# Simple question about motion of electrons

1. Feb 19, 2012

### mindauggas

1. The problem statement, all variables and given/known data

Why is electron movement through a loop wire considered not an accelerated motion?

If it is how come "an electric current flowing in a loop of superconducting wire can persist indefinitely with no power source" (Wiki)?

2. Feb 19, 2012

### wukunlin

the following are my understanding (which may or may not make sense)

1) you can model electrons "falling" from the negative terminal to the positive like any objects falling to earth due to gravity. In this model, electrons would, as you suggested, accelerate. But, in all real circuits, there are resistive elements which constantly consumes the energy of electrons, therefore the speed of the electrons maintain an equilibrium until the potential is removed.

2) newton's 1st law basically. superconducting objects have no resistive properties, so a moving electron has no reason to slow down or speed up (no power source), it will just keep going.

3. Feb 19, 2012

### mindauggas

I'm sorry, I should have made my question clearer. Since you mentioned classical mechanics it is useful to note that I too ask this question from CM paradigm - more precisely Classical electromagnetism. A "natural state of motion" is considered motion with unchanging/uniform velocity. Since velocity is a vector it has a speed component (magnitude) and direction. If either of those changes, the motion is considered not natural - it requires force acting on the object to occur.

Since classical EM is also based on this notion I ask then: why the movement of charge is not considered an accelerated movement in a loop-wire since the direction of vector is changing.

4. Feb 19, 2012

### jegannathan

Hi mindauggas, electrons in a conductor are moving only with a drift velocity which is attained due to the acceleration acting just for the relaxation time. Acceleration is there which equals to Ee/m. E-electric field, e-charge of an electron, m-mass of electron
So for the relaxation time which is the characteristic of the material electron attains a velocity called drift velocity. Vd = Ee tua / m. tua-relaxation time.
Hence though the acceleration is ever there, due to that, only drift velocity is found as a uniform velocity.
Explanation for the second query: Let us always recall "energy is conserved". As we provide the initial electrical energy to the electrons they get converted in the form of kinetic energy ie 0.5 m Vd^2. But at superconduction transition temperature there is absolutely no resistance and so no any kind of loss of energy. So they persist moving. But don't say without power source. Initially there was a power source which has handed over the required energy to persist the movement of electrons ie current flow.

5. Feb 19, 2012

### mindauggas

Ok, so is the electron motion through a loop-wire accelerated?

You say that acceleration (which occurs only in the relaxation phase) is there AND only drift velocity is considered uniform. So in a superconductor there is only a drift velocity? But I can see that electrons have to change direction since the wire is curved. So there is no acceleration due to change in direction in a superconductor?

No one suggested otherwise.