Solve Resistance Question: Find R of Resistor with 60W Lamp & 120V

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SUMMARY

The discussion centers on calculating the resistance (R) of a resistor in series with a 60W lamp connected to a 120V source, where the voltage across the lamp is 35V. The correct approach involves using the power formula P=IV to find the current through the lamp, which is 60W/35V = 1.714 A. Subsequently, the resistance of the resistor is calculated using R=(V_source - V_lamp)/I, resulting in R=(120V - 35V)/1.714A, which equals approximately 49.5 ohms. The initial calculation of 170 ohms was incorrect due to a misunderstanding of the current through the lamp.

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Homework Statement


A 60-W lamp is placed in series with a resistor and a 120-V source. If the voltage across the lamp is 35-V, what is the resistance (R) of the resistor?


Homework Equations


I am pretty sure I am using the right equation, but I am not getting the right answer. What am I doing wrong?


The Attempt at a Solution


P=IV --> 60W=I(120V) --> I=-.50 A
R=V/I --> (120V-35V)/.5A = 170 ohms (this is what I calculated but it isn't the right answer)
 
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Voltage across the lamp is 35V, so that in P=VI,V=35 for the lamp.

Current through the lamp = P/V =60/35
 
A lamp rated as 60W would dissipate 60W when connected directly to the 120V supply -- when you add a resistor in series with it, it no longer dissipates 60W.

- Warren
 

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