reading this section you will be able to do the following:
resistance and how we measure it.
the similarities between resistance in a wire and the resistance
in a water hose.
There is another important property
that can be measured in electrical systems. This is resistance,
which is measured in units called ohms. Resistance is a
term that describes the forces that oppose the flow of electron
current in a conductor. All materials naturally contain some resistance
to the flow of electron current. We have not found a way to make
conductors that do not have some resistance.
If we use our water analogy to help
picture resistance, think of a hose that is partially plugged
with sand. The sand will slow the flow of water in the hose. We
can say that the plugged hose has more resistance to water flow
than does an unplugged hose. If we want to get more water out
of the hose, we would need to turn up the water pressure at the
hydrant. The same is true with electricity. Materials with low
resistance let electricity flow easily. Materials with higher
resistance require more voltage (EMF) to make the electricity
The scientific definition of one ohm
is the amount of electrical resistance that exists in an electrical
circuit when one amp of current is flowing with one volt
being applied to the circuit.
Is resistance good
Resistance can be both good and bad.
If we are trying to transmit electricity from one place to another
through a conductor, resistance is undesirable in the conductor.
Resistance causes some of the electrical energy to turn into heat
so some electrical energy is lost along the way. However, it is
resistance that allows us to use electricity for heat and light.
The heat that is generated from electric heaters or the light
that we get from light bulbs is due to resistance. In a light
bulb, the electricity flowing through the filament, or the tiny
wires inside the bulb, cause them to glow white hot. If all the
oxygen were not removed from inside the bulb, the wires would
An important point to mention here
is that the resistance is higher in smaller wires. Therefore,
if the voltage or EMF is high, too much current will follow through
small wires and make them hot. In some cases hot enough to cause
a fire or even explode. Therefore, it is sometimes useful to add
components called resistors into an electrical circuit
to restrict the flow of electricity and protect the components
in the circuit.
Resistance is also good because it
gives us a way to shield ourselves from the harmful energy of
electricity. We will talk more about this on the next page.
is the opposition
to electrical current.
is measured in units called ohms.
is sometimes desirable and sometimes undesirable.