In the circuit represented in Fig. 268 the energy expended between the points M and N in 1 minute (60 seconds) is 8 × 2 × 60 = 960 joules.
291. Electric Power.—Since power refers to the time rate at which work is done or energy expended, it may be computed by dividing the electrical energy by the time, or the electrical power = volts × amperes. The power of 1 joule per second is called a watt. Therefore,
Watts = volts × amperes, or
Watts = E × I.
Other units of power are the kilowatt = 1000 watts and the horse-power = 746 watts. In the example given in Art. 290 the power of the current is 8 × 2 = 16 watts, or if the energy of the current expended between the joints M and N were converted into mechanical horse-power it would equal 16/746 of a horse-power. Electrical energy is usually sold by the kilowatt-hour, or the amount of electrical energy that would exert a power of 1000 watts for one hour, or of 100 watts for 10 hours, or of 50 watts for 20 hours, etc.
Important Topics
1. The storage battery, its construction, electrolyte, action, uses, advantages, disadvantages.
2. Electric energy, unit value, how computed?
3. Electric power, three units, value, how computed, how sold?
Exercises
1. In what three respects are voltaic and storage cells alike? In what two ways different?