Maximum power in this resistor circuit

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Homework Help Overview

The discussion revolves around a resistor circuit problem related to maximum power transfer and the implications of negative resistance values. Participants are exploring the conditions under which resistance can be defined and the mathematical reasoning behind determining maxima and minima in this context.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants are questioning the validity of negative resistance values and discussing the implications of local versus global minima in their calculations. There are inquiries about how to determine the range of resistance values and the conditions under which maximum power transfer occurs.

Discussion Status

Some participants have provided guidance on checking the endpoints of the resistance range and the implications of the derivative at those points. There is an ongoing exploration of the assumptions made regarding the resistance values and the conditions for maximum power transfer.

Contextual Notes

Participants note that the range of resistance values is not explicitly provided in the problem statement, leading to discussions on logical inference of the range and the physical constraints on resistance values.

Physics lover
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Homework Statement
The question is in Attempt at a solution
Relevant Equations
V=IR
P=V^2/R
P=I^2×R
20190912_085611.png


My attempt-:
20190912_085938.jpg


So the resistance is coming to be negative.Can anyone tell me my mistake.Thanks.
 
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Physics lover said:
Homework Statement: The question is in Attempt at a solution
Homework Equations: V=IR
P=V^2/R
P=I^2×R

View attachment 249522

My attempt-:
View attachment 249523

So the resistance is coming to be negative.Can anyone tell me my mistake.Thanks.
Formally your answer is correct. Input source impedance is (3||6 Ohm)=2 Ohm, therefore from matching theorem your load must be also 2 Ohm for maximal transferred power. You just need to trim negative resistance answer to closest physical value (i.e. 0).
 
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Physics lover said:
So the resistance is coming to be negative.Can anyone tell me my mistake.Thanks.
Your error is in assuming all minima are local minima, i.e. where a gradient is zero.
Sometimes the global minimum occurs at one end of the valid range. In general, you should check the extremities of the range even if you do find a local minimum.
 
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haruspex said:
Your error is in assuming all minima are local minima, i.e. where a gradient is zero.
Sometimes the global minimum occurs at one end of the valid range. In general, you should check the extremities of the range even if you do find a local minimum.
Can you please explain me how can i do that here means how can i find the range of R.
 
Physics lover said:
Can you please explain me how can i do that here means how can i find the range of R.
As @trurle posted, it cannot be negative.
 
Physics lover said:
Can you please explain me how can i do that here means how can i find the range of R.
The range of R should either be given by the problem(for example in this problem it could have been given that the variable resistor is in the range for example ##[4,404]##) or can be logically inferred like for example in this problem the range of R is logically inferred to be ##[0,+\infty)##. So you look for maximum or minimum at the end points and how to prove that the value at the end point is a maximum (or minimum).
 
haruspex said:
As @trurle posted, it cannot be negative.

You worked out the derivative. You looked for zeros. Good. Now check the value of the function and the derivative at the end points.

Suppose the derivative is positive at R=0. Can the maximum be at R=0? Suppose the derivative is negative at R=0. How about 0 as a maximum then?

You should also think about a maximum R. Though I don't see one listed in your problem statement. Variable resistors have a maximum value. You should check what is happening for large R.
 
ok thanks to everyone.I understood it
 

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