But that’s cheating. If a region of space contains electrons (or ions, or any other sort of charged particles,) THEN IT ISN’T A VACUUM ANYMORE.
I could also “prove” that water is a good conductor, all I have to do is dump salt into it. But that says nothing about the conductivity of PURE water.
A perfect vacuum is a perfect insulator, since there are no charge carriers present. Yes, electrodes can contaminate a vacuum, and once some ions or electrons have been injected, then a voltage will cause a current. But the electron cloud surrounding a hot filament in a vacuum tube is not “a vacuum.”
Side issue: lightning is impossible in a vacuum since lightning is the electrical breakdown of a gas. If you place a huge e-field across a gas, the gas turns to plasma and you’ll get an outbreak of glowing streamers. If you place a huge e-field across a vacuum, nothing happens since there are no gas molecules present which could ionize.
Forgive me if I don’t accept your argument. Defining a vacuum as something that can’t have an electron in it and then claiming that a vacuum is a perfect insulator is not a useful representation of how things work.
You could also claim that light can travel through glass, but not a vacuum, because once there are photons in it, it isn’t a vacuum anymore.
The electric discharge doesn’t heat you because it’s hot. The electric discharge heats you because there is electricity flowing through something that resists the current (your body). The heat that gets transferred to you is not generated in the spark, but rather in your tissues.
The air gets heated along the way too, and some of the oxygen gets converted to ozone. In fact, it’s the rapid expansion and contraction of air due to the heating that makes the “zap” sound (or the thunderclap on the larger scale of lightning). None of this heat gets transferred to you however.
If you want to know the temperature that the air gets heated to as the electrons move through it, it’s pretty darn hot (I don’t know the exact numbers so I’ll just go by what the others have posted). If you want to know the heat that gets to you from the spark it’s pretty much nothing. If you were a superconductor you wouldn’t generate any heat from the electricity flowing through you. Since you are not a superconductor, as the electricity flows through you, it generates heat, a small amount of heat in the case of a little spark, a rather substantial amount of heat in the case of a lightning bolt.
There’s two bits of heat here. (1) The spark heats the air and (2) the electricity (not really a spark at this point) heats your tissues as it goes through them.
I have to strongly disagree. “How things work” for conductors is based on charge density and mobility. Injecting electrons into a vacuum CONVERTS IT INTO a conductor. (Just as dumping salt into pure water converts it into a conductor, just as triggering the formation of spark-plasma will convert air into a conductor.)
Measure a vacuum with an ohmmeter. It says infinite ohms. Do the same thing with air, or with pure water. These are insulators. Just because we’re able to convert them into conductors does not mean they were conductors BEFORE we futzed with them.
I’m not trying to be twisted. Our disagreement hinges on the defintion of “electrical conductor.” There are two different definitions in common use:
“something through which electric charges can pass” (based on the incorrect “empty pipes” analogy for electric circuits)
“something which contains mobile charges” (based on Ohm’s law)
So according to #1, a vacuum is a conductor because it doesn’t stop electrons from flowing. But according to defintion #2, a vacuum is an insulator because it doesn’t contain any movable charges. Which definition is correct? A physicist would go for #2 : Put a voltage across something, and if there is no current, then there must be no mobile charges available. Measure a vacuum with an ohmmeter, the meter will say infinite ohms.
A metal wire is nothing like an optical fiber, since wires are “pre-filled” with the material which flows and optical fibers are not. Wires aren’t transparent to electrons. Instead, wires are like water hoses which are already filled with water. If an optical fiber was like a wire, then the fiber would have to be full of unmoving photons all the time, and in order to transmit light, you’d have to apply some kind of pressure across the ends of the fiber, and only then would the light inside begin to move.
To create a current in a wire we just have to apply a voltage across it’s ends, and then the wire’s own electrons will start flowing. To create a current in a vacuum we FIRST have to convert the vacuum into a conductor by injecting mobile charge carriers. Only then can we create a current by applying a voltage across the vacuum. The injected electrons have changed the conductivity of the vacuum.
I just think that the statement “vacuum is a perfect insulator” is silly. Even if you eliminate conduction currents, there are still displacement currents to worry about.
You have to very narrowly define “vacuum”, “perfect” and “insulator” to make the statement true.
That makes the statement pretty much useless, unless you provide all of the qualifications that make the statement true.
No insulator blocks displacement currents, so it’s a bit of a stretch to call something a conductor on that basis.
I do agree that “vacuum” needs to be better defined. A laboratory “vacuum” of 10[sup]-3[/sup] torr pressure is a very good conductor; a 500-volt power supply can easily push current through it. A high-grade vacuum under 10[sup]-6[/sup] torr is, for most intents and purposes, a very good insulator. It’s safe to apply thousands of volts on a bare wire and rely on the vacuum to act as insulation. To push electrical current through a high-grade vacuum requires not only a high voltage, but a method of supplying free electrons such as a heated filament.
You are using a narrow definition of insulator. Your own statement implies that currents can pass through insulators. If I said to you something like “the signal current passes through the input capacitor to the op-amp”, you probably wouldn’t respond with “wait a minute, a capacitor is an insulator”. Maxwell’s version of Ampere’s law makes no particular distinction between the conduction component and the displacement component.
No heated filament is required. The corona effect occurs.
True, AC current can pass through an insulator. Is that a problem? In all the references I’ve seen, an insulator is defined as something with a high resistance. Not impedance, but DC resistance.
I thought “corona effect” refers to electrical conduction by ionized air molecules. If you have cold electrodes in high-grade vacuum and apply high voltage, how exactly are ions generated? And what kind of voltages are you talking about? I’ve used several thousand volts in 10[sup]-6[/sup] torr vacuum chambers successfully (i.e. without accidental discharges). I admit I’ve never looked into what happens if you use 10,000 volts.
Do you see why the statement “vacuum is a perfect insulator” isn’t a great one?
If you mean by “insulator” something with high resistivity, then okay. If you interpret “insulator” as meaning that current can’t pass through it, then it requires further explanation.
You don’t need ions. Electrons get stripped directly from the conductor.
You can’t just state that vacuum is a perfect insulator, without explaining all of the assumptions behind the statement. That is, that “vacuum” precludes the existence of electrons, that “insulator” refers only to resistivity and not to the conduction of current, or that the existence of time varying electric fields is precluded, and that “perfect” isn’t meant to refer to breakdown voltage or dielectric constant, etc.
Damn…you start out with ‘how hot is a spark’ and you get treatises on vacuum efficiency…
So how hot is it? LIttle bit o plasma, then the reaction to your skin is negligble because the short duration that the heat is applied isn’t enough to damage anything…
So far we have:
Static electricity sparks super heat the air, generating the ‘pop’
it hurts more due to direct electricity to the nerves rather than heat
it’s somewhere between 0 and 53,000F.
Soooo…back to my original question… HOW HOT IS A SPARK? (static electricity, socks on a shag carpet kinda spark)
So what is the field strength or voltage required to do this? If it’s true I’d like to know; I’ve never worried about this when conducting experiments in vacuum.
Sorry, I shouldn’t have said that. Yes, I realize it does happen, but only under extreme conditions. Something on the order of 200 kilovolts for a 1-mm gap.
The answer to your question, with respect to the heat applied to your skin (which seems to be what you keep asking) is that it is not hot at all. There is no heat transferred from the spark to your skin.
The temperature of the air is going to vary, since the spark can range from what you can feel but doesn’t even cause a visible spark, up to holy cow what a big blue bolt that was. At the bottom end you’re not going to get much above room temperature. At the top end it will be pretty close to that of a spark plug, which someone else posted was in the range of about 800 deg C. I poked around on google trying to find the temperature at which you actually start to see the blue light but wasn’t very successful.
You seem to keep asking for the heat that would be transferred to your skin, and the energy being transferred to your skin is not in the form of heat.
You keep saying things along the line of “the heat is applied…” There is no heat applied to your skin. The answer to the way you keep wording the question is the spark has no heat. It’s kind of like asking how much heat is in a 120 volt wall socket. There’s lots of energy there, but it’s not in the form of heat.
More specifically I think i’m asking “what is the temperature of a static electric spark such that you can see the visible blue flash with the naked eye”
Daylon, If I had seen your thread yesterday I could have got an expert answer for you quickly. We have an Electrostatic Discharge Laboratory where I work, and I’m sure they can answer your question.
Unfortunately, I will be out of town next week, and unable to pose your question to them.
If you can wait about 10 days, I’d be glad to get an answer for you.
I’ll check this thread when I get back to the office. Maybe I can have an answer for you on the 27th or 28th.
A lightning bolt contains no molecules either, and it’s difficult to convince me that a lightning bold has no temperature. Can that be?
Can it be that burns caused by lightning strikes are not from the bolt itself, but from the effect of frighteningly high voltage passing through objects ( trees, cows, people ) whose bodies provide resistance, and therefore generate heat in the resistance of that voltage?
It’s pretty hot, and in fact it may move a lot of current. Peak temperatures, as noted upthread, may be on the order of 50,000F, and peak current may be tens of amps. However, the duration of high current is on the order of nanoseconds, and the total mass of the air that is heated to 50,000F is very, very small, so there’s not a lot of energy contained in the spark gap.
