Why don't I get shocked constantly?

Question

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! :)

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.)

Answer 6

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.

Answer 7

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.

Answer 8

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.