Different emf across the same resistance

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    Emf Resistance
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Discussion Overview

The discussion revolves around the concept of potential difference (p.d.) across a resistor when different electromotive forces (emf) are applied. Participants explore the relationship between emf, resistance, and energy conversion, seeking analogies and clarifications on why varying emf results in different energy dissipation across the same resistance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about why the same resistance can convert different amounts of energy when different emf values are applied, questioning the underlying principles.
  • One participant states that the different electric currents resulting from varying emf lead to different energy dissipation across the resistor, referencing Ohm's law.
  • Another participant explains that a larger emf results in a larger p.d. across the resistor, leading to greater energy transfer to thermal energy, as per conservation of energy.
  • Concerns are raised about why all the emf must be used to maintain a larger p.d. across the resistor, questioning the constancy of p.d. when changing the emf source.
  • A participant emphasizes that the p.d. across a resistor is determined by the emf source, not the resistor itself, and that increasing emf necessitates an increase in p.d. and current.
  • There is a clarification request regarding the meaning of "p.d." as it could refer to either potential difference or power dissipation.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and acceptance of the principles involved, with some agreeing on the relationship between emf and p.d., while others remain uncertain or seek further clarification. The discussion does not reach a consensus on the analogies or explanations that would satisfy all participants.

Contextual Notes

Participants mention Ohm's law and conservation of energy, but there are unresolved questions about the constancy of p.d. and the role of the emf source, indicating a need for further exploration of these concepts.

truewt
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Hi,

I'm having some difficulties understanding some elementary physics concept.

When different emf is supplied to a resistor of the same resistance, the p.d. across changes in different scenarios. By definition, p.d. is the amount of electrical energy converted to other forms of energy per unit charge. Why does the same resistance converts different amounts of energy? Can anyone kindly come up with an analogy? Because I can't find any explanation to this. And why does the charge-carriers want to lose all the energy given to them by the source?
 
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truewt said:
Hi,

I'm having some difficulties understanding some elementary physics concept.

When different emf is supplied to a resistor of the same resistance, the p.d. across changes in different scenarios. By definition, p.d. is the amount of electrical energy converted to other forms of energy per unit charge. Why does the same resistance converts different amounts of energy? Can anyone kindly come up with an analogy? Because I can't find any explanation to this. And why does the charge-carriers want to lose all the energy given to them by the source?

The same resistance dissipates different electric energies because the electric currents are different. Haven't studied Ohm's law?

\Delta V = R*i

i is the current.

It's not that the charge-carriers want to lose all the energy given to them by the source, it's that they lose it because of the material' resistance: this acts slowing down to a constant (average) speed the charge-carriers put in motion from the electric field. At constant speed no work is done on them, so all the electric work have to go to the conductor (as heat) and you have completely dissipated the electric energy.
 
truewt said:
Hi,

I'm having some difficulties understanding some elementary physics concept.

When different emf is supplied to a resistor of the same resistance, the p.d. across changes in different scenarios. By definition, p.d. is the amount of electrical energy converted to other forms of energy per unit charge. Why does the same resistance converts different amounts of energy? Can anyone kindly come up with an analogy? Because I can't find any explanation to this. And why does the charge-carriers want to lose all the energy given to them by the source?

A lager emf will maintain a larger pd across the resistor. This means that the decrease in electrical potential energy (EPE) of the charge carriers is greater (as they pass through the resistor), and so by conservation of energy there is a greater energy transfer (to internal thermal energy) to the resistor. This also answers your last point - the charge carriers lose EPE via the potential drop and this lost EPE appears as thermal energy.
 
tonyh said:
A lager emf will maintain a larger pd across the resistor. This means that the decrease in electrical potential energy (EPE) of the charge carriers is greater (as they pass through the resistor), and so by conservation of energy there is a greater energy transfer (to internal thermal energy) to the resistor. This also answers your last point - the charge carriers lose EPE via the potential drop and this lost EPE appears as thermal energy.

My problem is, why must all the emf supplied by different cells be used to maintain a larger pd across the resistor? Why can't the pd across a resistor remain constant while you change the emf cell?

I do know the actual facts; it's that I somehow can't seem to accept them as I can hardly find any analogies that can satisfy me..

Thanks for any replies.
 
truewt said:
My problem is, why must all the emf supplied by different cells be used to maintain a larger pd across the resistor? Why can't the pd across a resistor remain constant while you change the emf cell?

I do know the actual facts; it's that I somehow can't seem to accept them as I can hardly find any analogies that can satisfy me..
The pd (V) across a resistor is set and maintained by the emf source *not* the resistor. Imagine a simple circuit consisting of just an emf source and a resistor (R). A given emf (E) will produce a certain pd (V) across R. The value of R will then determine the current (I). If you increase the emf, V *must* also increase (by conservation of energy) and so too will I.
The key point is the first sentence. Hope this helps :smile:
 
"p.d." = potential difference, a measure of voltage?
"p.d." = power disipation, a measure of power?
 

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