I am aware that there are thousands of volts passing through spark plugs as a result of being stepped up in the coil. But doesn’t the increase in voltage result in a corresponding decrease in amperage (I think there are about .02 - .06 amps moving through the plug)? And isn’t it the amperage that determines the severity of the shock to the victim? Does the amperage increase when it passes into your body?
A simplified way of looking at it is that the voltage determines how far the arc will jump and the amperage determines how bad the damage will be. A spark plug gets up to tens of thousands of volts but the low current is the reason it doesn’t do any real damage when you get zapped by one. There’s a lot of grey area when you start trying to separate voltage and current effects since a change in one usually affects the other.
My basic understanding is that voltage is a measure of how hard the electricity is being pushed and amperage is a measure of the quantity of electricity being pushed. So it’s a very small amount of electricity being pushed very hard.
No, because your body is an extra load to the spark plug. The added resistance is enough to bring the current down to a non-fatal but still painful level.
For the same amount of energy, if you increase the voltage you decrease the amperage. This is how your typical power transformer works.
If you just increase the voltage and then apply it to a load though, you’ll increase the current that flows through the load as well.
Let’s say you’ve got a 10 volt source, and a 1,000 ohm resistor (I’m intentionally choosing values that make the math easy). The current is I=V/R or 10/1000 or 0.01 amps. Simple enough. (look up Ohm’s Law if you are lost already)
Now let’s put in a voltage transformer that increases the voltage by a factor of 10. Like so:
10 volt source -> transformer -> resistor
Now you have 100 volts applied to your 1,000 ohm resistor. The current through the resistor now is 100/1000 or 0.1 amps. You have ten times as much current as you did before.
At your 10 volt source, though, you have even more current. You now have 10 watts (P = VI) of power going through the resistor, so your source has to supply 10 watts of power. Instead of 0.1 amps at the 10 volt source, you have 1.0 amps.
At the source you have 10 volts at 1.0 amp.
After the transformer you have 100 volts at 0.1 amps.
So, as you can see, when you increase the voltage you decrease the current, in this case by a factor of 10. But, because we have changed the circumstances at the load, we now have much more current overall than we had before.
That’s the basic concept that I think you are missing, and this is what HongKongFooey meant when he said when you change one it affects the other.
That said, coils aren’t simple transformers. They store energy in a magnetic field and release it. A spark plug also isn’t a simple resistor, and neither is the human body. In these cases things get a lot more complicated. The simplest model for a human that we tend to use is a resistor in series with a resistor and capacitor in parallel, and even with that model the resistor values are different depending on the voltage involved. If you measure your body’s resistance with a multimeter (which uses a very low voltage), you’ll find that the resistance is very large, on the order of many megs. At home AC outlet voltages you’ll find that the resistance of a human body is only a few k, a very significant difference.
Generally speaking though it is the amount of current that you can get to flow that causes the damage. A shock from a 120 volt line under wet conditions may cause as much current to flow as a shock from a 1200 volt line under dry conditions, and in either case the amount of damage would be the same (because the current flow is the same) even though the applied voltages would be different. Under equal conditions though a 1200 volt shock will give ten times as much current as a 120 volt shock (and assuming that the body is a simple resistor, which it isn’t).
I shocked myself on a spark plug cleaning machine once. It hurt like hell. But it was gone in a minute.
Yea, can remember working on an engine with my dad back in the old days. We had the distributor cap off, I was holding it with a couple of fingers inside it. He turned over the engine and I got the jolt of my life (not counting women). It didn’t feel good but it didn’t kill. I was sort of proud of the fact that I took 10-12 thousand volts.
Amps, that’s a different story.
And that’s the shocking story.
Way back in the dark ages, flat-head engines were the rule; spark plugs on such engines were very accessible and you could practically count on being accidentally shocked if you had to do anything under the hood while he engine was running. If you were trying to isolate the source of a “knock” inside the engine, shorting out one spark plug at a time was an accepted practice; I knew several guys who would simply put their thumb on a spark plug and their index finger tip on the cylinder head. They claimed that if you performed these two operations fast enough, the electrical shock was minimized. I never got the hang of it, though.
My dad once gave me a shock while I was holding the spark plug of the lawnmower to the chassis, looking for the spark.
Nowadays most cars have CDI - capacitative discharge electronic ignition, instead of sparks and coils. Shorting the plug could damage the electronics. Not recommended.
IIRC, too, the old idea is a coil is energized creating a magnetic field; when the gap is opened, the magnetic field collapses; that collapsing field induces a huge surge - igh voltage - that can jump the gap in the plug. So it’s not just stepped up voltage, you are also storing the power for a while, and releasing that same energy in a much shorter time. OUCH.
In many ways, the OP is a victim of widespread misunderstandings about electricity.
It IS true that the severity of shocks is determined by their current. (amperage) The threshold for death is in the 10-100mA range.
What is NOT true that the voltage doesn’t matter. Due to the resistance of the human body, significant voltage is required to force the current to flow. Thus a 12V car battery that can supply several hundred amperes to a starter motor will only produce a few microamps if you grab the terminals with each hand. It may be correct to say that it is the current that kills you, but that current won’t happen if there is not high voltage to push it.
US houshold AC at 115VAC is seldom sufficient to induce a lethal shock, even though outlets can supply 15 - 20A.
I just wanted to add a footnote that amperage is a measure of the rate of current flow. To measure how much charge passes through a given point, you need to know the current (amperage) and amount of time.
automotive ignition systems have interesting transient behavior. Over the space of a millisecond, the voltage can vary from 10+ kV before the air in the spark gap ionizes, down to ~50 volts (yes, just fifty) once a good arc is established, and back up to 500-1000 volts near the end of the event. The initial current can be well over a hundred amps (lasting for tens of nanoseconds), and tapers down to zero at the end.
A lot of that transient behavior is due to the nature of the air in the spark gap. It has very high resistance initially - especially when the engine is near TDC, and cylinder pressure is elevated - but once it breaks down/ionizes, the resistance drops dramatically, and so does the voltage drop required to move current across the gap.
As you might expect, the nature of the voltage and current observed when an ignition coil discharges through a human body will be completely different. Your body has different capitance and resistance than the spark gap described in the previous paragraph. Hard to give numbers, but I would expect dramatically lower peak voltage across your body (no spark is happening across your body; if there’s a spark going on, it’s before/after the current enters/leaves your body). Current? Something less than the 100+ amp peak observed in a normal spark plug, but not milliamps, either.
Part of the injury one experiences might be a burn, if there is a spark between one’s skin and either the coil or ground. This is why static electricity shocks hurt.
He had one of those flat head engines that in the rain would get water pooling up in the well surrounding the spark plugs. So when his car shorted out and died, he would take a copper tube and blow the water out of the wells then fire it right back up.
One day he decided to pre-emptively blow the water out before it got deep enough to total kill the engine…forgot to shut the car off first.