Lesson 5: SI Units of Measurement

Volt (V)

The SI unit of electric potential, the volt, is named in honor of Alessandro Volta. One volt is defined as the potential difference that will drive one ampere of current against one ohm of resistance. Volta’s invention of the voltaic pile, the first chemical battery, was crucial in providing a continuous source of electric current, making the volt a fitting tribute to his contributions.

Ampere (A)

The SI unit of electric current, the ampere, is named after André-Marie Ampère. One ampere is defined as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible cross-sectional area, placed one meter apart in a vacuum, would produce a force equal to 2×10−72×10−7 newtons per meter of length. Ampère’s work in electromagnetism and his formulation of Ampère’s law were foundational in understanding the relationship between electricity and magnetism.

Watt (W)

The SI unit of power, the watt, is named after James Watt, a Scottish engineer and inventor. One watt is defined as one joule of energy transferred or converted per second. Watt’s improvements to the steam engine were instrumental in the Industrial Revolution, and his work on power measurement laid the groundwork for the modern understanding of energy and power.

Ohm (Ω)

The SI unit of electrical resistance, the ohm, is named after Georg Simon Ohm, a German physicist. One ohm is defined as the resistance between two points of a conductor when a constant potential difference of one volt applied to these points produces a current of one ampere. Ohm’s law, which states that $V=IR$ (voltage equals current times resistance), is fundamental to the study of electrical circuits.

Coulomb (C)

The SI unit of electric charge, the coulomb, is named after Charles-Augustin de Coulomb, a French physicist. One coulomb is defined as the amount of charge transferred by a current of one ampere in one second. Coulomb’s law, which describes the force between two charges, is a cornerstone of electrostatics.

Farad (F)

The SI unit of capacitance, the farad, is named after Michael Faraday, an English scientist. One farad is defined as the capacitance of a capacitor that, when charged with one coulomb of electricity, has a potential difference of one volt between its plates. Faraday’s work in electromagnetism and electrochemistry was ground-breaking, and his discoveries are fundamental to the field.

Henry (H)

The SI unit of inductance, the henry, is named after Joseph Henry, an American scientist. One henry is defined as the inductance of a circuit in which a change in current of one ampere per second induces an electromotive force of one volt. Henry’s research in electromagnetism and his discovery of self-induction were pivotal in the development of electrical engineering.

In summary, these SI units of measurement honour the pioneering scientists whose discoveries and inventions laid the foundations for modern electrical science and engineering. Each unit represents a fundamental aspect of electricity and magnetism, essential for understanding and working with electrical systems. These scientists, operational in the first half of the 18th century, laid the groundwork for the subsequent “War of the Currents” that would unfold towards the end of the century. This pivotal conflict between direct current (DC) and alternating current (AC) systems would shape the future of electrical power distribution and set the stage for the modern electrical era.