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Current flowing through a poor conductor produces heat by an effect
similar to mechanical friction. That heat represents energy that
comes from the charge traveling across the voltage difference.
Remember that separated charges have the potential to do
work and provide energy.
The work involved in heating a resistor is not very useful,
unless we are making a hotplate; rather it is a byproduct of
restricting the current flow.
Power is measured in units of watts (W), named after James
Watt, the Englishman who invented the steam engine, a device
for producing lots of useful power.
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The power that is released into the resistor as heat
can be calculated as P=VI , where I is the current flowing
through the resistor and V is the voltage across it. Ohm's law relates
these two quantities, so we can also calculate the power as The power
produced in a resistor raises its temperature and can change its value
or destroy it. Most resistors are air-cooled and they are made with
different power handling capacity. The most common values are 1/8, 1/4,
1, and 2 watt resistors, and the bigger the wattage rating, the bigger
the resistor physically. Some high power applications use special water
cooled resistors. Most of the resistors on the RoboBoard are 1/8 watt.
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