Download Physics Chapter 22 Notes Induction and alternating current

Physics Chapter 20 Electromagnetic Induction and alternating current
Instead of just using a battery or a power supply, a current can be made by moving a
portion of a closed electric circuit through an external magnetic field. The process of
inducing a current in a circuit with a changing magnetic field is called electromagnetic
induction. If the circuit is moved toward or away from the magnet or the magnet moves
toward or away from the circuit, a current is induced. The magnitude of the induced emf
depends on the velocity with which the wire is moved through the magnetic field, the
length of the wire, and the strength of the magnetic field.
The angle between a magnetic field and a circuit affects induction. The induced current
is largest when the plane of the loop is moving perpendicular to the magnetic field.
A change in the number of magnetic field lines induces a current.
An approaching or receding magnet changes the direction of the current in a solenoid.
Lenz’s Law states that “The magnetic field of the induced current opposes the change in
the applied magnetic field”. In other words, as a magnet approaches a solenoid or vice
versa, the current moves away from the magnet. When a magnet moves away from the
solenoid, the current moves toward the magnet.
To calculate the magnitude of the emf (electromotive force), you must use Faraday’s
Law of magnetic induction. For a single loop of a circuit, this may be expressed as:
emf = -N[AB (cos ) ]
t
where N = number of loops in the circuit
A= circuit loop area
B = magnetic field normal to plane of loop
If the field is perpendicular to the loop, then the angle of B is zero and the cosine of zero
is one.
With an electrical generator, mechanical energy to turn a loop is converted to electrical
energy.
This is like one of those Honda gasoline generators used by folks when their electricity is
out.
The output of a typical generator has a sinusoidal wave pattern. The emf thus alternates
from positive to negative, resulting in an output current from the generator which changes
direction at regular intervals. This variety of current is called alternating current or ac.
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The important quantity for current in the ac circuit is the rms current. The rms (or rootmean-square) current is the same as the amount of direct current that would dissipate the
same energy in a resistor as is dissipated by the instantaneous-alternating current over a
complete cycle.
Alternating current can be converted to direct current (dc).
Motors are devices that convert electrical energy to mechanical energy.
A current of 100 milliamps can be fatal. If the current is larger than about 10 mA, the
hand muscles contract and the person may be unable to let go of the wire.
Never use electrical appliances near water or with wet hands. Use a battery-powered
radio near water because batteries cannot supply enough current to harm you.
A transformer is used to change one ac potential difference (V1) to a different ac
potential difference (V2).
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