From Wikipedia, the free encyclopedia
For other uses, see
Ampere (disambiguation).
Current can be measured by a galvanometer, via the
deflection of a magnetic needle in the magnetic
field created by the current.
The ampere, in practice often shortened to amp,
(symbol: A) is a unit of
electric current, or amount of
electric charge per second. The ampere is an
SI base unit, and is named after
André-Marie Ampère, one of the main discoverers of
electromagnetism.
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Contents
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1
Definition
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2
Explanation
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3
Proposed future definition
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4
See also
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5
References
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6
External links
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Definition
The ampere is that constant current which, if maintained in
two straight parallel conductors of infinite length, of
negligible circular
cross section, and placed 1
meter
apart in vacuum, would produce between these conductors a force
equal to 2×10–7
newton per meter of length.[1]
Electric current is the time rate of change or displacement
of
electric charge. One ampere represents the rate of 1
coulomb of charge per second.
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The ampere is defined first (it is a
base unit, along with the
metre,
the
second, and the
kilogram), without reference to the quantity of charge. The
unit of charge, the coulomb, is defined to be the amount of
charge displaced by a one ampere current in the time of one
second.
Explanation
Because it is a base unit, the definition of the ampere is
not tied to any other electrical unit. The definition for the
ampere is equivalent to fixing a value of the
permeability of vacuum to μ0 = 4π×10−7
H/m. Prior to 1948, the so-called "international ampere" was
used, defined in terms of the
electrolytic deposition rate of
silver.[2]
The older unit is equal to 0.999 85 A.
The ampere is most accurately realized using an
ampere balance, but is in practice maintained via
Ohm's Law from the units of
voltage and
resistance, the
volt
and the
ohm, since the latter two can be tied to physical phenomena
that are relatively easy to reproduce, the
Josephson junction and the
quantum Hall effect, respectively.
The unit of
electric charge, the
coulomb, is defined in terms of the ampere: one coulomb is
the amount of electric charge (formerly
quantity of electricity) carried in a current of one ampere
flowing for one
second.[3]
Current, then, is the rate at which charge flows through a
wire or surface. One ampere of current (I) is equal to a flow of
one
coulomb of charge (Q) per second of time (t):
-
Proposed future definition
Since a coulomb is approximately equal to 6.24150948×1018
elementary charges, one ampere is approximately equivalent
to 6.24150948×1018 elementary charges, such as
electrons, moving past a boundary in one second.
As with other
SI base
units, there have been proposals to redefine the
kilogram in such a way as to define some presently measured
physical constants to fixed values. One proposed definition
of the kilogram is:
- The kilogram is the mass which would be accelerated
at precisely 2×10-7 m/s2 if
subjected to the per metre force between two straight
parallel conductors of infinite length, of negligible
circular cross section, placed 1 metre apart in vacuum,
through which flow a constant current of exactly 6 241
509 479 607 717 888 elementary charges per second.
This redefinition of the kilogram has the effect of fixing
the
elementary charge to be e = 1.60217653×10-19
C
and would result in a functionally equivalent definition for the
coulomb as being the sum of exactly 6 241 509 479 607 717
888 elementary charges and the ampere as being the electrical
current of exactly 6 241 509 479 607 717 888 elementary charges
per second. This is consistent with the current 2002 CODATA
value for the elementary charge which is 1.60217653×10-19
± 0.00000014×10-19 C.
CIPM recommendation
International Committee for Weights and Measures (CIPM)
Recommendation 1 (CI-2005): Preparative steps towards new
definitions of the
kilogram, the ampere, the
kelvin and the
mole in terms of fundamental constants
The International Committee for Weights and Measures (CIPM),
- approve in principle the preparation of new definitions
and mises en pratique of the kilogram, the ampere and the
kelvin so that if the results of experimental measurements
over the next few years are indeed acceptable, all having
been agreed with the various Consultative Committees and
other relevant bodies, the CIPM can prepare proposals to be
put to Member States of the Metre Convention in time for
possible adoption by the 24th
CGPM in 2011;
- give consideration to the possibility of redefining, at
the same time, the mole in terms of a fixed value of the
Avogadro constant;
- prepare a Draft Resolution that may be put to the 23rd
CGPM in 2007 to alert Member States to these activities;
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This
SI
unit is named after
André-Marie Ampère. As for all SI units whose names
are derived from the proper name of a person, the first
letter of its
symbol is
uppercase (A). But when an SI unit is spelled
out, it should always be written in
lowercase (ampere), unless it begins a
sentence or is the name "degree
Celsius".
— Based on
The International System of Units, section
5.2.
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See also
- SI
-
Ohm's Law
-
Electric shock
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Ampere's law
-
Ammeter
References
- ^ Paul
M. S. Monk, Physical Chemistry: Understanding our
Chemical World, John Wiley and Sons, 2004
online.
- ^ Robert
B. Northrop, Introduction to Instrumentation and
Measurements, CRC Press, 1997
online
- ^ Kuzman
Ražnjević, Physical Quantities and the Units of the
International System (Si), Begell House Publishers, 1995
online
External links
-
The NIST Reference on Constants, Units, and Uncertainty
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A short history of the SI units in electricity
Categories:
SI base units |
Units of electrical current
