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