<text><span class="style10">lectro-magnetism</span><span class="style7"> </span><span class="style10">(2 of 5)Static electric charges</span><span class="style7">In dry weather, a woolen sweater being pulled off over the hair of the wearer may crackle; sparks may even be seen. This is caused by an </span><span class="style26">electric charge</span><span class="style7">, which is the result of electrons being pulled from one surface to the other. Objects can gain an electric charge by being rubbed against another material. Experiment has shown that there are two types of charge. These are now associated with the negative and positive charges on electrons and protons respectively. Similar electric charges (i.e. two positives, or two negatives) repel each other and unlike charges (i.e. a positive and a negative) attract. (Note that the terms `positive' and `negative' are merely conventions for opposite properties.) No smaller charge than that of the electron has been detected (but see quarks).The force of repulsion or attraction is known as the </span><span class="style26">electric force</span><span class="style7">. It is described by </span><span class="style26">Coulomb's law</span><span class="style7">, an inverse-square law similar to the law for the gravitational force. Coulomb's law states that the attractive or repulsive force (F) between two point (or spherically symmetrical) charges is given by: where k is a constant, Q1 and Q2 are the magnitudes of the charges, and r is the distance between them. The force acts along the direction of r. The unit of charge is called a </span><span class="style26">coulomb</span><span class="style7"> (C) and is the quantity of electric charge carried past a given point in 1 second by a current of 1 ampere.</span></text>
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<text><span class="style10">. Electric fields</span><span class="style7"></span></text>
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<text>ΓÇó STARS AND GALAXIESΓÇó QUANTUM THEORY AND RELATIVITYΓÇó WAVE THEORYΓÇó OPTICSΓÇó ELECTRICITY IN ACTIONΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó THE EARTH'S STRUCTUREΓÇó MEDICAL TECHNOLOGYΓÇó RADIO, TELEVISION AND VIDEOΓÇó SEEING THE INVISIBLE</text>