Solving a Series Circuit with Ohm's Law

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Homework Help Overview

The problem involves a series circuit containing a 60W lamp, a resistor, and a 120V power source. The voltage across the lamp is given as 25V, and the objective is to determine the resistance of the resistor.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of Ohm's Law and the relationship between voltage, current, and resistance in a series circuit. Questions arise regarding the correct voltage to use for calculating resistance and the implications of the current being the same through both components.

Discussion Status

There is an ongoing exploration of the correct approach to find the resistance, with some participants providing hints and guidance on using the total voltage and the voltage drop across the resistor. Multiple interpretations of the problem are being considered, and participants are actively engaging with each other's reasoning.

Contextual Notes

Participants are working under the constraints of the problem statement and discussing the necessary relationships between power, voltage, and current without reaching a definitive conclusion.

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Homework Statement



A 60W lamp is placed in series with a resistor and a 120V source. If the voltage across the lamp is 25V, what is the resitance R of the resistor?

Homework Equations



I derived R= V2/P from ohm's law to solve this problem.

The Attempt at a Solution



R= V2/P
=(25V)2/60W
=10.4 ohms

Is this answer correct? Thank you in advance for your time and help.
 
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That's the resistance of the lamp.
Remember the current in the lamp an the resistor is the same, what's the voltage rop across the resistor?
 
120v - 25v = 95v?
 
I guess I just don't understand what voltage to use and why. :frown:
 
You have 120V total, 25V across the lamp and so (120-25) across the resistor
To work out the resistance you need the current through the resistor
This is the same as the current through the lamp

hint: Power = current * voltage difference
 
I=P/V
=60W/(120V-25V)
=0.63 A

R=V/I
=95V/0.63A
=151 ohms?
 
sounds good
 
Thank you so much for your help! :approve:
 

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