Electricity From Magnetism
Electromagnetic Induction
 Electromagnetic induction – the production of an emf
(energy per unit charge supplied by a source of electric
current) in a conducting circuit by a change in the
strength, position, or orientation of an external magnetic
field
 The process of creating a current in a circuit loop by changing
the magnetic flux in the loop
 The circuit must be closed
 The circuit and magnetic field must move relative toward
each other
 Creates an electrical current without a battery or generator
Electromagnetic Induction
 According to the right-hand rule, magnetic force, the
magnetic field, and the motion of the circuit are all
three perpendicular to one another
 Magnetic field is directed out of the fingers
 Magnetic force is directed out of the palm
 The charge moves in the direction of the extended
thumb
 The magnitude of the induced emf depends upon the
velocity with which the circuit is moved through the
magnetic field, the length of the wire, and the strength
of the magnetic field
Electromagnetic Induction
Electromagnetic Induction
 The magnitude of the induced emf and current
depend upon the orientation of the loop with respect
to the magnetic field
 Largest when the plane of the loop is perpendicular to
the magnetic field

Zero when the plane of the loop is parallel to the field
 Changing the size of the loop will induce an emf
 Changing the strength of the magnetic field induces
an emf
 Basically, emfs are induced when something changes
Electromagnetic Induction
 Ways of Inducing a Current in a Circuit
Electromagnetic Induction
Characteristics of Induced Current
 The induced current in a circuit produces its own
magnetic field
 Lenz’s Law – The magnetic field of the induced
current opposes the change in the applied magnetic
field
 The direction of the new magnetic field is based on that
of the original magnetic field
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If the circuit and magnet are getting closer together, the new
magnetic field will be in the opposite direction
If the circuit and magnet are getting further apart, the new
magnetic field will be in the same direction
Characteristics of Induced Current
 Faraday’s Law of Magnetic Induction
 Average induced emf = -(the number of loops in the circuit)*[the rate of change
of (circuit loop area)*(magnetic field component normal to the plane of the
loop)]
 emf = -N[AB(cos)]/t
 N is always a whole number
 A is area in square meters (m2)
 B is magnetic field strength in Teslas (T) or N/(A*m) or (V*s)/m2
  is the angle between the magnetic field and the loop in degrees
 t is the time interval in seconds (s)
 emf is equivalent to voltage or potential difference and is measured in volts (V)
 Either A, B, or  will change. Subtract the two values
 Average induced emf = -(the number of loops in the circuit) * (the time rate of
change in the magnetic flux)
 emf = - N * ΔΦM / Δt
 Once you calculate the emf value using Faraday’s law, you can use Ohm’s law to
find either current or resistance or vice versa
Characteristics of Induced Current
 Example: A coil with 25 turns of wire is wrapped
around a hollow tube with an area of 1.8m2. Each turn
has the same area as the tube. A uniform magnetic
field is applied at a right angle to the plane of the coil.
If the field increases uniformly from 0.00T to 0.55T in
.85s, find the magnitude of the induced emf in the
coil. If the resistance in the coil is 2.5, find the
magnitude of the induced current in the coil.
Characteristics of Induced Current
Applications of Induction
 Use electromagnetic induction to produce temporary
or continuously changing current
 Door bells – pressing the button interrupts an electric
circuit


This causes the circuit’s magnetic field to decrease
In response this induces a current in a nearby circuit
 The induced current generates an opposing magnetic field that
causes a plunger to strike a chime
Applications of Induction
 Tape recorders – magnetic tape moves past a recording head
and a playback head



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A microphone transforms sound into a fluctuating electric current
The current is amplified and passes through a wire coiled around an
iron ring
 This is the recording head
A gap is cut in the ring
 The magnetic field does not pass through the gap as easily as it
does the rest of the coil
 The magnetic field magnetizes the metal oxides on the tape in
a pattern that corresponds to the frequency and intensity of the
sound entering the microphone
In playback mode, the reverse happens
 The pattern on the tape generate variable magnetic fields which
correspond to certain frequencies and intensities of sound waves