Concept Summary. Potential Difference Current A sustained flow of electric charge past a point is...

Concept Summary

Potential Difference

CurrentA sustained flow of electric charge past

a point is called an electric current.Specifically, electric current is the rate

that electric charge passes a point, so

Current = or I = q/tCharge

time

Measuring Current If 1 Coulomb of charge (6.25 x 1018

electrons) passes a point each second, the current is 1 Ampere.

So, 1 Ampere = 1 Coulomb/sec

Voltage SourceA battery or electrical outlet is a source

of electric potential or voltage - not charge.

The electrons that move in a conductor are supplied by the conductor - not the voltage source.

The net charge on a current-carrying conductor is zero.

Electromotive ForceAn old-fashioned term for electric

potential or voltage is “electromotive force” or “emf”.

Electrical ResistanceMost materials offer some resistance to

the flow of electric charges through them. This is called electrical resistance.

ResistanceResistance of a conductor depends on:

Material - Gold is best Length - longer conductors have more

resistance. Cross section - thick wires have less

resistance than thin wires Temperature - higher temperature means

more resistance for most conductors

Ohm’s LawFor many conductors, current depends

on:Voltage - more voltage, more current

Current is proportional to voltageResistance - more resistance, less

current Current is inversely proportional to

resistance

Ohms’ Law In symbols:

V = IRVI R

Direct Current If the voltage is maintained between

two points in a circuit, charge will flow in one direction - from high to low potential. This is called direct current (DC)

Battery-powered circuits are dc circuits.

Alternating Current If the high & low voltage terminals

switch locations periodically, the current will flow “back and forth” in the circuit. This is called alternating current (AC).

Circuits powered by electrical outlets are AC circuits.

AC in the US In the US, current changes direction

120 times per second, for a frequency of 60 cycles per second or 60 Hertz.

Normal outlet voltage in the US is 110-120 volts, although some large household appliances run on 220-240 volts.

Converting AC to DCAC is converted to DC using devices

called diodes, which allow charges to move in only 1 direction.

Speed of Electrons

Electrons in a circuit do not move quickly - they actually “drift” at about 1 mm/s.

It is the electric field that moves quickly - at about the speed of light - through the circuit and carries the energy.

Electric PowerPower = energy/time = current x voltage

P = IV = I2R 1 Watt = (1 Amp)(1 Volt) 1 kilowatt = 1000 Watts A kilowatt-hour is a unit of energy

The End