What's different on electron level when voltage is higher?


by kraku
Tags: electron, voltage
kraku
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#1
May9-13, 02:48 PM
P: 2
I'm trying to form a mental image what's happening with the electrons when you think of the following equation:

----
Power = volts*amperes

volts = joules/coulomb
amperes = coulomb/second
----

Amperes I understand that they tell you how many electrons go through a certain point every second.

But when I look at the volts, it says "joules/coulomb"? This indicates that the electrons in the circuit can somehow behave differently or be different. If I change the volts but the amperes stay the same (ie. same amount of electrons running through the circuit), how can the electrons cause different behavior if they have the same charge and (I assume) same speed?

Do the volts describe some kind of power by which the electrons are forced forward, even though with their regular speed. An analogy to this would be a bulldozer vs. human pushing an object at the same speed: bulldozer has much more power behind it's movement and doesn't stop easily when it hits obstacles.

What's going on here? What are the electrons doing differently when the voltage is higher?
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Nugatory
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#2
May9-13, 02:55 PM
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Quote Quote by kraku View Post
I'm trying to form a mental image what's happening with the electrons when you think of the following equation:

----
Power = volts*amperes

volts = joules/coulomb
amperes = coulomb/second
----

Amperes I understand that they tell you how many electrons go through a certain point every second.

But when I look at the volts, it says "joules/coulomb"? This indicates that the electrons in the circuit can somehow behave differently or be different. If I change the volts but the amperes stay the same (ie. same amount of electrons running through the circuit), how can the electrons cause different behavior if they have the same charge and (I assume) same speed?

Do the volts describe some kind of power by which the electrons are forced forward, even though with their regular speed. An analogy to this would be a bulldozer vs. human pushing an object at the same speed: bulldozer has much more power behind it's movement and doesn't stop easily when it hits obstacles.

What's going on here? What are the electrons doing differently when the voltage is higher?
A better analogy would be that volts are like height when we're pushing the electrons uphill (or letting them roll downhill). If the hill is twice as high, it will take twice as much energy (Joules) to push the same amount of charge (Coulombs) up the hill (or twice as much energy will be released if we let the same amount of charge roll downhill).
pumila
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#3
May9-13, 03:05 PM
P: 114
Amperage is proportional to the number of electrons per unit time.

Voltage is related to the kinetic energy of each electron - increasing the voltage in the circuit increases the electric field at the electron, increasing its acceleration within that field and so increasing its kinetic energy.

kraku
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#4
May9-13, 03:28 PM
P: 2

What's different on electron level when voltage is higher?


@pumila:

With kinetic energy do you mean that the electron moves faster and faster or that it quickly reaches a certain speed but is harder to slow down like described in my bulldozer analogy?

So basically with higher voltage the electrons just move faster? Does this mean that voltage transformers "fling" the electrons with higher/slower speed along the electric circuit?
pumila
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#5
May9-13, 04:12 PM
P: 114
Electron energy is proportional to the voltage, and speed to the root of voltage.

Higher voltages 'fling' electrons faster - but bear in mind that electron speeds in most circuits are in fact quite low.
Integral
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#6
May9-13, 06:53 PM
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Careful, there is some near misinformation in this thread.

The electric field only has a significant effect on KE of an electron when in a vacuum, as in a CRT. In a circuit where the mobile electrons are part of an electron gas they have a very high thermal velocity, thus a high KE, but they move very slowly under the influence of the E-field. Their energy is lost by the frequent collisions (actually near collisions) with other electrons and the metallic crystal structure. This is the heat generated due to current flow.
MisterX
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#7
May10-13, 04:32 PM
P: 532
It seems there have been some potentially misleading statements in this thread. There is multiple things that can be meant by voltage, but let's concentrate on the electrostatic potential, also known as the Galvani potential. This is defined as the line integral of the electric field.

It is wrong to suggest the electron drift velocity is proportional to the square root of the applied voltage. Consider a section of wire with constant conductivity, with voltage V across it. The E field would be V/d, and the drift velocity is directly proportional to E, and thus directly proportional to the applied voltage V.

I think it is misleading to claim the voltage is generally related to the electron drift velocity or the kinetic energy of electrons. We might consider a capacitor, where there are two parallel plates, one with a positive charge and one with negative charge. We could assume that all the charge has zero kinetic energy and there would still be a voltage difference between the plates.
pumila
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#8
May10-13, 05:04 PM
P: 114
An electron flow through a resistive circuit component is different to electron flow in space charge devices. In the former you will get a balance speed where the kinetic energy increase from the driving voltage is balanced by the kinetic energy loss from interaction with the conducting matrix. In the latter there is no such kinetic energy loss.

Sorry if I seemed to oversimplify things but a complete answer is several chapters long.
pumila
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#9
May11-13, 02:22 AM
P: 114
Thinking about it, the originator is more likely interested in the effects within a resistor, so my reply should probably be ignored - just been doing a lot of crunching with low energy electron beams so took that context. I think there is also a difference in approach - I would take the emf as the source energy and see how the energy changed, to thermal in a resistor, staying as electric field energy in a capacitor, or to magnetic energy in an inductor. Never mind semiconductors or space charge devices, it gets too complicated. The inductor presents some challenges - Feynman (in his chapter on electromagnetic inertia) and others rejected the idea of electromagnetic inertia, so we treat the kinetic energy separately from magnetic field energy (must be both as current is flowing) but then we need to know where the induced magnetic field energy of the standalone electron comes from to add to the extended magnetic field energy of inductor to close the equation and Feynman is silent on this. So the only way is to cheat and treat the total induced magnetic field energy as an enhanced kinetic (inertial) energy which seems to work in practice as a mathematical wheeze although it is in violation of Feynman et all in terms of the model. At least the equations can be closed. But there are probably several other approaches, let me know if you know one, I am collecting them at moment.


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