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 flow.
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 or bad?
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 burn up.
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.
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