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power can be calculated from the relationship of P = IV, which gives SUMMARY OF EQUATIONS
the power unit of watts.
6.1
Magnets have two poles about which their attraction is concen-
quantity of charge = (number of electrons)(electron charge)
trated. When free to turn, one pole moves to the north and the other
q = ne

to the south. The north-seeking pole is called the north pole and the
south-seeking pole is called the south pole. Like poles repel each other 6.2
charge on
charge on
and unlike poles attract. ×

one object
second object
The property of magnetism is electric in origin, produced by ___

electrical

charges in motion. Permanent magnets have tiny regions called force = (constant) × distance between objects squared
magnetic domains, each with its own north and south poles. An _
q q
1 2
F = k
unmagnetized piece of iron has randomly arranged domains. When 2
d
magnetized, the domains become aligned and contribute to the overall 9 2 2
where k = 9.00 × 10 newton∙ meters / coulomb
magnetic eff ect.
6.3

A current-carrying wire has magnetic field lines of closed, concen-
work to create potential
tric circles that are at right angles to the length of wire. Th e direction __

electric potential =
of the magnetic field depends on the direction of the current. A coil charge moved

of many loops is called a solenoid or electromagnet. Th e electromagnet W _
V =
q
is the working part in electrical meters, electromagnetic switches, and
the electric motor. 6.4
quantity of charge
When a loop of wire is moved in a magnetic field, or if a magnetic __

electric current =

field is moved past a wire loop, a voltage is induced in the wire loop. time
The interaction is called electromagnetic induction. An electric genera- _
q

I =
tor is a rotating coil of wire in a magnetic fi eld. The coil is rotated by t

mechanical energy, and electromagnetic induction induces a voltage, 6.5
thus converting mechanical energy to electrical energy. A transformer volts = current × resistance
steps up or steps down the voltage of an alternating current. Th e ratio V = IR
of input and output voltage is determined by the number of loops in 6.6
the primary and secondary coils. Increasing the voltage decreases the
electrical power = (amps)(volts)
current, which makes long-distance transmission of electrical energy
P = IV
economically feasible.
Batteries connected in series will have the same current and the 6.7
(watts)(time)(rate)
voltages add. In parallel, the voltage in the circuit is the same as each __

cost =
source, and each battery contributes a part of the total current. 1,000 W/kW
A series circuit has resistances connected one aft er the other so 6.8
volts

volts
the same current flows through each resistance one after the other. A __ ___

primary
secondary
=

parallel circuit has comparable branches, separate pathways for the (number of loops) primary (number of loops)
current to flow through. As more resistances are added to a paral- secondary

V p
V s
_ _

lel circuit, it has an increase in the current, the same voltage is main- =
N p N s
tained across each resistance, and total resistance of the entire circuit
6.9
is lowered.
Household circuits are parallel circuits, so each appliance has ( volts primary )( current primary ) = ( volts secondary )( current secondary )
the same voltage available to do work and each appliance draws V p I p = V s I s
current according to its resistance. Fuses and circuit breakers pro- 6.10 Resistances in series circuit
tect circuits from overheating from overloads or short circuits. A R = R + R + R + …

1
3
2
total
short circuit is a new path of lesser resistance. Other protective
6.11 Resistances in parallel circuit
devices are three-pronged plugs, polarized plugs, and ground-fault 1 _ _ _ _
1
1
1
= + + +
interrupters. R R R R …
3
1
total
2
6-33 CHAPTER 6 Electricity 171