What is the apparent difference between the two cases of H=I^2RT and H=(V^2)T/R?

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Homework Help Overview

The discussion revolves around the equations H=I^2RT and H=(V^2)T/R, focusing on the apparent differences in how energy consumption (H) relates to resistance (R) in each case. Participants explore the implications of these equations in the context of electrical energy conversion and the relationships between current (I), voltage (V), and resistance (R).

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants discuss the nature of the equations and their applicability to the same physical situation. Questions arise regarding the assumptions made when interpreting the equations, particularly in relation to changes in resistance and their effects on current and voltage.

Discussion Status

The discussion is ongoing, with participants examining the assumptions underlying the equations and how they lead to different interpretations. Some guidance has been offered regarding the need to clarify the assumptions that lead to contradictory conclusions, but no consensus has been reached.

Contextual Notes

Participants are navigating the complexities of electrical principles, including Ohm's Law, and the implications of varying resistance on power calculations. The discussion highlights the need for careful consideration of assumptions in deriving conclusions from the equations.

shihab-kol
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Homework Statement



H=I^2RT and H=(V^2)T/R.
In one case H is directly proportional to R while in the other it is not where H is the energy consumed.
Explain the apparent difference between the two cases

Homework Equations



None

The Attempt at a Solution


In the first case electricity is being converted to heat while in the second electric energy is being obtained from something else.So the equations are different
My version is you can derive the two equations from one another

Thus they are same
But it's this alright?
 
Last edited:
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shihab-kol said:
In the first case electricity is being converted to heat while in the second electric energy is being obtained from something else.
No, they describe the same circumstance.
Consider the other difference in the equations. One has V where the other has I.
What happens to these when the resistance changes?
 
haruspex said:
No, they describe the same circumstance.
Consider the other difference in the equations. One has V where the other has I.
What happens to these when the resistance changes?
Ok
So for getting V we have to include it
So is my edited post correct?(post 1)
 
shihab-kol said:
Ok
So for getting V we have to include it
So is my edited post correct?(post 1)
No, as I thought I had indicated, it is completely wrong. The two equations apply to the same circumstance. What you need to explain is why they are consistent, despite appearances.
Try to answer my question in post #2.
 
I has to decrease and V increase if we increase R and vice versa
What I meant by my previous post was that you could derive both formulas from one another .For including V we have to include the inverse of R, thus both equations mean the same thing.
So,is it alright?
 
shihab-kol said:
What I meant by my previous post was that you could derive both formulas from one another .For including V we have to include the inverse of R, thus both equations mean the same thing.
Yes, but I don't feel it gets to grips with the apparent paradox.
shihab-kol said:
I has to decrease and V increase if we increase R and vice versa
Sort of.
P=V2/R seems to say that if you increase R P decreases. What assumption is being made in saying that?
P=I2R seems to say that if you i ncrease R P increases. What assumption is being made in that case?
 
P=V^2 /R says what you say .But if we increase R then V and so V^2 has to increase
Thus,P has a change
P=I^2R means as R increases I^2 has to decrease balancing the equation and more or less making it equal to the first one.
So,is this it?
P.S.- These are due to Ohm's Law
 
shihab-kol said:
But if we increase R then V and so V^2 has to increase
shihab-kol said:
as R increases I^2 has to decrease
Not necessarily.
You don't seem to be grasping the question. The question is saying "if we look at the equation P=V2/R then we might expect that if R increases then P decreases". Do you agree with that statement? What assumption is being made in making that statement?
 
haruspex said:
Not necessarily.
You don't seem to be grasping the question. The question is saying "if we look at the equation P=V2/R then we might expect that if R increases then P decreases". Do you agree with that statement? What assumption is being made in making that statement?
We are saying that P is inversely proportional to R.
In decreasing R we have to increase I and I is directly proportional to P {P=VI} .
So, yes I think P will increase.
 
  • #10
shihab-kol said:
In decreasing R we have to increase I
To deduce that you must make a certain assumption. What assumption is being made?
 
  • #11
haruspex said:
To deduce that you must make a certain assumption. What assumption is being made?
That voltage is constant.
 
  • #12
shihab-kol said:
That voltage is constant.
Right!
Now look at the other equation, P=I2R. This appears to say that if R decreases P decreases. What assumption is being made now?
 
  • #13
haruspex said:
Right!
Now look at the other equation, P=I2R. This appears to say that if R decreases P decreases. What assumption is being made now?
That current is constant
 
  • #14
shihab-kol said:
That current is constant
Right. So how would you answer the question now? What explains the apparent paradox?
 
  • #15
Sorry for the delay.

Is it that in same time,t since both V and I are constant R is also a constant?
 
  • #16
shihab-kol said:
Sorry for the delay.

Is it that in same time,t since both V and I are constant R is also a constant?
No. Forget I, V and R for the moment and think in these general terms:
We have two equations, both valid.
If we make a certain assumption in one we reach a certain conclusion.
If we make a different assumption in the other, we reach a contradictory conclusion.
What can we say about the two assumptions?
 
  • #17
haruspex said:
No. Forget I, V and R for the moment and think in these general terms:
We have two equations, both valid.
If we make a certain assumption in one we reach a certain conclusion.
If we make a different assumption in the other, we reach a contradictory conclusion.
What can we say about the two assumptions?
That anyone assumption is wrong?
 
  • #18
shihab-kol said:
That anyone assumption is wrong?
Not exactly. I think it will take forever to get there this way, so I shall just tell you how I would answer:
The two assumptions cannot be true at the same time. So it should not be surprising that the two assumptions lead to contradictory conclusions.
 

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