Work and Energy
Energy is one of the fundamental conserved physical quantities found in nature. Potential energy is the energy resulting from an object's position when it is acted on by a conservative force. Kinetic energy is the energy of motion. The sum of the potential and kinetic energy of an object is its mechanical energy. Energy is a scalar rather than a vector quantity.
Physicists find the amount of work by multiplying the amount of force applied by the parallel distance over which the force is applied. Energy is required in order to do work.
Work and all forms of energy are measured in units of joules. One joule is defined as one newton meter. Think of a joule as roughly the amount of work or energy required to lift an apple from the floor to a table.
Power
Climbing a flight of stairs requires a certain amount of work to overcome the gravitational force. This amount of work is the same whether one climbs the stairs slowly or runs up the stairs quickly. However you are more likely to be breathing heavily after running up the flight of stairs than after climbing them slowly. Why?
The difference is power. The total work or energy is the same either way, but running up the stairs quickly requires more power than walking up slowly.
The average power is the total energy used divided by the time over which the energy is used. Power = Energy/Time, or Power = Work/Time. Because work and energy are measured in joules and time is measured in seconds power is measured in joules per second. One joule per second is defined as one watt.
To get a feel for how much a watt is look at a light bulb. A typical light bulb might be a 100 watt bulb. That means it uses a power of 100 watts, or it uses energy at the rate of 100 joules each second. Similarly if an electric power plant is rated as a 500 megawatt power plant, it has a power output of 500 megawatts, which is 500 million watts. That means the power plant produces 500 million joules of energy each second.
Because power is the rate at which energy is used or generated, power already takes time into account (joules per second). Therefore it is not correct to specify how long it takes a 500 megawatt power plant to generate 500 megawatts of electricity. Saying that it generates 500 megawatts means that it generates 500 megajoules in a second.
What Are Kilowatt Hours
A kilowatt is 1000 watts and is a unit of power. An hour is a time unit. A kilowatt hour is therefore a power multiplied by a time. Power multiplied by time is energy, so a kilowatt hour is a unit to measure energy not power. Because there are 3600 seconds in an hour, 1 kilowatt hour equals 3,600,000 joules. Electric companies use kilowatt hours to measure power consumption, but scientists seldom use kilowatt hours. They prefer to measure energy in joules.
Power is a way to measure the rate at which energy is used or work is done.
Further Reading
Knight, R.D., Physics for Scientists and Engineers with Modern Physics, Pearson, 2004.
