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Physics · Electricity
Amps (Electric Current)
Reference entry · last updated 20260908
Amps, short for amperes, measure electric current: the rate at which charge passes through a surface, such as a wire’s cross-section. One ampere (A) equals one coulomb per second.[1]
1. First principles and definitions
Average current is charge passing a point divided by elapsed time. For steady current, \(I=Q/t\), where Q is charge and t is time.[1]
The ampere is an SI base unit. Its definition fixes the elementary charge at exactly 1.602176634 × 10−19 C. A current of 1 A corresponds to about 6.24 × 1018 elementary charges passing per second.[2]
Conventional current follows the direction positive charge would move. In a metal wire, electrons drift in the opposite direction.[1]
2. Current, voltage, and power
Voltage measures energy per charge. Current measures charge per second. In steady DC, their product is power in watts. For a resistor obeying Ohm’s law, current equals voltage divided by resistance.[3]
3. Worked DC example
Illustrative calculation: a device consuming 24 W at 12 V draws 2 A. That steady current carries 20 C past a point in 10 seconds.
A milliampere (mA) is one thousandth of an ampere: 500 mA equals 0.5 A.[4]
4. Amps and amp-hours
An amp-hour (Ah) measures charge. A steady current of 1 A for one hour transfers 3,600 C. Illustrative calculation: 2 A for 3 hours transfers 6 Ah.[1]
An Ah value alone does not give stored energy. If voltage stays constant, multiplying Ah by V gives Wh. Thus, an idealized 12 V source delivering 6 Ah supplies 72 Wh.[3]