Voltage and Current, why?

[Mrjoe3012]

In a circuit with a set resistance, let's say 1 ohm, the current flowing through the circuit is the same as the supply voltage. Why does an increase in voltage at the supply cause the electrons to flow faster?

 

[DrClaude]

Increasing the voltage increases the potential energy of the electrons, which is then converted to kinetic energy, hence the higher speed. Increasing the voltage is analogous to increasing the slope of a ramp on which a ball will roll.

Note however that electricity in a circuit should not be seen as individual electrons moving from the low voltage side of the supply to the high voltage side. Electricity is a collective phenomenon involving many electrons colliding with each other and the atoms in the conductor.

 

[Dale]

In a circuit with a set resistance, let's say 1 ohm, the current flowing through the circuit is the same as the supply voltage. Why does an increase in voltage at the supply cause the electrons to flow faster?

I am afraid that this is a rather circular question. You assume that the material has a resistance of one Ohm. Such materials are called ohmic and by definition in these materials an increase in voltage causes the current to flow faster. So the reason why we get that behavior is because you specified a material that exhibits that behavior.

Note that not all materials behave that way. So it is not generally true that increasing the voltage increases the current. It is only true in specific materials. These materials have a valence band and a conduction band that overlap so electrons can be easily moved from one atom to another.

https://energyeducation.ca/encyclopedia/Conduction_band

 

[sophiecentaur]

Increasing the voltage increases the potential energy of the electrons, which is then converted to kinetic energy, hence the higher speed. Increasing the voltage is analogous to increasing the slope of a ramp on which a ball will roll.

Note however that electricity in a circuit should not be seen as individual electrons moving from the low voltage side of the supply to the high voltage side. Electricity is a collective phenomenon involving many electrons colliding with each other and the atoms in the conductor.

The Kinetic Energy of the electrons is a minute fraction of the Energy that's transferred in any electric circuit (speeds in the region of mm/s and the total mass of conduction elecrons being one over many thousands of the mass if the wires). The 'water circuit' analogy is way off beam here. The Energy is in the Fields and not KE. Also the process is very mucy Quantum Mechanical and it is prpoblematical to treat electrons 'mechanically'.

 

[DrClaude]

The Kinetic Energy of the electrons is a minute fraction of the Energy that's transferred in any electric circuit (speeds in the region of mm/s and the total mass of conduction elecrons being one over many thousands of the mass if the wires). The 'water circuit' analogy is way off beam here. The Energy is in the Fields and not KE. Also the process is very mucy Quantum Mechanical and it is prpoblematical to treat electrons 'mechanically'.

This is a B-level question, so I was keeping things simple. The drift velocity of the electrons increases with the difference in potential. I did give a caveat in the second paragraph of my reply.

 

[ZapperZ]

The Kinetic Energy of the electrons is a minute fraction of the Energy that's transferred in any electric circuit (speeds in the region of mm/s and the total mass of conduction elecrons being one over many thousands of the mass if the wires). The 'water circuit' analogy is way off beam here. The Energy is in the Fields and not KE. Also the process is very mucy Quantum Mechanical and it is prpoblematical to treat electrons 'mechanically'.

Dr. Claude is correct and quite appropriate in the response. You do not want to treat this quantum mechanically, because even graduate level treatment of the Kubo formulation is a BEAST!

We get Ohm's law by treating conduction electron as classical particles. It is good enough to solve many problems. There is no reason to use a bazooka to kill a fly.

Zz.

 

[CWatters]

The voltage across the resistor creates an electric field in the resistor. That in turn applies a force on any charged particles (such as electrons) in the resistor.

 

[sophiecentaur]

This is a B-level question, so I was keeping things simple. The drift velocity of the electrons increases with the difference in potential. I did give a caveat in the second paragraph of my reply.

Yes - BUT - there is no significant KE in that slight increase in average speed. I am just anxious that the mechanism of Energy transfer is not treated as 'mechanical'. It's such an attractive notion that it can be grabbed with both hands and a person ca be stuck with that misconception for ever. That applies whatever level we're discussing at.

 

[ZapperZ]

Yes - BUT - there is no significant KE in that slight increase in average speed. I am just anxious that the mechanism of Energy transfer is not treated as 'mechanical'. It's such an attractive notion that it can be grabbed with both hands and a person ca be stuck with that misconception for ever. That applies whatever level we're discussing at.

But there are "misconceptions" at almost every level when we teach physics. Treating the Earth as an inertial frame in General Physics examples is a "misconception". However, what did we sacrifice? In many cases, these are not physicists that we are teaching to, and that what they need are general ideas and concepts. If we keep on insisting that we do not use any kind of simplification, then there is no way for us to teach any parts of physics without scaring everyone away due to its complexities.

Zz.

 

[sophiecentaur]

But there are "misconceptions" at almost every level when we teach physics. Treating the Earth as an inertial frame in General Physics examples is a "misconception". However, what did we sacrifice? In many cases, these are not physicists that we are teaching to, and that what they need are general ideas and concepts. If we keep on insisting that we do not use any kind of simplification, then there is no way for us to teach any parts of physics without scaring everyone away due to its complexities.

Zz.

Some things are misconceptions and some things are approximations. When you get down to it, an approximation can be justified but a misconception needs to be 'undone' before you can move on. Bringing in the KE of electrons is like assuming it's the KE of a bicycle chain that carries the energy from your feet to the road and that's just plain wrong.

 

[ZapperZ]

Some things are misconceptions and some things are approximations. When you get down to it, an approximation can be justified but a misconception needs to be 'undone' before you can move on. Bringing in the KE of electrons is like assuming it's the KE of a bicycle chain that carries the energy from your feet to the road and that's just plain wrong.

Then answer the OP at the level he/she can understand.

Zz.

 

[sophiecentaur]

Then answer the OP at the level he/she can understand.

Zz.

Cue the bicycle analogy then. It has legs.

 

[ZapperZ]

Cue the bicycle analogy then. It has legs.

This simply shows that you complain, but you offer no solution.

Zz.

 

[DaveE]

This is a B-level question, so I was keeping things simple.

Yes, as you should! It is a bit tiring to hear replies that essentially ignore the OP. High school math teachers don't start with calculus, and answers to this sort of question really shouldn't involve QM. If you think beginners should be welcome to ask basic questions here then you should respond with effective teaching, not confusing them with complexity.
Replies to replies to impress each other about how much physics you know? Maybe you should start a new thread?

 

[cosmik debris]

Not picking on anyone in particular but sometimes answer to questions are not appropriate to the level of understanding of the poster. It is not enough to go by the grading given by the poster.

"Mummy, where am I from"?
"Well, it's like this: once there was a beautiful garden full of apple trees...".
"Yes, I know that, but Millie is from Oxford, where am I from"?

Cheers

 

[sophiecentaur]

This simply shows that you complain, but you offer no solution.

Zz.

Unfair.
The KE idea has no legs at all. Forces between electrons (highly valid) has an analogy with bicycle chain. What can be the objection to that solution? Is it really good to use KE in this way? It will need to be unlearned (forcibly) for the next level of understanding. Otoh, forces between electrons (as in a chain) can explain the transfer of Energy in a future-proof way. Also, forces and work are easier than KE.

 

[ZapperZ]

Unfair.
The KE idea has no legs at all. Forces between electrons (highly valid) has an analogy with bicycle chain. What can be the objection to that solution? Is it really good to use KE in this way? It will need to be unlearned (forcibly) for the next level of understanding. Otoh, forces between electrons (as in a chain) can explain the transfer of Energy in a future-proof way. Also, forces and work are easier than KE.

Then produce a coherent answer to the OP at the level he/she can understand! "Cue bicycle chain" is meaningless. I want to this full "quantum mechanical" explanation that is understandable at this level.

Zz.

 

[sophiecentaur]

"Cue bicycle chain" is meaningless.

OK. Let me help you with this (I thought the cycle chain idea was well known). The analogy between electric power transfer and a bicycle chain is very good. Nominally massless 'particles' interacting to maintain a constant mean spacing (rigid connections) will transfer power according to the Forces and the Velocitiy. The mechanism is independent of mass because it just involves Mechanical Work. Details of the Mass of the chain links (or the mass of a drive belt or gear train) make no difference to the transferred power so the KE is not relevant.
The OP doesn't mention KE so why introduce it in any attempt to explain the mechanism?

Then answer the OP at the level he/she can understand.

A 'B' level answer would include Power = Force X Speed as well as Kinetic energy so why not supply an answer that actually makes sense? I think you would agree that KE doesn't actually come into an idealised conduction model.
Granted, my mention of Quantum Mechanics may be over the top but let's get the classical explanation as near right as possible.

 

[jbriggs444]

For simple circuits with power sources, wires and resistors, the fluid analogy is reasonable. Power is equal to pressure (voltage) times volumetric flow rate (current). When an electric current flows through a constriction, a higher pressure drop yields a higher flow rate.

One might alternately think about little balls rolling down an inclined plane which is covered with pegs. When a ball (an electron) hits a peg (an atom?), it is brought momentarily to a halt. It is accelerated by the slope of the plane (the electric field) until it hits the next peg. The average rate (current) at which the electrons roll down is proportional to the slope (electric field) of the plane. This is the Drude model.

In truth, thermal electrons are bouncing around randomly at a very high rate (around 100 km/sec). Their average drift velocity due to an applied voltage is measured in millimeters per second. But there are a lot of them, so a low average velocity can still amount to a significant current.

 

[sophiecentaur]

One might alternately think about little balls rolling down an inclined plane which is covered with pegs.

That's an understandable mechanism for Dissipation of Power in a resistive conductor. However, the 'slope ' equivalent for along a low resistance supply wire is more or less horizontal / zero. That doesn't take care of Power Transfer to a resistive load. There is no gain of KE on the way through the supply wire because the Potential Drop is virtually zero. Nonetheless, large amounts of Power is transferred along the wire and that is achieved by Forces between neighbouring electrons within the conductor. It seems that we could have been arguing at cross purposes because of the different contexts.

 

[jbriggs444]

That's an understandable mechanism for Dissipation of Power in a resistive conductor. However, the 'slope ' equivalent for along a low resistance supply wire is more or less horizontal / zero. That doesn't take care of Power Transfer to a resistive load.

Yes, you are absolutely correct. The two analogies address different aspects of the situation.

 

[sophiecentaur]

Yes, you are absolutely correct. The two analogies address different aspects of the situation.

A great example of a PF discussion where people take sides at the drop of a hat. (Mea culpa too)
Interestingly, if we consider two series resistors, one of them could be regarded as 'supplying Power' to the second in the chain. Dissipation in the supply Resistor can be modeled by KE to Thermal transfer and the transfer of Power through a resistor is a matter of Force times Distance (=Work).
I will leave it to the objectors to my idea to square it with the OP, if he/she is further confused by all this.

 

[David Lewis]

In a circuit with a set resistance, let's say 1 ohm, the current flowing through the circuit is the same as the supply voltage.

It's numerically the same because of the units chosen. Current and voltage can never be the same because they are different quantities.

Treating the Earth as an inertial frame in General Physics examples is a "misconception".

But it's not a model. It's a simplification. Nobody is fooled into thinking that, by ignoring gravity, it doesn't exist.