# Homework Help: Solving Umax with Pmax and Ohms

1. Aug 20, 2013

### Ren_Majer

Brief personal intro, I have last time studied physics about 12 years ago.

I am now starting in 2 weeks time in a university and there is mandatory physics course so I am trying to relearn and learn as much as I can before that time.

For that reason I am trying to solve text book tasks from my old physics books.

I have never been good at maths nor physics atleast not calculations. Thus I appreciate thorough-Step by Step explanations so I can understand the solution.

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

How large voltage U , can be attached to the circuit , when both the resistance have an Pmax of 0,25 W ?

R1 = 120Ω
R2=180Ω

They are in series.

Umax = 11

2. Relevant equations

P = U * I
U = R * I

3. The attempt at a solution

0,25*120= 0,002083333333
0,25*180= 0,003888888889

0,002083333333 + 0,003888888889=0,00347222222

0,00347222222*120 = 0,416666664
0,00347222222*180 = 0,624999996

4,16666664+6,24999996= 10,41666666

2. Aug 20, 2013

### milesyoung

Since they're in series, they have the same current through them. If you start with a low current and slowly ramp it up, do you know which of the resistors will reach its thermal limit first?

3. Aug 21, 2013

### Ren_Majer

No , it is not mentioned on the task nor which direction the current will go

4. Aug 21, 2013

### milesyoung

Just assume that you can control the current through the resistors, I promise you that I'll get to the point shortly

Say you have a current of 1 A through the resistors. How much power is then dissipated in each resistor?