S.MORRIS 2006

ELECTRICAL CIRCUIT ELECTRICAL CIRCUIT CALCULATIONSCALCULATIONS

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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 pathThis means that the current is the same throughout the circuitIt = I1 = I2 = I3Because 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!!

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A2

A1

4AV1

V3

V2Light bulb 1 has .5 ohms of resistance

Light bulb 2 has .9 ohms of resistance

Light bulb 3 has .6 ohms of resistance

Find V1, V2, V3, Vt, It, and Rt

It = I1 = I2 = 4ARt = .5 ohms + .9 ohms + .6 ohmsRt = 2 ohmsV1 = 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

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!!

V2

V1

6VIt

I2

I1

Light bulb 1 has 1 ohms of resistance

Light bulb 2 has 4 ohms of resistance

Find V1, V2, I1, I2, It, and Rt

Vt = V1 = V2 = 6 VI1 = 6 volts / 1 ohms = 6 amps I2 = 6 volts / 4 ohms = 1.5 amps It = 6 amps + 1.5 amps = 7.5 amps1/Rt = 1/1 ohms + 1/4 ohms 1/Rt = 1 ohm + .25 ohms1/Rt = 1.25 ohms1/1.25 = Rt

.8 ohms = Rt