(Basic Electric Circuits)_Current, Voltage and Resistance

In physics or mathematics, understanding definitions of basic terms is essential. Without a stable foundation, the next level of knowledge can not be built above it securely. To understand an advanced level of electric circuits, it is important to understand the basic definitions correctly. This series of posts will help to grip the basic electric circuit terms. Let's start with current, voltage and resistance. 

 

Electric Current

An electric current is a rate of flow of charge, usually carried by electrons. In an electrical conductor or space, charged particles such as electrons or ions can move freely. The unit of current is the ampere (A). A current of 1A is the flow of electric charge across the surface or control volume at the rate of one coulomb (C) per second (s) which means 6.25×10^18 electrons passing along the surface every second. 

The Coulomb (C) is defined as the unit of charge. One coulomb is equal to the amount of charge that flows along a surface or wire in one second when the current is 1A (equation 1).  

Equation 1. The relationship between the current (A) and the charge (C).

 

Voltage (Potential Difference)

A voltage across an electrical component is necessary to make an electric current flow. The definition is often explained as a waterfall. To make water flow, a potential height difference is needed between two points. In electric circuits, a cell or batteries often take the role and provide the potential difference between two points that the voltage needed for the current flow. Voltage can be expressed as the below equation 2. 

 

Equation2. The voltage of a cell or a battery is the power (W) that the cell can deliver to the rest of the circuit for every ampere of current passing through it. 

 

Resistance

When electrons pass through conductors in an electric circuit, the electrons repeatedly collide with the ions of the conductors and lose energy in these collisions. This state makes the current flow difficult and causes resistance. To overcome the resistance and keep flowing from one point to the other, the electrons must use some of their energy. In equation 3, the relationship between resistance, current and voltage is described. This relationship is called Ohm's law. Ohm's law explained that the current through a conductor between two points is directly proportional to the voltage across the two points. 

Equation 3. The amount of resistance is defined as the ratio of voltage (V) across two points in a circuit to current (A).