3. (a) Consider an incandescent light bulb of 40 W (Watts) connected to a 120 V

Question

3. (a) Consider an incandescent light bulb of 40 W (Watts) connected to a 120 V electrical outlet. What is the effective resistance of the light bulb (in ohms)? (b) If we connect 3 such light bulbs in series on the same outlet, what power do they consume? What is the total resistance through the circuit? (c) If we connect these 3 light bulbs in parallel instead, what is the total power and what is the total resistance? (c) Suppose this circuit includes a fuse with rated current IN = 100 A. In other words, the fuse blows and opens the circuits if there is current I IN . How many of these same 40 W light bulbs can we connect in parallel without blowing the fuse?

Explanation / Answer

3 a)

P = power of bulb = 40 W

V = Voltage = 120 Volts

R = Resistance of Bulb

Using the formula

P = V2/R

40 = 1202 / R

R = 360 ohm

b)

series combination of resistance of 3 bulbs is given as

Rseries = R + R + R = 3R = 3 x 360 = 1080 ohm

Power = P = V2/Rseries = 1202 / 1080 = 13.33 Watt

c)

Parallel combination of resistance of 3 bulbs is given as

1/Rparallel = 1/R + 1/R + 1/R

Rparallel = R/3 = 360/3 = 120 ohm

Power = P = V2/Rparallel = 1202 / 120 = 120 Watt

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