42.  A 60.0-W lamp is placed in series with a resistor and a 120.0-V source. If the voltage across the lamp is 25 V, what is the resistance R of the resistor?

If the voltage across the lamp is 25 V, then since the sum of voltages around the closed loop must be zero, we know the voltage across the resistor is found from

 

 

 

Using the power of the lamp and the voltage we can determine the current through the lamp which is also the current through the resistor.

 

 

 

Ohm’s law can now provide the resistance of the resistor.

 

 

 

 

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This page last updated on July 12, 2018