What units measure electric current in the SI system?

Electric current in the SI system is measured in amperes (A).

In the International System of Units (SI), the standard unit for measuring electric current is the ampere, often shortened to 'amp'. Named after the French physicist André-Marie Ampère, this unit is fundamental in the study of electricity and magnetism.

Electric current refers to the flow of electric charge, typically carried by moving electrons in a conductor such as a wire. The ampere is defined formally as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, and placed one metre apart in a vacuum, would produce a force of 2 × 10^-7 newtons per metre of length between these conductors.

In simpler terms, one ampere of current represents one coulomb of electrical charge (6.242 × 10^18 charge carriers, usually electrons) moving past a specific point in one second. It's worth noting that the direction of current flow is conventionally considered as the direction in which positive charges move. However, in metallic conductors, electrons carry the charge and move in the opposite direction, but this does not affect calculations.

The ampere is a base unit in the SI system, meaning it is one of the fundamental measures from which other units are derived. For instance, the unit of electric charge, the coulomb, is defined as one ampere-second. The unit of electrical power, the watt, is defined as one ampere times one volt.

In practical applications, electric current is often measured in smaller units like milliamperes (mA) or microamperes (µA), particularly in electronics. Conversely, in high-power applications like power transmission, current may be measured in kiloamperes (kA).

Understanding the ampere and how to measure electric current is crucial in physics, particularly in the fields of electromagnetism and electronics. It's a fundamental concept that underpins much of our modern technology, from the smallest electronic devices to the largest power stations.