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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 (W)
• Due to collisions between electrons and
atoms
Ohm’s Law
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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
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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