Solving Umax with Pmax and Ohms

[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 textbook 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.

Thank you for your time

Homework Statement

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.

Answer is :

Spoiler

Umax = 11

Homework Equations

P = U * I
U = R * I

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

 

[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?

 

[Ren_Majer]

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?

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

 

[milesyoung]

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

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?

 

[Nickie]

Umax= 10,41666666V

Hello, as a fellow scientist, I can understand your desire to relearn and brush up on your physics knowledge before starting your university course. It's great that you are taking the initiative to solve textbook problems and improve your understanding.

Now, to solve this problem, we can use the equations you mentioned: P = U * I and U = R * I. We know that the maximum power for both resistors (R1 and R2) is 0.25 W. We also know that they are in series, which means that the same current flows through both resistors.

To find the maximum voltage (Umax) that can be applied to the circuit, we need to find the maximum current (Imax) first. We can do this by substituting Pmax = 0.25 W and R1 = 120Ω into the equation P = U * I:

0.25 = U * I
0.25 = U * (Imax)
Imax = 0.25/U

Now, we can use this value for Imax and substitute it into the equation U = R * I, along with the value for R2 = 180Ω, to find Umax:

Umax = R2 * Imax
Umax = 180Ω * (0.25/U)
Umax = 45V

Therefore, the maximum voltage that can be applied to the circuit is 45V. I hope this explanation helps you understand the solution better. Keep up the good work and good luck with your studies!