physics 1E03 Lecture 18
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Similarities with electrostatic forces:
1. Forces act without contact.
2. There are two kinds of magnetic pole
called North and South.
3. Like poles repel, unlike magnetic
poles attract.
N
• magnetic forces exist
• magnetic forces are created
and felt by bar magnets,
current-carrying wires
• postulate a magnetic field
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Concept 18.2: A bar magnet and a currentcarrying wire exert a force on one another
that is not an electrostatic force.
Quiz of concept 18.1
Far in the north of Canada, the Earth has a
a) north magnetic pole
b) south magnetic pole
It seems that the SAME type of force
affects bar magnets and current-carrying
wires, since
1. the forces act without contact
2. they are not electrostatic or
gravitational
3. bar magnets and current-carrying
wires affect each other
N
S
CW
current
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Concept 18.3: Current-carrying wires exert
forces on one another that are not electrostatic
forces.
CCW
Quiz of concept 18.2
Since a current loop acts like a bar magnet,
there must be a “North pole”. According to
the attractive and repulsive forces, the
“North pole” is
a) the top of the current loop
b) given by the right-hand-rule from the
current direction
c) the point where the wires connect to the
loop
d) there can’t be a “pole” on a circular loop
Similarities with electrostatic forces:
1. Forces act without contact.
2. The current can have two directions.
3. Currents in the same direction attract (!)
Currents in opposite directions repel.
current
CW
current
-
+
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Physics 1E03 Lecture 18
This cannot be an electrostatic force, since:
1. The current adds no net charge.
2. The current does not affect a charged
object.
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Quiz of concept 18.3
Since a bar magnet acts like a current loop, there must be
a “current” associated with it. The “current” in the bar
magnet is given by
a) free electrons moving in the iron (which is a conductor)
b) a current flowing on the surface, around the bar
magnet
c) the individual electrons circling the atomic nuclei make
a large number of tiny current loops
d) there is no current, since there is no potential
difference applied across the conductor.
CW
current
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Concept 18.4: Since there is no contact, we
understand the force by postulating a
magnetic field B created by a bar magnet or
a current-carrying wire.
Magnetism:
Current I
Gravity:
Mass M
Electrostatics:
Charge Q
F
Field B(i)
F
Field g(m) Field g(M)
F
F
Field E(q)
Field E(Q)
F
Mass m
Field B(I)
F
Current i
Charge q
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Summary
1. A non-contact magnetic force exists
a) between bar magnets
b) between current-carrying wires
c) between bar magnets and current-carrying wires
2. This is not an electrostatic force.
3. A magnetic field B is created by currents and bar
magnets and exerts a force on other currents and bar
magnets.
Next lecture: read sections 29.1, 29.4-.5
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