THE AMPERE - UNIT OF CURRENT BASIC DEFINITION AND TUTORIALS


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 per 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 0. 001 mA, a millionth of an ampere. And it is increasingly common to hear about nanoamperes (nA), where 1 nA 0. 001 μA 0.000000001 A (a billionth of an ampere).

Rarely will you hear of kiloamperes (kA), where 1 kA 1000 A. 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. An electric iron draws approximately 10 A; an entire household normally uses between 10 A 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 will flow 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, flow; this might 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 amounts of charge. 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 at all.


The name of the unit is a tribute to the French scientist Andr´e Marie Amp`ere. The electrical engineering convention states that the positive direction of current flow is that of positive charges.

In metallic conductors, however, current is carried by negative charges; these charges are the free electrons in the conduction band, which are only weakly attracted to the atomic structure in metallic elements and are therefore easily displaced in the presence of electric fields.

WHO IS ANDRE MARIE AMPERE?

(1775–1836)
Born: Lyon, France

Ampere is best known for his pioneering work in the field of Electrodynamics. During his emotionally troubled life, he held several professorships: at Bourg, Lyon, and at the Ecole Polytechnic in Paris.

While Ampere worked in several sciences, the work of the Danish physicist Hans Christian Oerstad on the electric deflection of a compass needle, as demonstrated to him by Dominique Arago, caused Ampere’s great interest in electromagnetism.

His seminal work, Notes on the Theory of Electrodynamic Phenomena Deduced Solely from Experiment, established the mathematical formulations for electromagnetics including what is now known as

Ampere’s Law. It can be said that Ampere founded the field of electromagnetics. He is honored for this by the naming of the unit of electric current as the ampere.



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