• Magnets have two ends – poles – called north and south.
• Like poles repel; unlike poles attract.
• Only iron, cobalt, nickel, gadolinium, and some of
their oxides and alloys show strong magnetic effects.
• If you cut a magnet in half, you don’t get a north pole and a
south pole – you get two smaller magnets.
• Magnetic poles are different from
electric charges due to the fact that
electric charges can be separated and
isolated, however magnetic north or
south poles can not be separated and
isolated.
• Surrounding a magnet, there is a magnetic field. Like an
electric field, a magnetic field has both magnitude and
direction.
• Magnetic fields can be visualized using magnetic field lines,
which are always closed loops.
• The magnetic field at any point is
tangent to the magnetic field line at that
point.
• The strength of the magnetic field is
proportional to the number of lines per
unit area.
• The Earth’s magnetic field is similar
to that of a bar magnet.
• Note that the Earth’s “North Pole” is
really a south magnetic pole, as the
north ends of magnets are attracted to it.
• A uniform magnetic field is constant in magnitude and direction.
• The field between these two wide poles is nearly uniform.
• The magnetic field between two wide
poles of a magnet is nearly uniform,
except at the edges.
• A charged particle, placed in a magnetic field, will experience a magnetic force if….
-The charge is moving
- The velocity of the moving charge has a component that is
perpendicular to the magnetic field.
• This picture shows a charge moving parallel to the
magnetic field (B), which results in no magnetic
force on the charge.
• This picture shows a charge moving perpendicular
to the magnetic field (B), which results in a
maximum magnetic force on the charge.
• The magnetic force (F) is perpendicular to both
the velocity (v) and the magnetic field (B).
Right Hand Rule for Force
Fingers point in direction of
magnetic field B.
Magnetic
Force
Charge
Magnitude
Velocity of
charge
Magnitude
of magnetic
field
Angle formed
between magnetic
field direction and
velocity vector.
Thumb points in direction of
the velocity vector v.
Palm shows the direction of
the force F.
• The magnitude B of the magnetic field at any given point in space is defined as:
• SI Unit of Magnetic Field:
newton . second = 1 Tesla (T)
coulomb . meter
• 1 Tesla (T) is also written as 1 T = 1 N/(A . m)
Nicola Tesla (1856-1943)
• As a charge moves through an
electric field, its path is curved in the
direction of the electric field lines.
This is due to the electric force acting
parallel to the electric field and
perpendicular to the velocity vector.
• As a charge moves through a
magnetic field, its path is curved
upward along a vertical plane. This is
due to the magnetic force acting
perpendicular to both the magnetic
field and the velocity vector.
• The magnetic force always remains
perpendicular to the velocity and is
directed toward the center of the circular
path.
• The magnetic force does no work on the
charge, therefore the kinetic energy of the
charge does not change.
• The magnetic force acts as the
centripetal force on the charge. The
stronger the magnetic field, the shorter the
radius and therefore, a “tighter” circular
path.
• A magnet exerts a force on a current-carrying wire. The direction of the force is
given by a right-hand rule.
• The force on the wire depends on the current, the length of the wire, the magnetic
field, and its orientation.
Magnetic
Force
Current
Length of
Wire
Magnitude
of magnetic
field
Angle formed
between magnetic
field direction and
wire orientation.
• Using the right-hand
rule, a leftward directed
force results.
• Using the right-hand
rule, a rightward directed
force results.
• Experiment shows that an electric current produces a magnetic field.
• The direction of the field is given by a right-hand rule.
Right Hand Rule for Current:
Grasp the wire with your right hand so that your
thumb points in the direction of the conventional
current; then your fingers will encircle the wire in the
direction of the magnetic field.
• The right hand rule indicates the direction of the magnetic field in a looped wire.
The field is inversely proportional to the distance from the wire:
The constant μ0 is called the permeability of free space, and has the value:
• The magnetic field that one current creates can exert a force on another nearby
current.
• If currents are in the same direction, wires attract. If in opposite direction, wires repel.
• This is the opposite of the rule for charges: like charges repel, opposite charges attract.
• Using the two right hand rules, one for finding the direction of the B-field of a wire, and
the other for the direction of force on a wire, one can predict the results above.
• The magnetic field produced at the position of wire 2
due to the current in wire 1 is:
• The force this field exerts on a length l2 of wire 2 is: