Solving a Series Circuit with Ohm's Law

[starhallie]

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.

 

[mgb_phys]

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?

 

[starhallie]

120v - 25v = 95v?

 

[starhallie]

I guess I just don't understand what voltage to use and why.

 

[mgb_phys]

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

 

[starhallie]

I=P/V
=60W/(120V-25V)
=0.63 A

R=V/I
=95V/0.63A
=151 ohms?

 

[mgb_phys]

sounds good

 

[starhallie]

Thank you so much for your help!