(Nov 21, 2012)
This from November CX:
Ohm I God
How much tech do you need to know? It’s a frequently visited (and dismissed) topic at the bunker. There are those who think that theory of anything is a quick trip to boredom. OK, here’s a test: “Ohm’s law”. Have you stopped reading yet? Clearly not. Ohm’s law is came from the German mathematician Georg Ohm in 1827, who made a name for himself doing electrifying things with frogs legs, metal plates, jars of acid, and bratwurst. While outwardly it was thought that he was searching for electric truths, it’s also widely believed that he was trying to invent the Energiser Bunny. What he did discover was resistance, thus paving the way for Hugh Hefner who would later establish exactly how much resistance an energised bunny has.
Ohm built battery after battery, using nothing more than parts. He connected all kinds of objects together to see if they would light up, and finally made the big discovery for which he’s best known: I=V/R pronounced “eye equals vee over arrrhhh”. Sounds more like something that a pirate might say, and that’s another story. The formula says that current (in a circuit) equals volts divided by resistance. I could have discovered that on a quiet day between gigs, and then you’d all have to say “Maiz’s law” which is harder to say and sounds like a curse. So let’s give a pat on the back to Ohm.
One of the things they taught me in high school maths is that this type of formula is obscenely simple to solve. If one of the values is kept constant, then the value goes up and down proportionally with the third. They are locked together. Hold that thought.
Last October I was in a Hollywood setting with a Gaffer and his lighting electricians – the so called Best Boys. I looked with a mix of awe and amazement at the accepted Hollywood lighting protocols, which involve bakelite and wood, and exposed connections. Open live copper holes everywhere, just waiting to show that Ohm was right and do rather more than excite a frog’s leg. It struck me that in a million years the authorities in Australia would never accept US plugs, sockets and practices. But nobody appeared to be shocked or needing treatment, so I figured all was well until a conversation with one of the Best Boys went, so help me Julius, like this:
Me: “Isn’t that wiring really unsafe… with all those exposed connectors? They’d never approve those connectors in Australia.”
BB: “Actually, this wiring would be way safer for you guys down under”.
BB: “You guys have 240V, right?”
BB: “So when you grab hold of the wire, only half the current flows compared to 120V”.
Well, that certainly stopped my heart for a few beats. A Best Boy is the second highest person in the lighting chain (after the Gaffer) and would be expected to have a serious idea of his or her craft. I’d sort of assumed that included knowledge of Ohm’s law. The conversation made me reconsider that – was there a possibility that this bozo had obtained his electrical ticket by opening a box labelled “Kelloggs”? In which case, I hoped it had came with CPR training. Mr Best Boy had demonstrated the problem that comes from half-learning a useful piece of theory: someone had told him once that double-the-volts means half-the-current. Which would be true if you’re talking about, say, 100 watt 240V light bulbs compared to 100 watt 120V light bulbs. Mr Boy, showing that his brain might have been constrained to only a few watts itself, missed the obvious point. Where resistance doesn’t change, double the volts means double the current. A warm body in the southern hemisphere still has the same resistance as a warm body in the USA, so double the volts means a more rapid transition to dead. BANG. Do not pass go.
The moral is that a solid piece of tech theory is a very good thing, and a little bit of rote learning can cause great harm.
Homework for today: How many badly-educated US sparkies can you connect in parallel across a 240 volt circuit before they learn the difference between current flowing in a lamp and current flowing in a human body? First correct entry opened gets this month’s special prize, which is an open-circuit ELCB.
A REPLY FOLLOWS:
Hi John, while having a spare moment to ponder your question I have come up with the following regarding how many badly educated US sparkies you can connect in parallel across a 240V circuit before they learn the difference between current in a lamp and current in a human body.
Now we do have a bit of an issue here as at 240V AC if you connect one person they will die so that person has actually not learnt anything as he is now dead, learning something assumes you actually retain the knowledge for future use and I don’t think death actually then qualifies as a learning experience at least not for the person involved.
Now as your article actually referred to connecting sparkies in parallel then each and everyone of them will actually die if they have 240V across them as opposed to if you decided to hook them up in series. So to be correct I must assume that its enough to hook one sparkie up and the rest will learn from his death. Having said that the equation gets quite complex, the person hooked up learns nothing while those watching gets an education. Having said that the death of one sparkie may not teach the lesson as well as killing say 100 sparkies or even 1000 at that. So as you increase the amount of sparkies killed in this way the educational experience gets more powerful while the amount of US sparkies would naturally decrease. So this is really a bit of a trick question depending on the quality of the reporting media in the United States which is well outside my area of expertise. If I had to pick a number I’d say electrocute one and hope the next sparkie gets the lesson.
Now if on the other hand there was a misprint in CX (WHOA….) and you meant to say electricians in SERIES not PARALLEL, then we come across another range of variables.
IF we can assume that the sparkies are not sweating profusely, working in the rain or that Wikipedia is not correct….. then….another amazing web fact surfaces. The human body is not a linear resistor but actually has an impedance that also varies across the population (Impedance meaning that at lower voltages the skin resistance to an alternating frequency would be much higher than at higher voltages according to the IEC) . If we then assumed that these sparkies were pretty average and that thus their body impedance at 240V was just over 1470 Ohms then we would need three of them to ensure that they all lived to learn the lesson getting 54mA through them as at 60ma they’re likely to die and thus learn nothing. I have not taken into account the frequency difference of 50 to 60Hz as I think that is taking this answer a bit too far and I don’t want to be petty.
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