Ohm's Law for Simple Electric Circuits
by Ron Kurtus (revised 23 October 2019)
Ohm's Law is a most fundamental formula for simple electric 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 electric 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
Ohm's Law shows the relationship between voltage, current, and resistance in a simple electric circuit. The easiest form of the equation is:
V = IR
- 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
A simple electric 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.
Note: If an AC circuit includes components such as capacitors or inductors, Ohm's Law does not apply.
Simple DC Circuit
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.
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
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
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
Ohm's Law is the equation V = I R that shows the relationship between voltage, current and resistance in a simple electric circuit. It can apply to both AC and DC circuits.
Be determined to do your best
Resources and references
A Little History About Ohm - Short history
Ohm's Law - Explanation, including an Ohm's Law calculator
Basic Electric Laws - Includes circuit theory
Electric Circuit Formulae - High level equations for problem solving
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
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