Electric Current

Flow of Charge Potential difference causes flow of charge Similar to water flowing from high level to lower level Electric current is flow of charge In wires and other solid conductors, electrons are charge carriers In solutions and plasmas, + and - ions can both move

Flow of Charge Unit of current is ampere or amp, abbreviated A, symbol in formulas: I 1 ampere = 1 coulomb/second of charge flowing through a conductor Amount of current depends on potential difference or voltage (pressure) and resistance to flow of charges

Voltage Sources Dry cells (flashlight batteries), wet cells (car batteries), generators, solar cells Supply energy like a pump to move charges Provide sustained potential difference EMF (electromotive force) is potential energy per coulomb of charge moving between battery terminals; creates electrical “pressure”; measured in volts

Resistance Depends on conductivity of circuit elements Long wires have more resistance than short wires Thin wires have more resistance than thick wires Measured in ohms (  ) Due to collisions between electrons and atoms

Ohm’s Law Current = Voltage / Resistance I = V/R or V = IR So 1 ampere = 1 volt / 1 ohm Resistors are circuit elements with a specific resistance; often used to limit current

Electric Shock Current causes injury, not voltage Currents can be high if skin conductivity is high -- wet or salty Must be a potential difference for current to flow -- connection to high voltage not dangerous unless path to ground exists Grounded (3 wire) and polarized plugs help prevent shocks

Direct and Alternating Current DC: flow of charge in one direction only - batteries AC: electrons vibrate back and forth; don’t actually flow through circuit In USA, current alternates at 60 Hz with voltage of 120 V AC can be transmitted for long distances with little loss due to heat

Converting AC to DC Often need DC for electronic devices (stereos, computers, TV, etc.) Diode acts as one way valve turning AC into pulsed DC 2 or more diodes together can provide smoother DC Capacitors also used to smooth out DC signal

Speed of Electrons Electrical signal travels close to light speed, but not electrons Random motion of electrons (thermal motion) due to temperature is fast Motion through circuit is slow: drift speed less than 1 mm/s Collisions with metal ions cause generation of heat in wires

Speed of Electrons In AC, conduction electrons oscillate around fixed point Energy flows through wires, not electrons! Electrons already in wires, lamp filaments, appliances; they vibrate from energy provided by power company

Electric Power Power is work done per unit time Electrical power = current times voltage Unit of power is watt (W); 1W = (1A)(1V) Example: 60 W bulb on 120 V line draws 0.5 A current (W/V =A) Electrical energy use is measured in kilowatt·hours