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Concept Summary
Batesville High School Physics
Potential Difference
 Charges
can “lose” potential energy by
moving from a location at high potential
(voltage) to a location at low potential.
 Charges will continue to move as long
as the potential difference (voltage) is
maintained.
Current
 A 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 = Charge
time
or
I = q/t
Measuring Current
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
 If
Voltage Source
 A 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 Force
 An
old-fashioned term for electric
potential or voltage is “electromotive
force” or “emf”.
Electrical Resistance
 Most
materials offer some resistance to
the flow of electric charges through
them. This is called electrical
resistance.
Resistance
 Resistance
 Material
of a conductor depends on:
- 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 Law
 For
many conductors, current depends
on:
 Voltage - more voltage, more current
 Current
is proportional to voltage
 Resistance
- more resistance, less
current
 Current
is inversely proportional to
resistance
Ohms’ Law
 In
V
symbols:
= IR
V
I 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 110120 volts, although some large
household appliances run on 220-240
volts.
Converting AC to DC
 AC
is converted to DC using devices
called diodes, which allow charges to
move in only 1 direction.
Speed of 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.
 Electrons
Electric Power
 Power
= energy/time = current x voltage
= IV = I2R
 1 Watt = (1 Amp)(1 Volt)
 1 kilowatt = 1000 Watts
 A kilowatt-hour is a unit of energy
P
The End
