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== Current and Power and Energy
== The unit used to measure the flow of electricity, or current, is the ampere (A). The unit to measure power is the watt (W). Energy is measured in Joules (J). Amperes are really a measure of a rate of electron flow (electrons per second), or equivalently, how much charge is passed per second (Coulombs per second). 1 A = 1 Coulomb per second, and 1 electron has a charge of 1.602 * 10-19 Coulombs.
Power is measure in watts. A watt is an amount of energy per second, or Joules per second (1 W = 1 J/s). The number of watts is for an electrical circuit given by the number of amperes times the voltage drop: 1 W = 1 V * 1 A.
A power company supplying electricity wants to know how much energy (that's Joules) you have used. How much power (that's Watts) you use changes during the day... you turn on the toaster or microwave, your power usage goes up, and at night, you power usage goes down. But the amount of energy you use is the sum of that averaged over time (more precisely, it is the integral of your power consumption, or the area under your power consumption curve).
Energy is measured in Joules (in SI units), but power companies use funny units of kilowatt hours (kW hr). Since a watt is just a joule per second (J/s), a kilowatt hour is a measure of energy. Here is how you convert:
1 kW hr = 1000 W hr = 1000 J/s hr = 1000 J/s * 3600 s = 3.6 * 106 J or 3.6 million Joules
The power company doesn't use Joules on your electricity bill because we use a lot of them... instead they use kilowatt hours... but that's just a bunch of Joules.



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