Demo
An electric current can be produced by a changing magnetic field. (Faradays observation)
< Magnetic Flux >
The magnetic flux is the amount of the magnetic field passing perpendicularly through an area multiplied by the area.
F = = B A cos
where B is the magnetic field, A area, and the angle between the magnetic field and the normal to the area.
The unit of magnetic flux is Wb (Weber)
1 Wb = 1 T-m2 (tesla-meter-square).
[Note] Fo = 2.0678 x 10-15 T-m2 : Flux quantum (fluxoid)
Figure 20.3
The magnetic flux is proportional to the number of field lines that passes through a surface.
Whenever there is a change in flux through a loop of wire, an emf is induced in the loop.
Examples
Demo (Jumping ring)
Examples
When a conducting rode moves through a constant magnetic field, an emf is induced in a rod due to the magnetic force acting on a moving charge.
Figure 20.8
The separated charges on the ends of the moving conductor give rise to an induced emf is called a motional emf.
The motional emf when , , and are mutually perpendicular is given by
= e = B v
Figure 20.9
The effect in which a changing current in a circuit induces an emf in the same circuit is referred to as self-induction.
Figure 20.24 and 25
The emf due to self induction is
e = - L
where L is a proportionality constant called the inductance of the device. The negative sigh indicates that a changing current induces an emf in opposition to that change.
The unit of inductance is H (Henry)
1 H = 1 V-s/A
Because of their self-inductance, coils are known as inductors.
L = N =
Examples
The energy stored in the magnetic field of an inductor carrying current I is
Energy = L I2