# Understanding voltage and current

While reading "Electronics for dummies" went through the following block and I realised that I have some uncleared concepts about electricity:

Electrostatic discharge involves very high voltages at extremely low currents. Combing your hair on a dry day can develop tens of thousands of volts of static electricity, but the current is almost so negligible you seldom notice it. The low current prevents the static discharge from really hurting you when you receive a shock. Instead, you just get an annoying tickle

I thought that voltage is the driving force that drives current, and the magnitude of generated current depends upon the resistance attached between terminals of a voltage difference, if so then why is there a low current generated by tens of thousands of volts of static electricity? if 220 volts in socket can electrocute then why not this tens of thousands of volt can? the resistance is the same i.e. the body

• When dealing with static electricity it is better to think in coulombs, bodies in vacuum and newtonian work, forces and distances. Then introduce capacity and voltage. Concept of current, fields and curcuits should come much later. This matters are way too different and may cause a lot of confusion when mixed. Most of electronic laws including Ohms law are high abstractions and are built on electroneutrality principles. – user924 May 27 '12 at 18:09

This is like asking, if I pour a cup of water off a skyscraper, why can't that drive a turbine to produce some meaningful electricity? It's got the gravitational potential, so what's the problem? After all, hydroelectric dams not as tall as a skyscrapers generate many megawatts.

Static electricity can have the capacity to kill. This occurs in nature and is called lightning.

• If you don't like the coffee, pour it down the sink, don't throw it off the building! – Federico Russo May 4 '12 at 7:47

I like being graphical.

Your hair, when charged electrostatically, acts like small capacitors charged to high voltages. The energy stored in those little capacitors is finite and small, and so it can do little harm to you.

On the other hand, a 220 Vrms outlet has a much lower voltage, but it is an unlimited source of energy. Even acting upon the same load resistance, this is much more dangerous, because all that extra energy means it can cause more heating of your tissues, and therefore more damage.

• We have a 10pF at 15kV, I am not brave enough to touch it. Our bodies are on the order of pF to my knowledge, charging to more then 10kV is very very rare. Your 5kV number is very reasonable, good answer. – Kortuk May 5 '12 at 0:24
• @Kortuk Thank you. The Human Body Model (HBM) en.wikipedia.org/wiki/Human_body_model specifies 100 pF and 1500 ohm. That's kind of a worst case. – Telaclavo May 5 '12 at 10:07
• Excellent description with graphics. This should have been the accepted answer. – hkBattousai May 10 '12 at 18:06

Well the description is a bit unclear there.

With electrostatic discharges you get lots of both instantaneous current and voltage but little electric charge. That limits the time duration during which the current can pass and limits the amount of damage that can occur.

Over time, the current is indeed low, but the point that needs consideration here is that the current basically goes through to stages: The part where you have current and the part where you don't have current.

The part during which you have current lasts for only a short time and during that time, the current is result of the voltage and the resistance of air (which is pretty complex as air has non-linear resistance). Over time the current decreases as the electrostatic charge is depleted and the resistance of air changes due to air movement. The resistance of a volume of air through which the current is passing tends to decrease over time, but that air heats up and expands and moves away from the source of discharge meaning that the total resistance increases because the length of the conductor is increasing. This lasts for a very short time. At one point you reach the part where the resistance is too high to maintain the arc (or alternatively you reach the point at which the charge has been depleted) and then the arc breaks. From that moment on, you don't have any current.

Another point is electrocution. For that you need not only sufficient voltage but also sufficient energy. An electric outlet at say 220 V can provide "large" current for very long time (compared to how long the arc lasts) and that allows large enough transfer of energy which is expanded to damage tissue. That energy doesn't exist in case of usual electrostatic discharge.

How electrostatic discharge works can be seen in this simulation. Notice the time on the lower right part of the black screen and click on the switch and see how quickly the capacitor discharges. Something like that happens with electrostatic discharge too.

• I feel the core issue here is not being addressed. The reason you can be at 100k volts and not be dangerous is because our bodies are a very weak capacitor so a very small amount of charge separation represents a large voltage but has no energy behind it. – Kortuk May 3 '12 at 21:59
• @Kortuk♦ Didn't I say that in the paragraph before last? – AndrejaKo May 4 '12 at 5:17
• you said the arc did not last for a long time and that there was not enough energy. The core concept as to why you are safe from ESD is that the capacitor has no energy, I can generate an arc at work at 13kV that will blow your hand apart, it being an arc adds no safety, almost more danger due to the fact that you have super heated air hitting you with the voltage. – Kortuk May 4 '12 at 5:54
• @Kortuk♦ I was thinking at the That energy doesn't exist in case of usual electrostatic discharge. Also in the second sentence:but little electric charge. I did not claim that in general case the arc carries little energy. Also I said that the arc lasts for a short time because of both the movement of air and the low amount of electric charge present in usual situations where we have static electricity discharge. Since the amount of charge is proportional to the energy of the capacitor I thought that I covered the little energy area. – AndrejaKo May 4 '12 at 6:26
• You did nothing wrong I just feel your answer is not cutting to the heart of the problem, you have a decently long answer discussing the arcing properties when it all centers around the capacitor issue. – Kortuk May 4 '12 at 21:07

Recall that current is the amount of charge that moves through a slice of conductor per unit time. I think the text's mistake is to conflate charge with current. Ohm's Law still holds, the current itself will be high...for the duration of the ESD event, which is on the order of microseconds or around there. But the charge itself is very low, so the current cannot be sustained. If you were to measure the current in units of "charge per microsecond", you'd see a high current for a brief period, but if you measure current in "charge per second" (i.e. amps) then it doesn't look so large.

So even though there is 5000 V on the cap, there's so little charge that it can't cause much damage; once the ESD event occurs, the charge is all gone, and no more current flows. And while there's "only" 220 V coming out of the wall, for all intents and purposes it has unlimited charge, and it will continue pumping charge into whatever is connected to it for the duration of the connection.

When we talk about voltage, we refer to the potential difference between two points while current is the rate of flow of charge. The notion of conductors and insulators is very relevant here. In conductors, there exists free electrons which allow the flow of current but in insulators, there are very few free electrons so current flow is restricted. With a large potential difference, if the material is an insulator like your hair then little current can flow to hurt you. But if those large voltages were developed in a conductor, there is a rush of current. Think of a conductor as a valve that is open and an insulator as a closed valve. Imagine the water pressure as the potential difference and the water through the valve as current. when the valve is closed i.e an insulator then little or no water flows through but when the valve is open i.e a conductor there is a flow of water i.e current which depends on water pressure i.e potential difference/voltage.

That is assuming a source that can provide the current. Electrostatic build up has a limited potential, lighting would be on the opposite end and a rotor craft somewhere in the middle. In any case you can not use more than whats there getting that level of static to kill you is hard but not impossible.