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Q & A: Voltage and current

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Most recent answer: 10/22/2007
Q:
What is the difference between voltage and potential difference, and voltage and current? How is it that a car battery which is only 12V can cause a person to die if touched with wet hands, for example, and other things of thousands of volts won’t cause much harm?
- Siddharth (age 15)
Shanghai, China
A:
Voltage and potential difference are different names for the same thing. The potential difference is the amount of energy you get when carrying a unit of electrical charge from one place to another. You need to specify both places in order to define the voltage difference between them. For example, the two ends of a battery have a potential difference between them -- they are at different voltages.

Current is something else entirely. It is the rate at which electrical charge flows. The standard unit for charge is the Coulomb, and the standard unit for current is the Ampere. One Ampere is one Coulomb per second. The unit for voltage is the Volt. All of these are named after pioneers of electricity and magnetism. One Watt of power is made available by flowing a current of one Ampere across a potential difference of one Volt.

If you touch the battery with wet hands, then a lot of current may flow from one of the terminals of the battery to the other. The resistance of your body is low due to the fact that blood is mostly saltwater, and saltwater conducts electricity rather well. The contact with the skin is usually poor. Wetting your hands will make the contact better, since the water will be salty from your skin's sweat. But the battery can supply lots of current, and this can burn the tissue inside the body.

For a few thousands of volts of static electricity built up on your body by walking on a carpet on a dry day, the potential for damage is less. This is because very little charge is involved, and once you allow the current to flow (by touching a doorknob or a faucet, for example), the voltage decreases rapidly. The whole spark is over in a tiny fraction of a second and the voltage is gone. Power is voltage times current, but the total energy deposited is power times time, and if this is short enough, very little energy is deposited. The battery can keep supplying current for a long time, depositing more and more energy, making the burns worse.

Tom

(published on 10/22/2007)

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