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Ohm's Law for Simple Electrical Circuits

by Ron Kurtus (revised 6 December 2013)

Ohm's Law is a most fundamental formula in electrical circuits. It states that the electric current passing through a conductor is directly proportional to the potential difference across the conductor. It was first formulated in 1827 by German physicist Georg Ohm while doing experiments on how well metals conducted electricity.

Ohm's Law is best demonstrated in a simple DC electrical circuit. Although it also applies to AC circuits, care must be taken to account for other possible variables.

The relationship between current, voltage, and resistance in a circuit allows you to calculate one variable if you the values of the other two.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Equation

Ohm's Law shows the relationship between voltage, current, and resistance in a simple electrical circuit. The easiest form of the equation is:

V = IR

where:

Thus, if you know the current and resistance, you can use the formula to find the voltage.

By using Algebra, you can rearrange the variables to suit your needs. For example, if you know the voltage and resistance and want to find the current, you can use:

I = V/R

Or, if you know the voltage and current and want to find the resistance, you can use:

R = V/I

Configuration

A simple electrical circuit consists of metal wires running to and from a power source, along with a source of resistance, such as resistors or a light bulb in a series with the source. A typical power source is a DC battery, although a DC or AC generator can also apply. If an AC circuit includes components such as capacitors or inductors, Ohm's Law does not apply.

Simple DC Circuit

Simple DC Circuit

Using equation

An importance of Ohm's Law is that if you know the value two of the variables in the equation, you can then determine the third. You can measure any of the parameters with a voltmeter. Most voltmeters or multi-meters measure voltage, current and resistance for both AC and DC.

Find voltage

If you know current and resistance, you can find voltage from V = I R. For example, if the current I = 0.2 A and the resistance R = 1000 ohms, then

V = 0.2 A * 1000 Ω = 200 V

Find current

If you know voltage and resistance, you can use algebra to change the equation to I = V / R to find the current. For example, if V = 110 V and R = 22000 ohms, then

I = 110 V / 22000 Ω = 0.005 A

Find resistance

If you know voltage and current, you can use algebra to change the equation to R = V / I to find the resistance. If V = 220 V and I = 5 A, then

R = 220 V / 5 A = 44 Ω

Summary

Ohm's Law is the equation V = I R that shows the relationship between voltage, current and resistance in a simple electrical circuit. It can apply to both AC and DC circuits.


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Resources and references

Ron Kurtus' Credentials

Websites

A Little History About Ohm - Short history

Ohm's Law - Explanation, including an Ohm's Law calculator

Basic Electrical Laws - Includes circuit theory

Electrical Circuit Formulae - High level equations for problem solving

DC and AC Electricity Resources

Physics Resources

Books

Teach Yourself Electricity and Electronics by Stan Gibilisco; McGraw-Hill; (2001) $34.95 - Guide for professionals, hobbyists and technicians desiring to learn AC and DC circuits


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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