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Ampere: Definition & Calculation

Instructor: Thomas Zesiger

Thomas has taught electronics and communications engineering, math, and physics and has a master's degree in electrical engineering.

The ampere is the unit of electric current. It is abbreviated as A and may also be called amp or amps. Learn how to calculate amperes and see examples of amperes in everyday life.

Ampere Definition

Have you ever wondered what powers some of your home appliances and modern electronic devices? The answer is a little more complicated than just 'a power cord.' Current is the force that drives these systems and allows them to perform their necessary functions. Electrical and electronic engineers measure this important system parameter and its associated unit for the purposes of design, maintenance and repair.

So how do these professionals measure current? When a voltage is applied across the ends of a conductor, electrons are forced to move in the same direction along the wire. This electron motion is transmitted along the path similar to the way motion in a whip is transmitted from one end to the other.

Current in a wire
Current in a wire

Current is measured by the number of electrons that pass a point on the wire in one second. Current, or the flow of electrons, is similar to the flow of water in a pipe, which is also measured in a similar way.

Water flow in a pipe
Water flow in a pipe

The ampere is the unit of electric current agreed upon by an international commission. It is abbreviated as A, and those working in the field often refer to the unit in a shortened version as an 'amp' or in 'amps'. The unit is named after the French physicist and chemist Andre Marie Ampere.

It is this flow of electrons that allows the operation of electrical devices such as lamps, motors, and heaters. Current is also used for plating metals in a process called electrolysis, which is the transferring of ions through water. It is this plating ability of current that we can use to accurately measure and define it.

Measurement

An electrolytic cell is the apparatus used in electrolysis. The electrolytic cell is used to define the ampere. The amount of metal deposited in an electrolytic cell is directly proportional to the current through it and also proportional to the time the current is kept flowing.

A solution of silver nitrate (the electrolyte) in water is used. The steady state current that deposits silver at the rate of 0.001118 gram per second (g/s) from this solution is taken as the unit of current and is called one ampere. One ampere is also defined in another way as the flow of one coulomb of charge per second, or 6.25 x 10^18 electrons per second. A 100-watt light bulb uses approximately 1 amp while an electric clothes iron operating at 600 watts used approximately 5.5 amps. The current required for most modern electronics like stereos and televisions is usually under 1 amp and is only about 2-3 amps at most.

Based on this definition of the ampere, measurement of current is made in a standardizing laboratory using a silver voltameter. A voltameter is a device used for measurement of electric charge. A silver voltameter consists of a platinum crucible holding a solution of fifteen to twenty parts of silver nitrate to one hundred parts of distilled water by weight, together with two plates (anode and cathode) of pure silver suspended in it. There are also certain specifications for the volume of the silver nitrate electrolyte and the current density, or surface distribution, over the electrode surfaces. While electrons are flowing, silver dissolves at the anode and is deposited at the cathode.

Diagram of a voltameter
Diagram of a voltameter

Other types of voltameters, such as those using copper (copper sulfate electrolyte) or zinc (zinc sulfate electrolyte), may be used for the measurement. These types of voltameters are much less accurate, however, and are consequently more suited for academic use. They are completely inadequate for 'official' measurements or any work involving standardization of the ampere unit.

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