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# Lect_30 - Lecture 30 Electric Charge Coulomb's Law The...

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Lecture 30 Electric Charge Coulomb’s Law The facts When some objects are rubbed with fur, tissue paper, certain fabrics, etc, they sometimes attract or repel each other. The repulsive/attractive force depends on the distance between the objects, on the materials used, on how hard you rub… DEMO: Rods, balloons… The proposed model Benjamin Franklin proposed that with the rubbing, objects acquired some kind of “ electric charge ”. There are two types of electric charge, which he called positive and negative . The force works in the following way: equal charges repel each other opposite charges attract each other the force gets weaker as the distance between the charged objects increases Structure of matter Later on it was established that matter is made of electrons, protons and neutrons. neutrons have no charge so we won’t worry about them protons are positive and are more or less fixed in their position electrons are negative and some of them are more of less free to move around. Most of the time, # of protons = # of electrons, so objects are neutral.

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Charging an object When you rub a rod with a fur, a fraction of the surface electrons in one object is transferred to the other object. Gain (loss) of electrons is equivalent to loss (gain) of protons. (This is not what usually happens, but sometimes it is easier to think it this way). Gain of electrons negatively charged object Loss of electrons positively charged object - + + + - - + + - - Negative rod Positive fur Example: ACT: Two charges A charged ball Q 1 is fixed on a horizontal surface. When another ball of charge Q 2 and mass m 2 is brought near, it achieves equilibrium at a distance d directly above Q 1 .
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