Magnetic Fields & Magnetic
Field Strength
• We have seen that magnets can exert a force
on objects without touching them. For this
reason we speak of a magnetic field around
a magnet, in the same way that we speak of
an electric field around a charged object.
• A magnetic field is represented by drawing in "magnetic
lines of flux".
• These are lines drawn so that a tangent at any point will
show the direction of the magnetic field.
• The direction of a line of flux is from the North Pole to the
South Pole, outside a magnet, from South to North inside
the magnet.
• (One line of flux may cause many lines of iron filings.)
• Weber - one magnetic line of flux is called a
Weber (Wb)
Wilhelm Eduard Weber
• Magnetic Flux - the entire group of lines of
flux about a magnet, or in a region is called
the magnetic flux.
• Symbol is  - phi)
Magnetic Field Strength
• An electric field exerts a straight attractive
or repulsive force upon a charged object.
• A magnetic field will exert a particular force
upon a charged moving object.
• This force is perpendicular to the motion of
the charge. The force is proportional to the
velocity of the moving charge.
Two methods of defining
magnetic field strength.
• in terms of amount of force exerted per unit
of moving charge - N/C
• Use the idea that magnetic field strength is
directly related to the concentration of flux
lines. i.e. lines of flux per unit of area.
no. of lines
Magnetic Field Strength 
area

B
A
where
B is measured in Tesla (T)
is measured in Webers (Wb)
A is measured in square metres. (m2)
• e.g. In a 25m2 area on the Earth, there are
1.25 x 10-3 lines of flux measured. What is
the field strength of the Earth's magnetic
field?
B

A
1.25 x10 3
Bearth 
25
5
Bearth  5.0 x10 T
*note this is a constant value