In our every day lives we use electrical energy to do work for us. This electrical energy is provided either by our power company through the outlets in our wall, or we can buy stored electrical energy in the form of batteries. Different electrical appliances use electrical energy at different rates. Power is the rate at which work is done. The power rating of an appliance tells us the rate at which it uses electrical energy. For example, a 1500 watt hair dryer uses 1500 joules of electrical energy per second. In any electrical appliance their is a simple relationship between the power output of a component, the potential difference across that component, and the current running through it. This is because charge times potential difference is energy, so (charge per unit time) times (potential difference) gives (energy per unit time). Thus, we have P=VI. Substituting V=IR from ohm's law, the power relationship can be rewritten as P=I2R. From this formula, the resistance can be calculated.
