## Electricity and electric current?

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
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Boris Lagutin

### Electricity and electric current?

Could somebody explain why sparks happen while two electrodes (anode, cathode) touch each other? I think I understand the process that happens in a light-bulb: wolfram (tungsten) makes it harder for electrons to pass through a filament so that heat releases. However, it's unclear for me why sparks in the electrodes contact place appear.

Thank you.
joe
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Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

The spark is quite different from an incandescent bulb. In the bulb, ohmic losses in the conductor heat the filament up to a few thousand kelvin, and it then glows.

To understand the spark, first let's note that there is no such thing as a perfect insulator: when an external electric field becomes comparable with the field around the nucleus (~10^8 V/m), it can remove electrons to create a plasma, which then conducts. Depending on humidity, air starts to break down when the fields are ~10^7 V/m.

So, if two electrodes have a potential difference of say 100 V, then when they are a few microns apart, the field is large enough to ionise air, which then conducts between them. This conduction heats up the remaining air and the plasma glows brightly: a spark. So the spark happens very slightly before they touch.

(In class, I use a van der Graaf generator to produce very high voltages and get a spark to jump a couple of cm, then do a similar calculation to estimate the voltage.)
Boris Lagutin

### Re: Electricity and electric current?

Hello,

Could someone explain how the electric field propagates through the conductive wire of 10 kilometers length after switching on (please, see my drawing)? Assume the battery has 10 Volts.

P.S. If somebody mentions an electromagnetic wave, then, please, explain how this electromagnetic wave appears and propagates through the wire (10 kilometers on my drawing) after switching on.

Thank you.
Attachments
electric_field_propagation_1.PNG (219.66 KiB) Viewed 13606 times
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

With the switch open, the wire on one side of the switch is at 10 V, and on the other it is zero. So 10V potential difference is across the switch.

Just before the switch contacts touch (distance ~ µm), the field is large enough to ionise air, so a spark allows current to flow. This changes the potential in each contact (they have finite capacitance). When they touch, there is no PD across the switch.

Now some electrons have moved from the wire on one side onto the wire on the other so, in a short distance near the switch, there is an electric field in the wires. We have 5V potential difference in each wire. Initially, this PD is concentrated over a short distance but, as the electrons move, this distance expands. Within a ms, we end up with a 5 V PD spread uniformly along each wire.

Why so quick? the capacitances involved are very small, so only a tiny current is necessary to change the potential, the wires (I expect) have low resistance and electrons have low mass so are accelerated very easily.
Boris Lagutin

### Re: Electricity and electric current?

Joe,

Do I understand you correctly that you write that the electric field in the portion of the wire connected with the battery anode (negative lead) has already existed before switching on (my drawing)?

Thank you.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

Do I understand you correctly that you write that the electric field in the portion of the wire connected with the battery anode (negative lead) has already existed before switching on (my drawing)?
No. I wrote:
With the switch open, the wire on one side of the switch is at 10 V, and on the other it is zero. So 10V potential difference is across the switch.
Boris Lagutin

### Re: Electricity and electric current?

Ok.

If I don't mistake, the electric potential cannot move electrons by itself. The electric field applies a force on electrons in the wire on my drawing above after switching on, doesn't it?

Thank you.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

the difference in electrical potential produces a field and that is what moves the electrons.
Initially, this PD is across the switch.
After switch closing, the PD very rapidly spreads along the wire.
I've given a more detailed description above.
Boris Lagutin

### Re: Electricity and electric current?

Thank you, Joe.

I am interested in dynamics (step by step) of the electric field propagation especially in the case pictured on my drawing above.

1) How does the electric field produced by the electric potential (10 Volts) move along the wire (10 kilometers) step by step after switching on?

2) Which configuration (form) does the electric field produced by the electric potential have along the wire while the electric field is moving in the wire?

3) How does the electric field produced by the electric potential (10 Volts) interact with the individual electrons' (charges) electric fields in the wire while the electric field produced by the electric potential is moving along the wire after switching on?

4) Why does the electric field produced by the electric potential (10 Volts) propagate ALONG the conductive wire between cathode and anode? What stimulates the electric field propagates ESPECIALLY ALONG the conductive wire?

Thank you.
Last edited by Boris Lagutin on Wed Nov 30, 2016 11:55 am, edited 2 times in total.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

The potential difference (and thus the field) is initially across the switch.
It then very quickly spreads long the wires until, in steady state, there is almost no PD across the switch and it is all across the wires (and the internal resistance of the battery, probably negligible if the wires are km long).
Boris Lagutin

### Re: Electricity and electric current?

Ok. I reformulate the set of my questions.

Why cannot the electric field produced by the electric potential propagate along a totally-non-conductive wire after switching on (my drawing above)? Is the ability to propagate along the conductive wire of the electric field produced by the electric potential (10 Volts on my drawing above) related with "free electrons" in the conductive wire or other stuff?

Thank you.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

Metals have the property that the outer electrons are 'shared' among all the atoms: they can move easily.
In insulators (like most plastics) the electrons are 'tightly bound' to their atoms. So they do not conduct electricity (at least not until the field reaches many MV per metre).
Boris Lagutin

### Re: Electricity and electric current?

Hello,

Could somebody clarify how "free electrons" (outer electrons) in a metal wire are related with the propagation along the wire of the electric field produced by the electric potential (10 Volts, see my drawing above) ?

Thank you.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Electricity and electric current?

There's a subtle point here. Before you close that switch, the left hand wire is at 10V, the RH wire at 0 V.
A ms after you close it, the LH wire is at 10 V (bottom) and 5 V (top), varying smoothly between the two. The RH wire is at 0 V (bottom) to 5 V (top). (There is also a current flowing through the wire, but that is not directly related to your question.)
So a tiny amount of static charge has been lost from the LH wire and added to the RH wire. If the wire is not touching anything, then the static charge is entirely at the surface of the insulation of the wire, not in the metal. The capacitance of these cables is tiny, so the charge that moves is probably too small to measure with a microammeter. But that tiny surface charge redistribution is what changes the field.
Boris Lagutin

### Re: Electricity and electric current?

Joe,

I have many questions about your last post. However, I just ask one question. If I understand you right, you claim that some tiny surface charge redistribution causes the electric field produced by the electric potential (10 Volts on my drawing) to propagate along the wire after switching on. The speed of the electric field propagation produced by the electric potential (10 Volts) in the wire after switching on is about the speed of light, but the electrons' (charges) speed in the wire (see my drawing above) is much less than that of the electric field propagation. What speed does your tiny surface charge (electrons) redistribution mentioned in your last post have?

Thank you.