Static Electricity CHARGE Principles of Physics CHARGE Charge is the ability to attract or repel Q (large charges) q (small charges) Units: coulombs (C) Types of charge: positive or negative neutral → no charge CHARGE Charged particles: Electron – around nucleus Charge = -1 elementary charge (-1e) Proton – in nucleus Charge = +1 elementary charge (+1e) MAGNITUDE OF CHARGE Amount of charge a particle has is proportional to the number of excess electrons or protons Q = Ne Q = charge (C) = q N = number of excess charges e = elementary charge = charge of a proton when + = charge of an electron when - MAGNITUDE OF CHARGE Example: A charged sphere has an excess of 15 protons. What is the charge of the sphere? Q = Ne = +15e Like charges REPEL Unlike charges ATTRACT ELECTROSTATIC FORCE A force exists between all charged bodies When the charges are like the force causes them to move away When the charges are unlike the force causes them to toward each other According to Newton’s 3rd law, if one charged body acts on another, the other acts back on it with the same force F1 +Q F2 F2 +q F1 -q ELECTROSTATIC FORCE A force exists between all charged bodies Inverse relationship between force and distance Force increases when distance decreases Force decreases when distance increases Direct relationship between force and charge Force increases when charge increases Force decreases when charge decreases MATERIALS Conductors: metals lots of loosely connected, easily moved electrons Insulators: plastic, ceramics Tightly bound electrons Semiconductors: Silicon, Germanium Semi-tight electrons – more movement at high temps Conductivity affected by doping (adding materials that help control conduction) GROUND Source (donator) or sink (acceptor) of an infinite amount of charge without significantly changing its own charge Examples: 1. Earth 2. You TRANSFER OF CHARGE Methods 1. Inducing a charge (Induction) Neutral objects can act charged when placed near a charged object 2. Charge by Conduction Charging by touch 3. Permanently charge by induction Charging by touching to ground INDUCING A CHARGE + + + + + + + + + -- -- -- -- -- -- -- -- -- When a negatively charged object is brought near a neutral object, electrons move away from it The object attract because the unlike charges are near + + -- ---+ -- + + --+ -- + + -- + + + --+ -+ + -- + -+ + -- + -+ + -- When a positively charge object is brought near a neutral object, electrons in the neutral object move toward it The objects attract because unlike charges are near WHY DOES THE BALLOON STICK TO THE WALL? Balloon (rubbed over hair) Wall (Neutral) The charges in the wall separate CHARGE BY CONDUCTION Charge transfers directly from one object to another Results in equal charge on all objects Charges distribute evenly between objects CHARGE BY CONDUCTION Example: A sphere with a charge of -4e touches a sphere with a charge of -6e. What is their charge after they separate again? Together they have: -6e + -4e = -10e After they separate they each have: -10e/2 = -5e CHARGE BY CONDUCTION Example: A sphere with a charge of -4e touches a sphere with a charge of +8e. What is their charge after they separate again? Together they have: +8e + -4e = +4e After they separate they each have: +4e/2 = +2e PERMANENTLY CHARGING BY INDUCTION Charging by touching to ground PERMANENTLY CHARGING BY INDUCTION Transfer of opposite charge, but same amount Example: If charged object has a charge of +24e, then object charged by induction will have -24e.