ker88839_ch02.qxd
1/9/06
11:22 AM
Page 30
30
Chapter Two
our range of hearing. These ultrasonic vibrations are used, among other things, to detect sonar reflections from submarines and to drill holes in diseased teeth.
1 Horsepower = a Horse pulling a 200 lb weight 165 feet in 1 minute
165 feet Time 1 Minute
HISTORICAL FIGURES IN ELECTRICITY In 1767, Joseph Priestly established that electrical charges attract with a force inversely proportional to distance. In 1800, Alessandro Volta invented the first battery. Michael Faraday (1791–1867) opened the doors of the science we now know as electromagnetism when he published his law of induction, which simply states that a magnetic field induces an electromotive force in a moving conductor. Thomas Edison (1847–1931) invented the incandescent lamp in 1879, but perhaps even more importantly, built the first central power station and electrical distribution system in New York City in 1881. This provided a means of introducing electrical power into industry and the home. The discovery of the electron by J.J. Thomson (1856–1940) in 1897 introduced the science of electronics and quickly resulted in the invention of the diode (1904), the triode (1907), and the transistor (1946). Andre Marie Ampere established the importance of the relationship between electricity and magnetism. In 1800, Alessandro Volta discovered that if two dissimilar metals were brought in contact with a salt solution, a current would be produced, this invention is now known as the battery. The German physicist George Simon Ohm (1787–1854) proved the mathematical relationship between electrical potential (voltage), electrical current flow (measured in amperes) and the resistance to the current flow (measured in ohms: symbol Ω). Another person who influenced electrical technology was a Scottish inventor named James Watt (1736–1819). James Watt worked in coal mines and saw the power of a horse as it
200 time 165 = 33,000 lb/ft of work 1 horse can do 33,000 lb/ft of work in 1 minute
Figure 2-18.
200 Pounds
Horsepower.
was used to lift coal from deep in the earth. He developed the steam engine to take over the task of lifting heavy loads instead of using the power of a horse. At the same time, he calculated the work that a horse could do and determined that a horse could walk 165 feet in one minute pulling a 200-pound weight (165 ft. 200 lb. 33,000 ft.-lb. per minute, or 550 foot-pounds of work per second) and called this amount of work one horsepower. One horsepower is needed to lift 550 pounds 1 foot off the ground in 1 second, one horsepower equals 33,000 foot-pounds of work per minute. The term brake horsepower comes from the method of testing the early engines. In the metric system, the power of engines is measured in watts or kilowatts after James Watt. Watt’s Law states that a watt is the power done by moving one ampere through a resistance of one ohm using one volt in one second. Horsepower can also be expressed in units of electrical power or watts; the simple conversion is 1 horsepower = 746 watts. The term watt is most commonly used to express electrical power, such as the wattage of light bulbs. A light bulb is an example of where watts are commonly used. A 100-watt light bulb requires more electrical power to light than a 60watt bulb. Electricity is sold in kilowatt hours. A kilowatt is 1000 watts and a kilowatt hour is one kilowatt of power being used for one hour.
