A Primer on Electricity (Part III)
Last week I put together volts, amps and resistance to explain with a bit more clarity the dangers of electricity. Towards the end of the article, I mentioned that there are situations where you have high voltage and low resistance, but there is little or no danger. We actually experience these scenarios regularly without even thinking about it.
To explain this, it’ll help to give our understanding of volts a bit more nuance. At the core a voltage represents a potential. Commonly, it represents a potential for charge to flow (amps), but charge need not necessarily be flowing for a voltage to be present. If you have voltage potential without any flow of charge, we call that voltage “static”. Static voltages surround us all the time, every day. As everyone knows, sometimes these charges make their presence known (sometimes painfully) in the form of a static shock. That’s a bit of a misnomer though, because when the shock happens, the voltage (or more specifically the charge) is no longer static. It moved, likely from your hand to a door knob or possibly to another person.
What many people don’t understand is that even these benign, common, static charges and shocks we feel often represent tens of thousands of volts. As I discussed last week, at very high voltages like that, not even the normal resistance of your body will protect you, so how are these voltages so harmless? Once again, it comes down to flow of charge (amps). A static voltage potential may have the pressure of tens of thousands of volts behind it, but the actual charge available to be transferred is incredibly small. For example, when simulating static discharge that might be experienced by a human touching a circuit board, engineers might use a capacitor storing 100 picofarads of charge at a potential of 10000-40000 volts. Wait, but what’s a picofarad? A farad is a unit of ability to store charge. Whereas amperes represent charge in motion, a farad is simply some amount of charge that can be stored. One farad can be represented as one coulomb of charge across a potential difference of one volt. I mentioned coulombs in my first article, but for reference, one coulomb is a lot of charge. We almost always talk about farads in fractions. 100 picofarads is 100 trillionths of a farad.
Still with me? Let’s put this together. When you have a static charge, even if it’s thousands of volts, when the potential is neutralized (in the form of a discharge) there are very few coulombs of charge that will be moving. As we’ve learned, charges moving = amps. Less charges moving = less amps. This is why everyday static charges don’t hurt us. There just isn’t enough flow of charges.
Static charges can be created in several different ways. In the case of the barbecue lighter I mentioned last week, the charge is created by a piezoelectric element (read more about piezoelectrics here!). Everyday static charges are created when electrons become stripped from their orbits on atoms in materials. This happens on a huge scale with lightning. This is why lightning is so dangerous. Despite being the same kind of static charge that shocks us when we rub our feet on a carpet and touch a door knob, in the case of lightning, there is much, much more charge available when the discharge happens. So we aren’t talking about a few picofarads, and fractions of amperes. We’re talking about thousands of amperes. At that point, the current doesn’t matter, because it’ll be the superheating of material (including your body) that will be the cause of death.
When working with sensitive electronics even a small static discharge delivered by a human can be enough to destroy components. Electronics (unlike humans) are directly sensitive to high voltage potentials. A high voltage discharge (regardless of amps delivered) can cause insulation breakdown in the very delicate and thin insulation between components. Damaging this insulation may cause a short circuit resulting in component failure. This is why when working on exposed electronics, technicians use a wrist strap that is grounded. Grounding allows any charge to flow easily and safely from the technician into a table, or other metal structure, rather than into the sensitive electronics.
I had really only intended this series to be a very brief introduction to electricity and only had three pieces planned. Is there more you’d like to know about? Any specific question you’ve always had? Let me know and I’ll do my best to explain in the next issue!