concept summary. potential difference current a sustained flow of electric charge past a point is...
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Concept Summary
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Potential Difference
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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
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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
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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.
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Electromotive ForceAn old-fashioned term for electric
potential or voltage is “electromotive force” or “emf”.
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Electrical ResistanceMost materials offer some resistance to
the flow of electric charges through them. This is called electrical resistance.
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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
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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
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Ohms’ Law In symbols:
V = IRVI R
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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.
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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.
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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.
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Converting AC to DCAC is converted to DC using devices
called diodes, which allow charges to move in only 1 direction.
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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.
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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
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The End