Two light bulbs (A and B) are connected in parallel to the same 120V battery as

Question

Two light bulbs (A and B) are connected in parallel to the same 120V battery as shown in the diagram below. When connected in this fashion, the power dissipated by each light bulb is indicated in the figure.

a.) Before you perform a calculation, predict which light bulb will have a greater resistence (or if you expect them to have the same resistance).

b.) What is the voltage drop across light bulb A? Use this value to calculate the resistance of light bulb A.

c.) What is the voltage drop across light bulb B? Use this value to calculate the resistance of light bulb B.

d.) Revisit your prediction in part (a.) in light of your calculations.

Explanation / Answer

a)

resistance is given as

R = V2/P

since V is same for both bulbs, resistance is greater for bulb with smaller power

so bulb A has greater resistance .

b)

since Bulb A is in parallel with the battery

Va = voltage across bulb A = 120 volts

Pa = power of bulb A = 30 W

resistance is given as

Ra = V2a /Pa = (120)2/30 = 480 ohm

c)

since Bulb B is in parallel with the battery

Vb = voltage across bulb B = 120 volts

Pb = power of bulb B = 60 W

resistance is given as

Rb = V2b /Pb = (120)2/60 = 240 ohm

d)

resistance of bulb B is greater

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