Different emf across the same resistance

In summary, the same resistance dissipates different electric energies because the electric currents are different. The pd across a resistor is set and maintained by the emf source *not* the resistor.
  • #1
truewt
78
0
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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  • #2
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?

[tex]\Delta V = R*i[/tex]

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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
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:
 
  • #6
"p.d." = potential difference, a measure of voltage?
"p.d." = power disipation, a measure of power?
 

1. What is emf and how does it relate to resistance?

Emf stands for electromotive force and it is a measure of the energy per unit charge that is supplied by a source, such as a battery or generator. It is directly related to resistance through Ohm's Law, which states that the current through a conductor is directly proportional to the voltage and inversely proportional to the resistance.

2. Why would there be different emf values across the same resistance?

There can be different emf values across the same resistance due to variations in the source of the emf. For example, different batteries or generators may have different emf values, even if they are connected to the same resistance.

3. Can the emf across a resistance change over time?

Yes, the emf across a resistance can change over time. This can be due to changes in the source of the emf, such as a battery losing its charge, or changes in the resistance itself, such as a wire heating up and increasing its resistance.

4. How does the emf across a resistance affect the flow of current?

The emf across a resistance directly affects the flow of current. A higher emf results in a higher current, while a lower emf results in a lower current, as long as the resistance remains the same. This is because the current is directly proportional to the emf and inversely proportional to the resistance.

5. Is there any way to change the emf across a resistance without changing the resistance itself?

Yes, the emf across a resistance can be changed without changing the resistance itself by changing the source of the emf. For example, replacing a low voltage battery with a higher voltage battery will increase the emf across the resistance, resulting in a higher current.

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