Physics 242 – Electric Charges Types and Sources of Electric Charge Two kinds of electric charge – positive negative electrons protons Bohr Model of Atom Quantum Mechanical Model Materials with equal numbers of neg. and pos. charge are said to be electrically neutral.

Physics 242 – Electric Charges Types and Sources of Electric Charge THANKS BEN! Ben Franklin actually did a great deal of experimentation in the area of electricity. However, Ben Franklin chose the wrong sign convention for charge – it has stuck ever since.

Physics 242 – Electric Charges Properties of Electric Charge Charge is quantized. Positive and Negative charges have same magnitude e = p = 1.6 x C SI unit of charge is the Coulomb Charge is conserved. p → e + π 0 Proton Decay

Physics 242 – Electric Charges Atoms and Materials Classification of Materials – Conductors, Insulators, Semiconductors Conductors Electrons are free to move about the material Insulators Electrons are bound to the atoms - can’t move (conduct). Different elements will have different types of bonds – different strengths.

Physics 242 – Electric Charges How do we charge an object? A. Tribocharging - Charging by rubbing Positive (+) Air Human Hands Rabbits Fur Glass Human Hair Mica Wool Cats Fur Silk Aluminum Paper Cotton Rubber Balloon Hard Rubber Gold/Platinum Polyester Vinyl Saran Wrap Negative (-) Triboelectric Series Electrostatic Voltages Means of Static Generation10 – 20 % Humidity 65 – 90 % Humidity Walking across carpet35,0001,500 Walking across vinyl floor12, Worker at bench6, Vinyl envelopes for work instructions Common poly bag picked up from bench 20,0001,200 Careful when you fill up your gas tank!

Physics 242 – Electric Charges How do we charge an object? A. Tribocharging - Charging by rubbing Conductors – charge will distribute around the surface to try until equilibrium is achieved. - charge only resides on surface, not inside. Insulators – charge cannot move, so it remains localized around the contact region. On symmetric objects, charge distributes uniformly On non-symmetric objects, charge collects at sharp points.

Physics 242 – Electric Charges How do we charge an object? B. Induction – Inducing a charge without touching. Conductors Charging by induction (two conductors) Charging by induction (1 conductor and ground) Charge is physically moved in/on the materials.

Physics 242 – Electric Charges Conductors – An Example +3q -3q +3q Outside the conductor, the net charge is +3q Sphere equivalent to a point charge Positive charge doesn’t move, neg. charge does all the moving.

Physics 242 – Electric Charges How do we charge an object? B. Induction – Inducing a charge without touching. Insulators Free Standing Atoms are randomly oriented Charge does not actually move In/on the insulator – redistributes. Polarizes the material.