Why don't we get shocked when we touch a car battery's terminal? --- Akshay, Nagpur, India
Marshall Brain Answers:
If you watch the following video, you can see that a standard car battery contains bunch of energy.
On the right is a screwdriver connected to a red jumper cable. The black jumper cable connects to a metal saw. When the saw touches the screwdriver to complete the circuit, the screwdriver gets hot enough to glow like the filament in a light bulb.
So what would happen if you were to grab hold of the red and black ends with your bare hands? Pretty much nothing. 12 volts is a low voltage, and the resistance in your body is fairly high, and at that voltage, not enough energy would flow through your body for you to feel anything.
Here's another way to think about it. If you touch the two ends of a AA flashlight battery, you don't feel a thing. These batteries deliver 1.5 volts. You can touch the two terminals of a normal 9-volt battery and you won't feel a thing (If you touch the two terminals with your tongue you will feel a tingle. A 12 volt battery is in that same league. The voltage is not enough to affect the human body.
When the battery is in a car, there is something else happening as well. If you touch the positive terminal of the battery and you don't touch any other part of the car, nothing at all will happen because you are not completing the circuit. Some other part of your body would have to be touching the negative terminal, or a piece of metal in the car (because the whole metal body of the car is connected to the negative terminal of the battery).
However, you don't want to mess with a car battery because it does have that massive amount of current available. If you happen to be wearing a metal bracelet, necklace or ring and if it happens to make contact between the positive and negative terminals (in any configuration) you will get a nasty burn.
This leads to the obvious question: when does voltage become dangerous? The 120 volts AC coming out of a normal household outlet can be fatal. At 120 volts, enough current flows through the body to cause damage (e.g. it can disrupt the electrical system of your heart). This page explains why:
To illustrate how a 120V wall-outlet voltage can affect a person, you divide the 120V voltage by the resistance to yield the current. For a high body resistance, dividing 120V by 100 k yields 1.2-mA current. For medium and low body resistance, divide the 120V by 10k and 1 k, respectively, yielding 12 and 120 mA, respectively. At 1.2 mA, a human body would be on the threshold of feeling a tingling sensation; at 12 mA, the human body would feel the beginning of a freezing, or "can't-let-go," feeling. At 120 mA, the feeling would be extreme pain and possible ventricular fibrillation.
I just got out my ohm meter. With dry fingers my body's resistance measures about 1 million ohms. When I lick my fingers it drops to about 100,000 ohms. If you are standing in a puddle of water with a bare electric wire in your wet hand, that's when you worry about fatal shocks. And that is why we have GFCI outlets in bathrooms and on exterior outlets - they protect against that kind of thing.