# Homework Help: Series Wiring

1. Oct 4, 2009

### starhallie

1. The problem statement, all variables and given/known data

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?

2. Relevant equations

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

3. The attempt at a solution

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

2. Oct 4, 2009

### mgb_phys

That's the resistance of the lamp.
Remember the current in the lamp an the resistor is the same, whats the voltage rop accross the resistor?

3. Oct 4, 2009

### starhallie

120v - 25v = 95v?

4. Oct 4, 2009

### starhallie

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

5. Oct 4, 2009

### mgb_phys

You have 120V total, 25V accross 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

6. Oct 4, 2009

### starhallie

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

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

7. Oct 4, 2009

sounds good

8. Oct 4, 2009

### starhallie

Thank you so much for your help!