Transcript Electricity - Ccphysics.us

Electricity
Static
Source of Electric Charge
 The Greek Thales noted that when rubbed
by a cloth, amber (small a) would attract
small particles.
 The Greek word for amber (small a) is
elektron,
 The only charge we’re concerned with in
discussions of basic electricity.
Positive and Negative
 Positive charges do exist, but most are
locked in the nucleus of the atom.
 Positive in electricity means less negative.
 Abdul’s principle: opposites attract (likes
repel).
 Coulomb’s Law: like Newton’s Law of
Gravity with charge replacing mass.
 Also an inverse square relation.
Electric Field Lines
Conduction
 Moving charges by touching them (more
later).
Conductors and Insulators
 Very similar to the idea in thermodynamics
 No real insulators, just poor conductors.
 Most good conductors are metal: first gold,
silver, then copper being the top three.
 Rubber and glass are good insulators, BUT
 ANYTHING will conduct electricity with
enough push.
Electrostatic devices
 Whimhurst generator (the spinning disks
with the small sparks)
 van der Graaf generator (the big sphere with
the big sparks)
 Electroscope: gold foil strips separated by
excess charges.
Induction
 Moving charges w/o touching (using
Abdul’s principle.)
 Moves the charges in the van der Graaf
generator and the electroscope.
Lightning
 Negative charges collect on the bottom of a
storm cloud in a process similar to a van
der Graaf generator.
 Positive charges collect on the highest
point on the ground.
 Intervening air breaks down and becomes
ionized, capable of carrying electricity.
 Strike starts from the ground and joins the
stroke from the cloud at about 30 feet.
Voltage
 This is the “push” behind moving charges.
 Not the whole story when it comes to
dangerous shocks.
Current Electricity
 AC (alternating current) or DC (direct)
– Westinghouse and Tesla
 DC first used in industry
– Edison
Sources of DC
 Solar cells (also known as
photovoltaic cells).
 Piezoelectric crystals.
 DC generator (discussed later)
 Batteries
Voltaic Pile
Batteries
 First known as the Voltaic Pile
– As in Alessandro Volta
 Alternating layers of two different metals
and wet cardboard.
 Modern batteries made from two different
metals and an acid or alkaline paste.
 Even a citrus fruit will do!
Batteries in Series and Parallel
 Parallel: + to + and – to –
 Like for jump-starting a car.
 More turning ability in a motor.
 Series: + to –
 Like in a flashlight
 More speed in a motor.
Current
 A flow of charge over time
 More charge, or in less time, more current.
 Measured in Amperes
 A small current with enough voltage to push
it IS dangerous!
Ohm’s Law
 George Simon Ohm says that the ratio of
voltage to current is resistance.
 Resistance, measured in Ohms, restricts the
flow of electricity in a circuit.
 Any electrical device—motor, light,
clock—has resistance. Even people!
Series and Parallel Circuits
 Think of a water circuit with a pump, hoses,
and someone to step on the hose.
 Only one path for the series circuit;
therefore the resistance builds up.
 Many paths in a parallel circuit, therefore
more current can flow.
Fuses
 Small strips of metal in series with a circuit
 If the current gets to be too much, the fuse
melts, breaking the circuit.
Short and Open Circuits
 A short circuit is where a conductor
connects the power source (battery) back
onto itself without going into the circuit.
 An open circuit is where a major wire is
broken and current cannot flow (like when a
fuse blows).