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Noobie question here; NOTE: I don't understand electrics much.

If a car's electrical system grounds to the bodywork (usually to the negative terminal) so that the electricity flows through the metal of the car, why don't I get shocked whilst touching the paintwork or touching the grounds themselves whist the battery is still connected and the car is running?

I've never understood electronics, and it just made me think that the bodywork is usually metal as well, so why doesn't that do anything in terms of shocking you?

The only thing I can somewhat guess is that I'm not touching the positive terminal of the battery and/or anything connected to it to put myself in line with the circuit, but that explaination doesn't sound right, because I can touch both terminals and not get anything. I don't understand how electronics work at all in a car, as you can probably tell, lol. Cheers in advance! :)

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    Because, even if you touch the body, you are not "in" the circuit. However, sometimes dry conitions will allow static to form and you can get a mild shock that way... – Solar Mike Sep 26 '17 at 11:20
  • Thankyou, all for your comments and answers, very helpful! :) All good answers and comments, definitely helped me reach a logical conclusion of what explanation would work for me, too! – yollooool Sep 27 '17 at 16:41
6

The reason is that very little current is flowing into the body of the car and that current is only at 12 volts which is not enough to penetrate dry human skin. For example, if a person puts a dry finger across the terminals of a 9v battery, they will not feel anything (but if a person puts the battery to their tongue, they will feel a tingle).

If current was actively flowing to the body of the car, then it would be a "short" and would cause the battery to be rapidly drained.

To easily go through average human skin, about 50 volts is needed. If the person is sweaty, the number can be much lower. If the current is exposed to the blood very little voltage is needed. In one case a guy electrocuted himself with a 9 volt battery by connecting probes to each terminal, then holding each probe in a hand and piercing his thumb with the probes. This allowed the current to directly access his blood and go through his heart.

  • I preferred this response for explaining A) what would be required and B) what would make it easier for getting a shock from the car. So in essence, because the output I would be exposed to there is so low in terms of how much would actively try to flow through me, it would not cause so much as a tingle unless parameters like ease to flow through water on my skin were apparent. This makes a lot of sense now, compared to the voodoo sorcery explanation I was sitting on! :P Thanks! – yollooool Sep 27 '17 at 16:40
  • Yeah, 12V just doesn't have the needed pressure to push through skin. As in the 9v battery, it's possible if you really work at it, but again flesh is so high impedance that not much current will flow at only 12V. This is why ~50V is considered the maximum for wiring designated "low voltage". Above that, it can bite you. – Harper - Reinstate Monica Sep 27 '17 at 20:46
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    "current is only at 12 V"? Nope. Voltage is only 12 V. You do not get shocked because the chassis of the car is usually at ground potential. You would not get shocked even if the battery was 12000 V. This answer does not make sense. – Vladimir Cravero Sep 28 '17 at 7:44
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    @finleyarcher The OP has already made it clear that he finds the "scientific" answers obtuse and unhelpful. – Cooter Davenport Sep 28 '17 at 15:25
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    Just a note to the OP @yollooool, some parts of the ignition system do have the capability of delivering a nasty shock; and shorting across the battery (or leads) can cause sparks/fire/electronics damage etc. Be careful, deenergize the system by disconnecting the battery when working, and/or use insulated tools. – Jimmy Fix-it Sep 29 '17 at 20:02
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  1. Unless you touch both battery posts, your body isn't completing the circuit from - to +
  2. The voltage is not high enough to overcome the resistance of your body (under most circumstances)

The most dangerous aspect of automotive batteries is that they give off hydrogen gas. If you create a spark close to the battery, its possible to ignite the gas, causing the entire battery to explode and shower you with sulphuric acid.

Read more here

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    It should be noted, if you are wearing jewelry and these items create the continuity (positive lead touches jewelry and subsequently touches ground). These metal items become red hot almost instantly and will cause severe burns. It's a good idea to remove such items (such as rings and necklaces) which might happen to find it's way to a connection. – Pᴀᴜʟsᴛᴇʀ2 Sep 26 '17 at 13:32
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    #3 is not true, electricity does not choose the path of least resistance, it will choose all paths, in inverse proportion to that paths resistance. – Glen Yates Sep 26 '17 at 14:15
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    the low voltage (#2) is probably the biggest factor here. You can in fact touch both battery posts with your fingers and you wont feel anything (A current will go through you but it is so small that you wont feel it) – agentp Sep 26 '17 at 14:20
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    @agentp, I've always been too scared to do that. I've seen what a battery can do to a screwdriver or wrench and I don't want any part of that! – JPhi1618 Sep 26 '17 at 14:26
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    @agentp But if you try that with your tongue, you will definitely feel it. – David Schwartz Sep 26 '17 at 19:02
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Electricity is like bullets. It can only hurt you if it goes through you. If the bullet wizzes by your face, or the electricity flows through the metal in the car in front of you, it doens't hurt you.

Now there's a few caveats to this. The first is that if you touch anything that is connected to the positive side of the battery and something metal, then electricity will flow through you from the positive side of the battery towards the metal (and eventually the negative side of the battery). Fortunately, it is typically nonlethal because you're not all that good of a conductor of electricity. However, Paulster2 made a good point in comments: if you can short a battery with a piece of jewelry, such as a ring, it is a much better conductor and can heat up very rapidly. That heat can burn you.

The other caveat is that if you touch the body of the car in two places, in theory current is flowing through you. Without getting into the electrical engineering details, a very tiny amount of current will flow through you in that case... much less than what happens when you scuff your feet across a carpet floor.

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    Bear in mind that touching the metal car body in two places is routinely done (in the US at least) when the police are patting down a person before hand-cuffing and putting in a patrol car. None of these people are electrocuted. – FreeMan Sep 27 '17 at 14:56
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    @FreeMan Very true. The only reason I mention it is because, if you ever do dig into the electrical engineering side of this problem, the idea that no electricity goes through you will conflict with what we know about circuits. What you find instead is exactly what you mention: the level of electricity is sufficiently small that we don't even notice, much less get hurt by it. – Cort Ammon Sep 27 '17 at 14:59
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If a car's electrical system grounds to the bodywork (usually to the negative terminal) so that the electricity flows through the metal of the car

The electricity (the electrical current) does not flow through the metal of the car. It flows to the metal.

There's a reason it's called a "current"; it's much like water. Water needs a height difference to flow. Electricity needs a voltage difference to flow. No voltage difference = No electrical current = No electricity.

The metal of the car serves as the "ground." Again, like water (which wants to flow to the ground), electricity wants to flow to the ground. If you touch the metal twice, there's no voltage difference and the electricity doesn't flow. There is a large, semi-lethal voltage difference between the positive (+) terminal of the battery and the negative (-) terminal of the battery.

The water-electricity analogy isn't perfect, but it's useful for simple questions like this.

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    Also, don't mix water and electricity :) – valbaca Sep 26 '17 at 20:34
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    Actually, it really wants to flow to the negative terminal of the battery, to even out the imbalance created at the other terminal. So if something in the circuit dumps current into the chassis, it will flow through the metal of the car to the point where the negative terminal is strapped to the body. And this is in fact regularly done, because returning through the chassis instead of using a dedicated return wire saves weight and components, and because a big hunk of steel has a pretty low resistivity. – hobbs Sep 27 '17 at 5:05
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    So there absolutely is a current flowing through the body of the car, but the reason it's used in the first place also dictates that there isn't a noticeable voltage across it: because it conducts so well. V=IR, if you make R small enough, V will be small too. – hobbs Sep 27 '17 at 5:08
3

Electricity is often compared with water because it has a lot of behaviors that map well to behaviors seen in water. One of those behaviors is the tendency to follow the path of least resistance. In the case you ask about, you (and the paint) are far less conductive than the car body and so the electricity prefers to flow through the car body.

That isn't to say that no electricity flows through you, it's just so little that you don't notice. Under typical conditions, touching a car body will encourage some (a microscopically small number of) electrons to migrate through your body. They also migrate through the tires, through the air and even through the battery casing. This is one of the reasons why typical batteries won't stay permanently charged when left unused. (The practical existence of conductance and resistance are not black and white absolutes but it's usually useful to treat them as though they were.)

3

Also, 12V of a standard car battery or 24V of a dual battery car (like a diesel) is not enough to overcome the resistance of your clean dry skin.

If you're wet or oily or have metal shavings poking into your bloodstream, or actively bleeding then things get a bit different.

3

The 12 V of a car battery is low voltage: you need quite wet hands for this to do anything. Somebody suggested touching the tongue would make this a different experience. Well, I'd still expect that stretching your tongue from one terminal to the other would do more of a damage than the current for a short touch would (you can briefly touch the terminals of a 9V battery with your tongue: this is quite unpleasant but not doing terminal damage).

However, things become entirely differently in the car's high voltage circuitry leading to the spark plugs (starting at the ignition coil(s)). Don't touch anything there, particularly not while having any connection to the ground. There are several ten thousands of Volts there and enough current to cause a whole lot more damage than static electricity (carpets can easily produce similar voltages but the discharge is much less energetic).

That's the area dangerous for human flesh. The car terminals, in contrast, are dangerous for metal parts: a wrench falling on truck battery terminals will die causing a lot of damage (car battery housings are usual not too happy about molten metal). So will any metal jewelry.

  • Panics after wearing a metal watch right near a battery I've personally never managed to zap my watch on a battery, though I will say YET. That'll be like a branding iron! :O Apprentices seem to be the main ones doing that, dropping wrenches on batteries and then wondering why there was a massive spark from the battery in front of them and a whole workshop of bemused co-workers.... – yollooool Oct 1 '17 at 17:15
-1

You don't get shocked for the same reason that birds perched on high-voltage cables are unaffected.

If you were to remove one of the wires from the battery and grasp it in one hand whilst placing the other hand on the terminal then you could potentially get hurt because then you'd be part of the circuit.

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    It's still 12V so even if you did become part of the circuit it would be difficult to notice it. – Sam Sep 27 '17 at 15:26
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    That old adage, it's not the voltage that kills you. You might be fine until the starter circuit was engaged and the system attempted to draw 660cca (Cold Cranking Amps) from the battery. Good old ohms law states it's current that kills you, even at small voltages. People survive lightening strikes and they're tens of thousands of volts. – Steve Matthews Sep 27 '17 at 15:30
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    @SteveMatthews Except that's bullshit, there's no way 12V/R can be high enough to do damage to a human. You can't even feel it unless you're drenched in oil. – Navin Sep 27 '17 at 15:39
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    allaboutcircuits.com/textbook/direct-current/chpt-3/… Add something like wet hands or metal rings / jewelery and I'm sure a car battery would be enough so make you feel a slight shock. A 9V battery is enough to give you a tingle if you lick it. The OP question was about shocks, not fearsome death. – Steve Matthews Sep 27 '17 at 15:42
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    @SteveMatthews, "You might be fine until the starter circuit was engaged and the system attempted to draw 660cca (Cold Cranking Amps) from the battery" That has literally nothing to do with it. It doesn't matter that they are "cold cranking" they are just electrons flowing. All that matters Ohms law, and Humans are in the MegaOhm range. – Sam Sep 27 '17 at 16:25

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