The basic unit of matter is the atom, which is made up of protons, electrons,
and other particles. Protons and electrons have a property called
electric charge; electrons have a negative electric charge and protons have a
positive electric charge. The charges interact, and like charges repel and unlike charges attract.
Electrons can be moved and electrostatic charge, or static electricity,
results from a surplus or deficiency of electrons.
A quantity of charge (q) is measured in units of coulombs (C), the
charge equivalent to the transfer of 6.24 x 1018 charged particles such as
the electron. The fundamental charge of an electron or proton is 1.60 x
10-19 coulomb. The electrical forces between two charged objects can be
calculated from the relationship between the quantity of charge and the
distance between two charged objects. The relationship is known as
Coulomb’s law.
A flow of electric charge is called an electric current (I). Current (I) is
measured as the rate of flow of charge, the quantity of charge (q) through
a conductor in a period of time (t). The unit of current in coulomb/second
is called an ampere, or amp for short (A).
An electric circuit has some device that does work in moving charges
through wires to do work in another part of the circuit. The work done
and the size of the charge moved defines voltage. A volt (V) is the ratio of work to charge moved, V = W/q. The ratio of volts/amps in a circuit is
the unit of resistance called an ohm. Ohm's law is V = IR.
Disregarding the energy lost to resistance, the work done by a voltage
source is equal to the work accomplished in electrical devices in a circuit.
The rate of doing work is power, or work per unit time, P = W/t.
Electrical power can be calculated from the relationship of P = IV, which gives the power unit of watts.
Magnets have two poles about which their attraction is concentrated.
When free to turn, one pole moves to the north and the other to
the south. The north-seeking pole is called the north pole and the southseeking
pole is called the south pole. Like poles repel one another and unlike
poles attract.
A current-carrying wire has magnetic field lines of closed, concentric
circles that are at right angles to the length of wire. The direction of
the magnetic field depends on the direction of the current. A coil of
many loops is called a solenoid or electromagnet. The electromagnet is
the working part in electric meters, electromagnetic switches, and the
electric motor.
When a loop of wire is moved in a magnetic field, or if a magnetic
field is moved past a wire loop, a voltage is induced in the wire loop. The
interaction is called electromagnetic induction. An electric generator is a
rotating coil of wire in a magnetic field. The coil is rotated by mechanical
energy, and electromagnetic induction induces a voltage, thus converting
mechanical energy to electrical energy. A transformer steps up or steps
down the voltage of an alternating current. The ratio of input and output
voltage is determined by the number of loops in the primary and secondary
coils. Increasing the voltage decreases the current, which makes
long-distance transmission of electrical energy economically feasible.
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