Ohm’s Law

What is Ohm’s Law?

Ohm’s law defines the relationship between voltage, current, and resistance by the equation below.  The second equation better represents voltage as the difference between two electric potentials.

$V = IR$ $V_{1} - V_{2} = IR$

Note that V1 and V2 are voltages measured with respect to ground and V is the voltage potential measured between them.

The equation derived from Ohm’s law is incredibly useful for an electrical engineer.  Ohm’s law allows many circuits to be fully analyzed with the aid of just a few measurements.  Many circuit analysis techniques you will learn involve some combination of KCL, KVL, and/or Ohm’s law.

Ohm’s law states that there must be a voltage drop (or voltage difference) across a resistor in order for any current to flow.  If V1 and V2 are the same, no current flows through the resistor.  In terms of currents, a current produces a voltage drop across a resistor; if there is no current, there is no voltage drop across the resistor.

A circuit component that follows Ohm’s law has a constant resistance.  Increasing the current through the component will produce a proportional increase in the voltage drop across it.  The plot of the current through the component versus the voltage drop across it will be linear, with the slope of the line determining the resistance of the component.

I-V Plot to describe Ohm’s Law

Figure 2 shows I-V plot of a 1 Ω and a 2 Ω resistor, both of which follow Ohm’s law.  Figure 3 shows the I-V curve of a component not following Ohm’s law.  You will learn about two components that don’t follow Ohm’s law later in the course (diodes and MOSFETs).

Written by Ryan Eatinger (reatinge@ksu.edu).  Thank you!