Question: Joule's Law of heating.

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    Heating Law
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Discussion Overview

The discussion revolves around Joule's Law of heating, specifically addressing the relationship between resistance, heat produced, and the implications of changing resistance in a circuit. Participants explore the equations related to Joule's Law and question the conditions under which it applies, including the role of current and potential difference.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that doubling resistance should halve the heating effect, citing that resistance is directly proportional to heat produced, but questions whether this is correct under constant potential difference.
  • Another participant suggests that both equations for heat (H = I²Rt and H = (V²t)/R) can be used interchangeably, depending on the context.
  • A participant emphasizes that Joule's Law is not universally applicable and implies that there are specific conditions under which it holds true.
  • One participant explains the relationship between power, current, and resistance, noting that changing resistance affects current due to Ohm's Law, which leads to a clarification of how the equations relate to each other.
  • A later contribution challenges the notion of Joule's Law as a 'law' and introduces a different perspective on the power equation, suggesting that it remains valid even when current varies non-linearly with voltage.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Joule's Law and the implications of changing resistance. There is no consensus on whether the law applies universally or under specific conditions, and the discussion remains unresolved regarding the interpretation of the equations involved.

Contextual Notes

Some participants note that the assumptions regarding constant current and the relationship between voltage and current are crucial to understanding the discussion. The implications of non-linear relationships between current and voltage are also mentioned but not fully explored.

vaishnavsm
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Hi!

My text says that when the resistance is doubled, the heating effect is halved.
Joule's law of heating states that Resistance is Directly proportional to Heat produced.
I.E, R α H.
∴ 2R α 2H.
So, shouldn't the Heat be doubled.

Or, since the Potential Difference is constant, should I use
H = (V2 t) / R
instead of
H = I2Rt
?
Does Joule's law apply everywhere, or are there conditions?
Just to be clear, this isn't homework.
Thanks!
Vaishnav
 
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vaishnavsm said:
Hi!

My text says that when the resistance is doubled, the heating effect is halved.
Joule's law of heating states that Resistance is Directly proportional to Heat produced.
I.E, R α H.
∴ 2R α 2H.
So, shouldn't the Heat be doubled.

Or, since the Potential Difference is constant, should I use
H = (V2 t) / R
instead of
H = I2Rt
?
Does Joule's law apply everywhere, or are there conditions?
Just to be clear, this isn't homework.
Thanks!
Vaishnav

You can use both equations as both are same. Putting V = IR in first gives the second equation.

When resistance doubled, what happens to the current through the resisrance?
 
vaishnavsm said:
R α H.

NOT always. It is applicable to a certain condition, that is?
 
My text says that when the resistance is doubled, the heating effect is halved.

Correct. That follows from..

Power = V2/R .... (1)

Joule's law of heating states that Resistance is Directly proportional to Heat produced.

Joules law states that the heat produced in a conductor by a constant current is equal to the product of the square of the current and resistance of the conductor and the duration of time the current passes. eg.. Q = I2 * R * T

So

Power = Q/T
So

Power = I2 * R .....(2)

It may appear that (1) and (2) are contradict each other but that's wrong. As darkxponent points out, if you change R you change I. That's because in a simple resistor circuit I is not independent of R...

From Ohms Law..

I = V/R

Substutute for I in (2) gives

Power = (V/R)2 * R

= V2 /R

Which is equation (1)
 
Thank YOU!

Thank You Guys!
I knew that you can replace V = IR to get the equation.
I didn't know joule's law stated constant current as a requirement.
Thanks SO Much!

Best Regards,
Vaishnav.

[CLOSED]
 
vaishnavsm said:
Thank You Guys!
I knew that you can replace V = IR to get the equation.
I didn't know joule's law stated constant current as a requirement.
Thanks SO Much!

Best Regards,
Vaishnav.

[CLOSED]

Would like to add a bit more to the thread. The basic power equation is VI, derived from Electromagnetics, so Joules Law is not a 'Law' at all. This, P=VI, equation is valid everywhere, even when I varies non-linearly with V(where impedance is not constant).

Consider a case where where I = k*V^2.
In this case Power equation will become. p = k*V^3
 

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