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S.MORRIS 2006 ELECTRICAL CIRCUIT CALCULATIONS More free powerpoints at www.worldofteaching.com

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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

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SERIES CIRCUITS A series circuit has only one path This means that the current is the same throughout the circuit I t = I 1 = I 2 = I 3 Because current is the same, voltage at each load depends on the resistance of the load. V 1 = I t * R 1 V t = V 1 + V 2 + V 3 and R t = R 1 + R 2 + R 3

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Do the Math!! ? ? A2A2 A1A1 4A V1V1 V3V3 V2V2 Light bulb 1 has.5 ohms of resistance Light bulb 2 has.9 ohms of resistance Light bulb 3 has.6 ohms of resistance Find V 1, V 2, V 3, V t, I t, and R t I t = I 1 = I 2 = 4A R t =.5 ohms +.9 ohms +.6 ohms R t = 2 ohms V 1 = 4 amps *.5 ohms = 2 volts V 2 = 4 amps *.9 ohms = 3.6 volt V 3 = 4 amps *.6 ohms = 2.4 volts V t = 2 V + 3.6 V + 2.4 V = 8 V

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PARALLEL CIRCUITS A parallel circuit has multiple paths Current gets split, and voltage stays the same throughout V t = V 1 = V 2 = V 3 Because voltage is the same, current at each load depends on the resistance of the load. I 1 = V t / R 1 I t = I 1 + I 2 + I 3

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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/R t = 1/R 1 + 1/R 2 + 1/R 3

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Do the Math!! V2V2 V1V1 6V ItIt I2I2 I1I1 Light bulb 1 has 1 ohms of resistance Light bulb 2 has 4 ohms of resistance Find V 1, V 2, I 1, I 2, I t, and R t V t = V 1 = V 2 = 6 V I 1 = 6 volts / 1 ohms = 6 amps I 2 = 6 volts / 4 ohms = 1.5 amps I t = 6 amps + 1.5 amps = 7.5 amps 1/R t = 1/1 ohms + 1/4 ohms 1/R t = 1 ohm +.25 ohms 1/R t = 1.25 ohms 1/1.25 = R t.8 ohms = R t

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