Unit of Electric Current: ampere, amp

The ampere or amp is the SI unit of electric current, enabling the volume of current in a circuit to be defined.

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It is important to be able to quantify the amount of current flowing in a circuit as it enables the performance of the circuit to be defined and for the circuit to operate as required.

To achieve this it is necessary to have a unit and this is the ampere or amp. The abbreviation for this is ‘A’. A current of ten amperes may be written as 10 amps or 10A.

Note: the name of the physicist Ampère is spelt with capital A and with the accent, the term of the unit of current is ampere or amp without the capital letter or accent.

Unit of current; ampere definition

The ampere is equivalent to a charge of one coulomb per second flowing in a circuit. Although this is the practical realisation of the ampere, the formal definition relates the level of current to basic SI parameters.

Ampere definition:

The formal definition of an ampere is the constant current which if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length.

Definition of the ampere
Conditions for the definition of the ampere

The ampere is the SI unit of electric current and it is one of the seven SI base units

Interestingly one ampere is approximately equivalent to approximately 6.24×1018 elementary charges such as electrons or holes moving past a given point or boundary in one second.

Physicists consider current to flow from relatively positive points to relatively negative points; this is called conventional current or Franklin current.

This definition utilises electromagnetism to define the unit of current. This leads on to implicitly determine the value for the magnetic constant µ0 = 4 π 10-7 Hm-1 = 4 π 10-7 m kg s2 A-2. Therefore, the base unit ampere - and thus all other electrical units - is related to the base units meter, kilogram and second via this fundamental constant.

History of the ampere

The unit of current; the ampere is named after André-Marie Ampère who one of the early pioneers in electrical science.

Note on André-Marie Ampère:

André-Marie Ampère (1775–1836) was a French mathematician and physicist. He undertook many experiments associated with the early science of electricity and in view of his pioneering work he is considered by many to be the father of electrodynamics.

Read more about André-Marie Ampère.

In view of the really fundamental work that was undertaken by Ampère, the unit of measurement for electric current, the ampere was named after him. This was in recognition of his great contribution to setting in place many of the fundamentals of modern electrical science. The name ampere was established as a standard unit of electrical measurement at an international convention signed in 1881.

Multiples and sub-multiples for amperes

The range of current carried in different scenarios varies enormously – by many orders of magnitude. It is therefore necessary to use the standard multiples and sub-multiples.

Multiples & Submultiples of amperes
Current Name Abbreviation
10-15 amperes femtoamps fA
10-12 amperes picoamps pA
10-9 amperes nanoamps nA
10-6 amperes microamps µA
10-3 amperes milliamps mA
amperes amps A
103 amperes kiloamps kA
106 amperes Mega amps MA

Current in amperes of typical devices

Different devices use different levels of current and on often wonders how many amps a device might use. The table below gives a list of the typical current in amps used by a number of different electrical and electronic items.

Typical current in amperes of common devices
Device Details Typical current
Electric fire 1kW bar running from 240V mains supply 4 amps
Desktop computer Computer being used and not in standby ~ 0.5 amps
Kettle Typical 2.5 kW kettle running on 240 volts 10 amps
Laptop computer On being charged ~0.2 amps
Television Example of typical 42 inch LED flat screen TV ~0.3 amps

The ampere is one of the seven base SI units and as such it is key to electrical and electronic science as well as many other areas of science. The definition is based upon the electromagnetic effect it induces giving it a fundamental definition.

Ian Poole   Written by Ian Poole .
  Experienced electronics engineer and author.



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