May 30, 2012
Current: Flow of charge
Symbolized by “I”
Units = Coulombs/second = Amperes (A)
Circuit: a closed loop that current flows
Resistance: Opposition to current
Units = Ohms (Ω)
LED: Light Emitting Diode
Used to test for presence of current.
Volts: the force that pushes current through a
Power: the energy delivered to the circuit per
Power = (current)(voltage)
P = (I)(V)
Units = Watts
Examples #1 & #2
car battery causes a current of 2.0
amps through a light while 12 V is across
the battery. What is the power used by
P = (2.0)(12)
P = 24 W
What is the current through a 75W lightbulb
connected to a 120 V outlet?
75 = (I)(120)
I = .625 A
How does a light bulb light?
As the current flows through the filament, it
Resistance: a material’s opposition to current
R = (V/I)
As resistance increases, current decreases
As voltage increases, current increases
The collisions between the current and
material results in energy loss (heat and light)
Examples #3 & #4
A car headlight has a resistance of 30Ω and is hooked to
a 12V battery. What is the current through the bulb?
30 = (12/I)
I = (12/30) = .40 A
A lamp draws a current of .50A when connected to a
120-V outlet. What is the resistance of the lamp?
R = (120/.50)
R = 240 Ω