Download Circuit Math Lecture - Petoskey Public Schools

ELECTRICAL CIRCUIT
CALCULATIONS
S.MORRIS 2006
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Basic Formula
• Almost all calculations in circuits use
Ohm’s Law
• V=IR
• Each circuit type has additional
formulas dependent on what happens
to voltage and current in the circuit
SERIES CIRCUITS
A series circuit has only one path
This means that the current is the same throughout the
circuit
It = I1 = I2 = I3
Because current is the same, voltage at each load depends on
the resistance of the load.
V1 = It * R1
Vt = V1 + V2 + V3 and Rt = R1 + R2 + R3
Do the Math!!
Light bulb 1 has .5 ohms of resistance
V3
A?1
Light bulb 2 has .9 ohms of resistance
Light bulb 3 has .6 ohms of resistance
4A
?
V1
A2
Find V1, V2, V3, Vt, It, and Rt
It = I1 = I2 = 4A
Rt = .5 ohms + .9 ohms + .6 ohms
Rt = 2 ohms
V1 = 4 amps * .5 ohms = 2 volts
V2 = 4 amps * .9 ohms = 3.6 volt
V3 = 4 amps * .6 ohms = 2.4 volts
Vt = 2 V + 3.6 V + 2.4 V = 8 V
V2
PARALLEL CIRCUITS
A parallel circuit has multiple paths
Current gets split, and voltage stays the same throughout
Vt = V1 = V2 = V3
Because voltage is the same, current at each load depends
on the resistance of the load.
I1 = Vt / R1
It = I1 + I2 + I3
PARALLEL CIRCUITS
Because a parallel circuit has multiple paths, the resistance
does not add up, instead it is split (but not the same as
current)
The formula for total resistance is:
1/Rt = 1/R1 + 1/R2 + 1/R3
Do the Math!!
Light bulb 1 has 1 ohms of resistance
6V
It
Light bulb 2 has 4 ohms of resistance
Find V1, V2, I1, I2, It, and Rt
V2
I2
V1
I1
Vt = V1 = V2 = 6 V
I1 = 6 volts / 1 ohms = 6 amps
I2 = 6 volts / 4 ohms = 1.5 amps
It = 6 amps + 1.5 amps = 7.5 amps
1/Rt = 1/1 ohms + 1/4 ohms
1/Rt = 1 ohm + .25 ohms
1/Rt = 1.25 ohms
1/1.25 = Rt
.8 ohms = Rt