Transcript Electric Current

Electric Circuits
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Electric Current
The amount of charge passing through a surface per
second.
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• Unit: ampere or amp (A) = coulombs/seconds
The “conventional” current (I) is defined by the
motion of positive charges → Benjamin Franklin
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Electric Circuits
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Drift Speed
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The average speed of charges that make a current
• Usually ~ 0.1 mm/s
The electric field is created throughout the wire so that
all charges respond almost instantaneously
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The charges collide with atoms in the wire and proceed
with a random walk
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• The atoms get hotter due to these collisions → light !
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Electric Circuits
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Ohm’s Law and Resistance
• Unit: ohms (Ω) = volts/amps
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Electric Circuits
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Resistivity
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What determines the value of R for a resistor?
• Type of material, length and thickness
• Temperature?
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Conductors, insulators, heating units, lights, etc.
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Electric Circuits
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Electric Circuits
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Energy and Electric Potential in a Circuit
The PE of a charge is unchanged after traveling around
a circuit
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PE is gained at the voltage source (ie. battery) and lost
at the resistor (ie. light bulb)
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The sum of the ΔV around the circuit equals zero
• Voltage “drops” across a resistor
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Electric Circuits
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Series circuits
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Electric Circuits
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Series circuits
1. The current is the same everywhere in the circuit.
Itotal = I1 = I2 = …
2. The total resistance of the circuit is
Rtotal = R1 + R2 + …
3. The sum of the voltages across each device equals the
source voltage
ΔVsource = ΔV1 + ΔV2 + …
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Electric Circuits
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Parallel circuits
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Electric Circuits
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Parallel circuits
1. The total current in the circuit equals the sum of the
current in each device.
Itotal = I1 + I2 + …
2. The total resistance of the circuit is
3. The voltage across each device is the same.
ΔVsource = ΔV1 = ΔV2 = …
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Electric Circuits
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Electric Power
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Energy transfer per unit time
• Unit: watts or joules/second
Since it takes work (W) to move a charge (q) through a
potential difference (ΔV) then we can say that
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Electric Circuits
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Electric Power
Overloading occurs when the circuit resistance drops
to where the circuit carries too much current.
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Fuses or breakers are used to control the amount of
current in a circuit
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