Lesson 3/Learning Event 1
From what we have covered so far, you see that the automotive electrical system can be compared
with a water system. Both systems work under pressure, and the pressure can be measured. We
also learned that pressure causes a flow in the system when the system is in use. This flow can
also be measured. The amount of flow in each system depends on the amount of pressure and
how much resistance is in the circuit or system. In the water system, the size of pipes and valves
can create resistance in the system, and the waterflow changes if the pressure is increased or the
resistance is decreased. The flow of electricity in a circuit also depends on several factors. One
important factor is resistance.
How can we know if the resistance in a circuit is correct? One way is to measure the voltage and
amperage, and if the voltage is correct but the amperage is not, you call safely assume the resistance
is wrong. We can measure resistance by itself using an ohmmeter.
OHMMETERS
Anything that moves meets resistance (opposition to movement). Remember what happens to the
space capsule when it returns after a trip in space? It gets so hot it has to have a heat shield to
keep the astronauts from being burned to death. That heat is caused by the resistance of air as the
space capsule moves through it. The faster the capsule moves, the greater the resistance of the air,
and resistance to motion always creates heat.
Resistance (opposition) to the movement of electricity in a wire also produces heat. Sometimes we
want a lot of resistance in a wire. For example, the wires in the burners on an electric stove get red
hot, and the wires in a light bulb get hot enough to shine and produce light. At other times, we
want as little resistance as possible in an electrical circuit, because the resistance does cause heat and
loss of voltage and reduces the amount of current that can flow.
The unit of measure for electrical resistance is the ohm. The instrument used to measure the
resistance in an electrical circuit is an ohmmeter.
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