Electricity Static

Source of Electric Charge 4 The Greek Thales noted that when rubbed by a cloth, amber (small a) would attract small particles. 4 The Greek word for amber (small a) is elektron, 4 The only charge we’re concerned with in discussions of basic electricity.

Positive and Negative 4 Positive charges do exist, but most are locked in the nucleus of the atom. 4 Positive in electricity means less negative. 4 Abdul’s principle: opposites attract (likes repel). 4 Coulomb’s Law: like Newton’s Law of Gravity with charge replacing mass. 4 Also an inverse square relation.

Electric Field Lines

Conduction 4 Moving charges by touching them (more later).

Conductors and Insulators 4 Very similar to the idea in thermodynamics 4 No real insulators, just poor conductors. 4 Most good conductors are metal: first gold, silver, then copper being the top three. 4 Rubber and glass are good insulators, BUT 4 ANYTHING will conduct electricity with enough push.

Electrostatic devices 4 Whimhurst generator (the spinning disks with the small sparks) 4 van der Graaf generator (the big sphere with the big sparks) 4 Electroscope: gold foil strips separated by excess charges.

Induction 4 Moving charges w/o touching (using Abdul’s principle.) 4 Moves the charges in the van der Graaf generator and the electroscope.

Lightning 4 Negative charges collect on the bottom of a storm cloud in a process similar to a van der Graaf generator. 4 Positive charges collect on the highest point on the ground. 4 Intervening air breaks down and becomes ionized, capable of carrying electricity. 4 Strike starts from the ground and joins the stroke from the cloud at about 30 feet.

Voltage 4 This is the “push” behind moving charges. 4 Not the whole story when it comes to dangerous shocks.

Current Electricity 4 AC (alternating current) or DC (direct) –Westinghouse and Tesla 4 DC first used in industry –Edison

Sources of DC 4 Solar cells (also known as photovoltaic cells). 4 Piezoelectric crystals. 4 DC generator (discussed later) 4 Batteries Voltaic Pile

Batteries 4 First known as the Voltaic Pile –As in Alessandro Volta 4 Alternating layers of two different metals and wet cardboard. 4 Modern batteries made from two different metals and an acid or alkaline paste. 4 Even a citrus fruit will do!

Batteries in Series and Parallel 4 Parallel: + to + and – to – 4 Like for jump-starting a car. 4 More turning ability in a motor. 4 Series: + to – 4 Like in a flashlight 4 More speed in a motor.

Current 4 A flow of charge over time 4 More charge, or in less time, more current. 4 Measured in Amperes 4 A small current with enough voltage to push it IS dangerous!

Ohm’s Law 4 George Simon Ohm says that the ratio of voltage to current is resistance. 4 Resistance, measured in Ohms, restricts the flow of electricity in a circuit. 4 Any electrical device—motor, light, clock—has resistance. Even people!

Series and Parallel Circuits 4 Think of a water circuit with a pump, hoses, and someone to step on the hose. 4 Only one path for the series circuit; therefore the resistance builds up. 4 Many paths in a parallel circuit, therefore more current can flow.

Fuses 4 Small strips of metal in series with a circuit 4 If the current gets to be too much, the fuse melts, breaking the circuit.

Short and Open Circuits 4 A short circuit is where a conductor connects the power source (battery) back onto itself without going into the circuit. 4 An open circuit is where a major wire is broken and current cannot flow (like when a fuse blows).