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Electromagnetic induction
Introduction
A stationary magnet does not cause a flow of current in a stationary conductor, just
as an immobile electric charge does not produce a magnetic field. The movement of
a magnet in relation to a conductor results in the flow of current across the conductor,
just as the movement of charges in a conductor produces a magnetic field.
The phenomenon of electric-current induction by a change in the magnetic field is
called electromagnetic induction, and the current produced in this way is called an
induced current.
The phenomenon of induction takes place when the electric conductor is open. At
each end of the conductor, there is a potential difference, which is known as the
induced electromotive force (EMF).
A state of equilibrium is reached when the forces affecting the charges – the Lorentz
force and the electric force – are equal:
Fm  Fe , qvB  qE .
Therefore:
E  vB .
The electric field strength E in the rigid wire is equal to the ratio of the potential
difference between the ends of the wire divided by the length of the wire. The
difference is called the electromotive force:
EMF
E
.
l
Therefore:
EMF  lBv .
When the flux density does not change, the EMF is proportional to the rate of change
in the area enclosed within a circuit and the flux density:
B ΔA
EMF 
.
Δt
When the area within the loop does not change, the EMF is proportional to the rate of
change of the flux density and the area within the loop:
AΔB
EMF 
.
Δt
The induced EMF can be obtained both as a result of changes in the area enclosed
within an electric circuit and also as a result of changes in the magnetic flux density.
The quantity:
Φ  BA
is called flux.
Applying this quantity, we can now express the principle, known as Faraday's Law:
EMF  
ΔΦ
.
Δt
Summarising, Faraday’s Law states that the magnitude of the induced
electromagnetic force in a closed loop placed in a magnetic field is proportional to the
rate of change of the magnetic field.
A the end of the lesson you should be able to:
 describe the phenomenon of induction,
 explain and use the Lentz’s Law and Faraday’s Law,
 describe the origin of eddy currents.
Electromagnetic induction
Worksheet
How do we induce current flow using a magnet?
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What does an induced current depend on?
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What is the direction of the induced current?
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