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Electric CurrentElectric Current Current
Rate of flow of charges through a conductor Usually the flow of electrons. depends on # of e- passing a point in a given
time measured in amperes (A) (1 A = 6250 million
billion electrons past a point every second)
Voltage DifferenceVoltage Difference
Voltage Difference The force that causes electric
charges to flow large separation of charge creates
high voltage measured in volts (V)
- - - - - - - - - - - -
- - - -
Low voltage High voltage
Electron flow
Current is from high voltage to lowElectrons flow from low voltage to high
CircuitCircuit
closed path through which electric current flowsA current will only flow as long as there is a
voltage difference. Sources of the voltage difference?
BatteriesBatteries To keep electric charges continually flowing in
an electric circuit, a voltage difference must be maintained. Power supplies such as batteries!
Dry Cell BatteriesDry Cell Batteries A cell = two electrodes surrounded by an
electrolyte The electrolyte enables the charge to flow
from one electrode to the other.
Dry Cell BatteriesDry Cell Batteries Electrode 1 – carbon rod; Electrode 2 – zinc container Electrolyte – moist paste of chemicals Completing the circuit causes a chemical reaction and
electrons are transferred. Carbon rod becomes positive, zinc accumulates
electrons (neg) creating a voltage difference and a current will flow in aclosed circuit
Wet-cell BatteriesWet-cell Batteries Contains two connected plates made of different metals
in a conducting liquid solution (electrolyte). Example: car battery
Wet-Cell BatteriesWet-Cell Batteries Lead-Acid batteries: contain a series of six wet cells
of lead and lead dioxide plates in a sulfuric acid solution.
Voltage difference approx 12V
Electric OutletsElectric Outlets
create a voltage difference Example: wall socket Usually has a higher voltage difference then a
battery. Approx 120 V in US for most sockets
Why do cell phones get hot on Why do cell phones get hot on a long call?a long call?
Materials resist the flow of electrons and convert electrical to thermal energy
Resistance Measured in Ohms (Ώ)
Resistors reduce the current through the circuit to prevent overload (some energy transferred to resistor)
ResistanceResistance
Copper - low resistance
Tungsten - high resistanceNot only is electrical energy converted to thermal energy but also to light!
ResistanceResistance Resistance depends on…
the conductor• less resistance in a
better conductor
wire thickness• less resistance in
thicker wires wire length
• less resistance in shorter wires temp
less resistance at low temps
Ohm’s LawOhm’s Law
Ohm’s Law
I = V / RV: voltage difference
(volts)
I: current (amperes)
R: resistance (ohms)
Ohm’s LawOhm’s LawA lightbulb with a resistance of 160 is
plugged into a 120-V outlet. What is the current flowing through the bulb?
GIVEN:
R = 160 V = 120 V
I = ?
WORK:
I = V ÷ R
I = (120 V) ÷ (160 )
I = 0.75 A
I
V
R
Alternating Current and Direct Alternating Current and Direct CurrentCurrent
AC – current from household electrical outlets. In US power grids alternate current changes
direction 120 times per second. DC – battery powered devices use DC. Current
never changes direction. An AC/DC adapter is a device that converts the
alternating current of an outlet into direct current. Lets you charge a cell phone battery!
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