The magnetic flux 
is equal to the product of the magnetic field density 
and the surface area through which the magnetic field lines pass and which is perpendicular to the lines. If the area 
is not perpendicular to the lines of force, we take the component of the area that is perpendicular to the lines of force of the magnetic field.
Magnetic flux
The magnetic flux unit is:
Let's imagine that a wire loop encloses the surface area . If the magnetic flux through the loop changes with time 
, a voltage is induced in it. The voltage 
induced is given as:
If there are 
turns of the loop, the induced voltage is:
The magnetic flux through the loop changes if the surface 
or the magnetic field density changes with time - see equation (1). Therefore:
Let's assume we have a coil with turns and a cross-sectional area . The current 
through the coil changes over time . Due to the change in current, the magnetic flux through the coil also changes. The result is an induced voltage , which is calculated using equation (3) above:
The first factor in the right-hand side of the above equation depends only on the coil itself (number of turns, area, and the substance of the coil). We call it inductance. Let us denote it by L:
The unit for inductance is 
(Henry):
The induced voltage can now be written in the form:
Why did we put a minus sign in the equation for induced voltage?
This is according to Lenz's rule which states that the direction of the induced voltage is such that the current it drives inhibits the cause of the induced voltage (change in magnetic flux).
