Charge, Current, and Voltage

Electricity is the energy that drives the modern world, but at its core, it is simply the movement and interaction of elementary particles. To understand electrical engineering, we must first grasp three most fundamental quantities: Charge, Current, and Voltage.

Electric Charge ($q$)

Electric Charge is a fundamental physical property of matter. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. In most electrical circuits, the charges that move are electrons.

The unit of charge is the Coulomb (C). One Coulomb is a massive amount of charge—it is equivalent to the charge of approximately $6.242 \times 10^{18}$ electrons.

In calculation formulas, charge is represented by the variable $q$ (or $Q$ for constant charge).

Electrical Current ($I$)

While charge is a fundamental property, Electrical Current is the rate of flow of that charge with respect to time. If you imagine a wire as a water pipe, current is the rate at which water flows through that pipe.

Mathematically, current is defined as the derivative of charge with respect to time:

One Ampere (A) is defined as the flow of one Coulomb of charge per second (1 A = 1 C/s).

Engineering Note: Due to historical convention, we use "Conventional Current," which flows from positive to negative. We maintain this rule in engineering analysis, even though in physics, electrons actually move from negative to positive.

Voltage or Potential Difference ($V$)

Current tells us that charges are moving, but why do they move? They move because energy is expended to push them.

Voltage, also known as Electric Potential Difference, is the energy required to move a unit charge between two specific points. If current is the flow rate of water, voltage is the pressure pushing that water.

One Volt (V) is defined as one Joule of energy per Coulomb of charge (1 V = 1 J/C).

Whenever we specify a voltage, it is always a comparative measurement between two points (a potential difference). A "12V battery" means there is a 12-volt potential difference between its positive and negative terminals.

Summary of Relationships

• Charge ($q$) is the fundamental "stuff" influenced by electrical forces.
• Current ($I$) is how fast that charge is moving.
• Voltage ($V$) is the energetic push that makes the charge move.

Understanding these three quantities is a prerequisite for understanding resistance and Ohm's Law, which link all of these quantities together in practical circuits.