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When a current moves through a
conductor in a magnetic field a force is
produced.
 Michael Faraday discovered that the
force on the wire is at right angles to
both the direction of the magnetic field
and the direction of the current.

Faraday’s explanation was lacking the
direction of the force.
 The force can be found by using the
Third right-hand rule.
 Point right hand fingers in the direction of
the magnetic field
 Point thumb in the direction of
conventional current
 Palm points in the direction of force

F
B
I
Experiments show that the magnitude of
the force, F, on the wire, is proportional
to the strength of the field, B, the current,
I, in the wire and the length, L, of the wire
in the magnetic field
 F=ILB
 B is the strength of the magnetic field
measured in teslas, T (1 T = 1N/A.m

Charged particles don’t have to be
confined to a wire, they can also move
in a vacuum.
 These are called cathode-ray tubes used
to form pictures on screens.
 Magnetic fields control the motion of the
beam (back & forth, up & down)
 The screen is coated with phosphor that
glows when it is struck with the electrons

The force produced by a magnetic field
on a single electron depends on the
velocity of the electron and the strength
of the magnetic field.
 F=qvB
 q is the charge measured in Coulombs
 v is velocity measured in m/s
 B is magnetic field strength measured in T
 The direction of the force is given by the
third right-hand rule keeping in mind the
rule is for positively charged particles, so
for electron the force would be
opposite.
