Ohm’s Law is a foundational concept in the study of electrical circuits, establishing a direct relationship between three fundamental electrical quantities. Named after the German physicist Georg Simon Ohm, this law provides a mathematical framework for understanding how electricity behaves in many common materials. Knowing the meaning of the symbols in the equation is the first step toward grasping how electric current flows and how circuits are designed.
Defining the Variables of Ohm’s Law
The letter ‘E’ in the common representation of Ohm’s Law, \(\text{E} = \text{I} \times \text{R}\), stands for Electromotive Force (EMF). This term describes the electrical potential difference, or the “push,” that drives the flow of charge in a circuit. While ‘E’ is the historically accurate symbol for the source voltage, many modern textbooks and engineers use the letter ‘V’ to represent the same concept, which is simply called Voltage. The use of ‘E’ is often reserved specifically for the voltage supplied by a source, like a battery, whereas ‘V’ may be used for the voltage drop across a component.
The symbol ‘I’ represents the Current flowing through the circuit. Current is the rate at which electric charge passes a specific point in a circuit. The symbol ‘I’ is thought to have originated from the French term intensité du courant, or the intensity of the current.
The letter ‘R’ represents Resistance. Resistance is the opposition a material offers to the flow of electric current. This property determines how easily or how difficult it is for the electrical charge to move through a component or conductor.
The Standard Units of Measurement
Voltage, represented by ‘E’ or ‘V’, is measured in volts, which is symbolized by the capital letter V. One volt is defined as the electrical potential difference that will send one ampere of current through a resistance of one ohm.
Current, represented by ‘I’, is measured in amperes, which is often shortened to “amps” and symbolized by the capital letter A. An ampere quantifies the flow rate of electric charge, representing the movement of a certain number of electrons past a point in one second.
Resistance, represented by ‘R’, is measured in ohms, and its symbol is the Greek capital letter Omega (\(\Omega\)). One ohm is the amount of resistance that restricts a current of one ampere when a voltage of one volt is applied.
Applying the Formula and Basic Analogies
The relationship between the variables can be expressed in three algebraic forms, depending on which value needs to be calculated. The core formula is \(\text{E} = \text{I} \times \text{R}\), which states that the electromotive force is the product of the current and the resistance. This relationship can be rearranged to solve for the other two quantities: \(\text{I} = \text{E} / \text{R}\) and \(\text{R} = \text{E} / \text{I}\).
A common and helpful way to visualize this concept is through the water-and-pipe analogy. In this model, the electrical potential difference, or Voltage (E), is analogous to the water pressure created by a pump. This pressure is the driving force that pushes the water through the system.
The Current (I) is comparable to the volume flow rate of the water, representing how much water moves through the pipe in a given amount of time. Finally, the Resistance (R) is like a restriction in the pipe, such as a narrow section or an aperture. A greater restriction (higher resistance) results in a lower flow rate (lower current) for the same pressure (voltage).