While it seems counterintuitive, current is a fundamental quantity in the SI system because it’s easier to measure and define than charge, despite being related to it. The definition of the ampere (the unit of current) relies on measurable magnetic forces, whereas directly measuring and quantifying charge is more challenging. Historically, current was also the first electrical quantity to be accurately measured.
Here’s a more detailed explanation:
Fundamental vs. Derived:
A fundamental quantity is a basic unit of measurement that cannot be expressed in terms of other units (like length, mass, time, etc.). A derived quantity is defined in terms of fundamental quantities (like velocity, which is length/time).
Current and Charge:
Current is the rate of flow of charge, meaning it’s the amount of charge passing a point in a given time. So, while current is related to charge, it’s the rate of flow that’s considered fundamental.
Measurability:
Measuring the amount of charge passing a point is complex and often requires indirect methods. Measuring current, however, can be done directly with devices like ammeters. The ampere is defined based on the magnetic force between current-carrying wires, which is a measurable phenomenon.
Historical Context:
Historically, current was the first electrical quantity to be accurately measured and defined. The concept of charge was later developed, and the definition of the coulomb (the unit of charge) is based on the ampere.
Intuitive Analogy:
Think of velocity (speed) and displacement (distance). Speed (velocity) is a fundamental quantity in the SI system, even though it’s related to distance (displacement). It’s the rate of change of position, and it’s easier to measure and define than the distance itself. Similarly, current is the rate of flow of charge, and it’s easier to measure and define than the amount of charge itself.