Engineering Fundamentals: Ohm’s Law

Ohm’s Law, discovered by Georg Simon Ohm and first published in 1827, is the earliest and arguably most important connection between current, voltage, and resistance. A straightforward and practical technique for studying electric circuits, Ohm’s Law is very commonly used and has been documented on a broad range of scales. For aspiring electrical engineers studying the basics or for seasoned professionals looking to refresh their knowledge, this scientific law is worth having a deep understanding of.

More About Voltage, Currents, and Resistance
Current flows are the continuous flow of free electrons through a circuit. This can be likened to liquid flowing through a hollow pipe. Voltage is the force that causes electrons to “flow” in a circuit. The higher the voltage, the stronger the force pushing electrons from one end of a circuit to another..

Resistance refers to the resistance of motion / amount of friction present as free electrons pass across conductors. The quantity of current in a circuit is determined by the voltage available to drive the electrons as well as the degree of resistance in the circuit to prevent electron flow. Resistance, like voltage, is a quantity that exists between two places. As a result, voltage and resistance values are frequently expressed as “between” or “across” two locations in a circuit. In order to make meaningful assertions about these characteristics in circuits, we need to be able to quantify them similarly to how we would for temperature, length, or any physical property.

For this reason, specific measurements are used.

> Current is measured in Ampere, or “Amps”.

> Voltage is measured in Volts.

> Resistance is measured in Ohms.

What Does Ohm’s Law State?
For any given temperature, Ohm’s law asserts that the amount of electric current flowing through a metal conductor in a circuit is always proportional to the voltage across it. The law is described in the equation

E = 1R

Where voltage (represented by E), is equal to the current (represented by I) multiplied by the resistance (represented by R). This equation can be re-written to solve for either I or R:

I = E/R or R = E/I

Closeup of green circuit board

Water and Pipe Analogy
To think of this concept another way, consider how Ohm’s Law can be thought of as water going through a pipe. In this analogy, the three variables can be described as water pump pressure (voltage), the water (current), and a constraint (resistance) respectively. Assuming that the resistance to the flow of water remains constant despite the rise in pump pressure, then it stands to reason that the flow rate must increase with it. If the pressure remains constant but the resistance rises (making the water flow more difficult), the flow rate must decrease. If the flow rate remained constant but the resistance to flow decreased, the pump’s necessary pressure would also decrease.

Electrical Power Calculation
Aside from voltage and current, power is another way to quantify free electron activity within a circuit.

Broadly defined, power refers to the metric that measures how quickly a certain quantity of work is completed. Within the context of circuitry, power is defined as a function of voltage and current in electric circuits. This is outlined using the equation

P = IE

In which P is representative of power, which is equal to the current multiplied by the voltage. The unit of measurement for power is measured in watts.

The mathematical link between the dissipation of power and current through a resistance was discovered by a man named James Prescott Joule. Published in 1841, this finding became known as Joule’s Law, and is represented by the equations of:

P = I2R, P = IE, and P = E2/R

Resistors are devices that are used in electric circuits to give exact quantities of resistance. Both the resistance (measured in ohms) and the capacity to disperse thermal energy are used to assess resistors (measured in watts).

Although estimated power ratings may be calculated from the physical dimensions of the resistor(s) in question, resistor resistance ratings cannot. The greater the resistor, the more it can properly dissipate power without causing harm to the circuit.

A load is defined as any equipment that uses electricity to accomplish a beneficial purpose. In schematic designs, resistor symbols are sometimes used to signify a non-specific load rather than one of a real resistance.

The information and formulas in the post are derived from Chapter 2 of “Lessons in Electric Circuits – Volume 1” by Tony R. Kuphaldt under the design science license.