The Ampere

Current is a measure of the rate at which charge carriers flow. The standard unit is the ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers flowing every second past a given point.

An ampere is a comparatively large amount of current. The abbreviation is A. Often, current is specified in terms of milliamperes, abbreviated mA, where 1 mA = 0.001 A, or a thousandth of an ampere. You will also sometimes hear of microamperes (μA), where 1 μA = 0.000001 A or 0.001 mA, which is a millionth of an ampere. It is increasingly common to hear about nanoamperes (nA), where 1 nA = 0.001 μA = 0.000000001 A, which is a thousandth of a millionth of an ampere.

A current of a few milliamperes will give you a startling shock. About 50 mA will jolt you severely, and 100 mA can cause death if it flows through your chest cavity. An ordinary 100-watt light bulb draws about 1 A of current in a household utility circuit. An electric iron draws approximately 10 A; an entire household normally uses between 10 and 50 A, depending on the size of the house and the kinds of appliances it has, and also on the time of day, week, or year.

The amount of current that flows in an electrical circuit depends on the voltage, and also on the resistance. There are some circuits in which extremely large currents, say 1000 A, can flow. This will happen through a metal bar placed directly at the output of a massive electric generator. The resistance is extremely low in this case, and the generator is capable of driving huge numbers of charge carriers through the bar every second. In some semiconductor electronic devices, such as microcomputers, a few nanoamperes will suffice for many complicated processes. Some electronic clocks draw so little current that their batteries last as long as they would if left on the shelf without being put to any use.