Problem 17.72 A tungsten lightbulb filament is designed to dissipate 40.0 W at a

Question

Problem 17.72

A tungsten lightbulb filament is designed to dissipate 40.0 W at a 120-V potential difference.

Part A:
What's the filament current, assuming negligible resistance in the rest of the circuit?
Express your answer to three significant figures and include the appropriate units.

Part B:
What is the filament's resistance when lit?
Express your answer to three significant figures and include the appropriate units.

Part C:
If the lit filament is at 1850 K, what's its resistance when off?
Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

power P = 40 W potential difference V = 120 V A) power P = (V)(I) therefore, current I = (P)/(V)                             = 0.33 A B) power P = I2R power P = I2R resistance R0 = P/I2                    = 367.3 C) temperature T = 1850 K and T0= 293 K (room temperature) resistance R = R0(1+(T-T0)) ........ (1) where, temperature coefficient of resistivity of Tungsten = 4.5*10-3 (0C)-1 substitute the given data in eq (1), we get                  R = 2.940 k substitute the given data in eq (1), we get                  R = 2.940 k   

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