Lesson 1/Learning Event 2
When the voltage forces the current to flow in a wire, the electrons meet RESISTANCE.
Resistance is caused by the friction of the electrons bumping into the atoms. If you rub your
fingertips across a table, the friction causes heat. As you move your fingertips faster, the heat
becomes greater. Likewise, electrons flowing through a wire cause heat. If the voltage is increased,
current flow and the amount of heat are also increased. If the current flow is increased enough,
the wire will become hot enough to literally burn up.
The heating action of current flow is one of the great uses of electricity. For an example, let's take
a look at a light bulb.
The bulb contains a filament made of a material that has a lot of resistance to current flow and can
withstand extreme heat. The filament ends are connected to two contacts at the base of the bulb.
(Usually the metal part of the base serves as one contact.) The filament is then enclosed in glass,
and most of the air is removed, because if air gets to the filament, it will burn up too easily.
Voltage from a battery or other source is applied to the two contacts at the base of the bulb.
Current then flows through the filament causing it to get white-hot which produces light.
For the bulb to function properly the material in the filament, the size of the filament, and the
amount of voltage supplied must be carefully balanced so just the correct amount of current will
flow. If the current flow is too small, the filament will not get heated enough and the bulb will not
glow brightly. If the current flow is too great, the filament will burn up.
Sometimes a special part known as a RESISTOR is placed in the electrical circuit to reduce the
current flow. The action of the resistor can be compared to the restricting action of a water valve
or faucet. Opening the valve more will cause more water to flow because there will be less
resistance to the flow of water. Likewise, reducing the resistance in an electrical circuit will cause
more electrical current to flow.
Resistors are usually made from carbon or special wire. Some resistors are "fixed"; that is, they are
made so you cannot change the resistance as you can with the faucet in a water system. Resistors
that you can adjust are known as "variable" resistors or RHEOSTATS. A rheostat usually has a
movable contact that you can move along the length of a resistor. By moving the contact, you
change the effective length of the resistor. The greater the distance the current travels to get
through the resistor, the greater the resistance of the rheostat.