black 0
brown 1
red 2
orange 3
yellow 4
green 5
blue 6
violet 7
gray 8
white 9
silver ±10%
gold ±5%l/Color codes used on resistors.m/An example of a resistor
with a color code.n/The symbol used in schemat-
ics to represent a resistor.short-circuit: a circuit in which a low-resistance pathway connects
the two sides of a voltage source. Note that this is much more
specific than the popular use of the term to indicate any electrical
malfunction at all. If, for example, you short-circuit a 9-volt battery
as shown in figure k, you will produce perhaps a thousand amperes
of current, leading to a very large value ofP=I∆V. The wire gets
hot!
self-check A
What would happen to the battery in this kind of short circuit?.
Answer, p. 1058
At this stage, most students have a hard time understanding why
resistors would be used inside a radio or a computer. We obviously
want a lightbulb or an electric stove to have a circuit element that
resists the flow of electricity and heats up, but heating is undesirable
in radios and computers. Without going too far afield, let’s use a
mechanical analogy to get a general idea of why a resistor would be
used in a radio.
The main parts of a radio receiver are an antenna, a tuner for
selecting the frequency, and an amplifier to strengthen the signal suf-
ficiently to drive a speaker. The tuner resonates at the selected fre-
quency, just as in the examples of mechanical resonance discussed in
- The behavior of a mechanical resonator depends on three things:
its inertia, its stiffness, and the amount of friction or damping. The
first two parameters locate the peak of the resonance curve, while
the damping determines the width of the resonance. In the radio
tuner we have an electrically vibrating system that resonates at a
particular frequency. Instead of a physical object moving back and
forth, these vibrations consist of electrical currents that flow first
in one direction and then in the other. In a mechanical system,
damping means taking energy out of the vibration in the form of
heat, and exactly the same idea applies to an electrical system: the
resistor supplies the damping, and therefore controls the width of
the resonance. If we set out to eliminate all resistance in the tuner
circuit, by not building in a resistor and by somehow getting rid of
all the inherent electrical resistance of the wires, we would have a
useless radio. The tuner’s resonance would be so narrow that we
could never get close enough to the right frequency to bring in the
station. The roles of inertia and stiffness are played by other circuit
elements we have not discusses (a capacitor and a coil).
Resistors
Inside any electronic gadget you will see quite a few little circuit
elements like the one shown below. Theseresistorsare simply a
cylinder of ohmic material with wires attached to the end.
Many electrical devices are based on electrical resistance and
Ohm’s law, even if they do not have little components in them thatSection 9.1 Current and voltage 545