PHYA22 Chapter 33.doc

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17 Apr 2012
Chapter 33: The Magnetic Field
Magnetism is not the same as electricity.
Magnetism is a long-range force.
Magnets have two poles, the north and the south poles. Two like poles exert
repulsive forces on each other and two opposite poles exert attractive forces on
each other.
Magnetic Materials: materials that are attracted to or that a magnet sticks to
(iron, nickel and cobalt) and is attracted to pole poles of a magnet.
Magnetic Dipole: a permanent north and south pole; cutting a magnet in half
yields two weaker but still complete magnets.
Magnetic Monopole: an isolated magnetic pole (never been observed yet).
The geographic north pole is actually a south magnetic pole and is offset
slightly from the geographic poles of the earth’s rotation axis.
The Discovery of the Magnetic Field
An electric current causes magnetism.
Right-Hand Rule: relates orientation of the compass needles to the direction of
the current.
oPoint your right thumb in the direction of the current.
oCurl your fingers around the wire to indicate a circle.
oYour fingers point in the direction of the magnetic field lines around the
x: current into the page
: current out of the page
Magnetic Field (B)
oA magnetic field is created at all points in space surrounding a current-
carrying wire.
oMagnetic field at each point is a vector. It has both a magnitude (B) and
a direction.
oMagnetic field exerts forces on magnetic poles. The force on a north
pole is parallel to B and the force on the south pole is opposite B.
The pair of opposite forces exerts a torque on the needle, rotating the needle
until it is parallel to the magnetic field at that point.
Magnetic forces cause a compass needle to become aligned parallel to a
magnetic field, with the north pole of the compass showing the direction of the
magnetic field at that point.
Because compass needles align with the magnetic field, the magnetic field at
each point must be tangent to a circle around the wire.
Magnetic Field Lines: imaginary lines drawn through a region of space so that a
tangent to a field line is in the direction of the magnetic field and the field lines
are closer together where the magnetic field strength is larger.
The Source of the Magnetic Field: Moving Charges
Moving charges are the source of the magnetic field.
Biot-Savart Law: Bpoint charge = [(µ0/4π)(qvsinθ/r2), direction given by the right-
hand rule], where q is the charge, v is the velocity, r is the distance from the
charge and θ is the angle between v and r.
The SI unit of magnetic field strength is tesla (T) and 1 T = 1 N/Am.
Permeability Constant (µ0): µ0 = 1.257 x 10-6 Tm/A
Right-Hand Rule in finding B:
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