Thursday, December 26, 2013

What is Power and Energy? Part 2

In part 1 (click here to review) of this two-part series on power and energy, I delved into the specifics of what energy is about. But what then exactly is power?

We learned in part 1 how energy can be increased or extracted from a system by work applied to or derived from that system. Work performed changed the energy level in that system. But how long a period was that work performed over? Perhaps it was performed over a minute, a day or a year? Power is a measure of the rate of which work is performed and energy added or removed from a system.

Average power = work performed / interval of time

When we hear the word power the thing that might come to the mind most often is the horsepower one’s car has (OK, let me preface that with the mind of most auto enthusiasts!).  While most commonly used to refer to mechanical systems, horsepower is still power, just the same as the electrical power we get from the wall outlets in our homes, which is also power.

Back in the early days of heat engines James Watt developed the term horsepower as means to compare his steam engines to the rate of work a horse could produce.  Mechanical work is the measure of a force (pounds) moved through a distance (feet). A horse was judged to be able to move 550 foot-pounds in one second, or produce 550 foot-pounds per second of power

Electrical power is also a measure of work performed per unit of time. In this case however it is moving an electrical charge of one coulomb against a potential of one volt in one second. Note also that one ampere equals one coulomb per second. One unit of electrical power equals one watt (in honor of James Watt!).  To summarize:
P (watts) = Q (coulombs) * V (volts) / t (seconds) = I (amps) * V (volts)

Recall in part 1how energy was measured in watt-seconds and kilowatt-hours. Divide by time interval it is used over and it becomes power in watts and kilowatts! So how are mechanical and electrical power related? Well when electrical motors came onto the scene it was necessary to relate the work they could do to that of heat engines which were rated in horsepower, where one horsepower is equal to 550 foot-pounds/ second. It was determined that a 100% efficient motor required 746 watts of electrical power to produce one horsepower of mechanical power. Note that this horsepower rating is based on English mechanical terms. The measure of horsepower based on metric terms ends up being slightly different; about 735 watts instead.

So you can just as easily state the power consumption of your electrical appliances in terms of horsepower as you can watts. Conversely, you can also state the power-generating rating of your automobile’s engine in watts (or kilowatts) instead of horsepower, and this is actually more often practiced nowadays, as the measure of a watt is recognized world-wide, while horsepower is not.

Happy Holidays everyone!

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