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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:

- What do the parameters in the equation mean?
- What is the circuit configuration?
- How do you apply Ohm's Law?

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:

**V**is the voltage in volts (**V**)**I**is the current in amperes or amps (**A**)**R**is the resistance in ohms (**Ω**- Greek letter Omega)

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

## 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.

Be determined to do your best

## Resources and references

### 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**

### 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

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## Ohm's Law for Simple Electrical Circuits