Current, Power and Resistance

The rate at which a current supplies power to a device is given by
```
            Power = (current)*(voltage)      Watts
```
A device which runs for time t at power level P uses up a total energy
```
            Energy = P*t                     Joules
```
For everyday appliances, a convenient unit for calculating energy usage is the kilowatt-hour:
```
            1 kWh = (1 kilowatt) * (1 hour) = 3,600,000 Joules
```
Sending power over long distances can cause large ohmic losses as the current heats up the wire. It helps to reduce the size of the current, perhaps by boosting the voltage. Devices called transformers can do this, if they are given alternating current.
Alternating current changes the direction of its voltage and current periodically.
In order to calculate the average resistance or average power in an AC circuit, one must use the root-mean-square values of voltage and current:
```
           V(rms)  =  V(peak) / sqrt(2)  =  0.707 V(peak)

           I(rms)  =  I(peak) / sqrt(2)  =  0.707 I(peak)
```