Back to Basics - Ohm’s Law 

By on

The Basics

georgOhm
Georg Ohm
Electrical engineering has a multitude of laws and theorems.  It is fair to say the Ohm's Law is one of the more widely known; if not the most known. Developed in 1827 by Georg Ohm the law defines the relationship between voltage, current and resistance in an electric circuit.

Voltage is the force that is used to drive current around an electric circuit.  This can be provided by batteries, generators, etc., is measured in Volts (V) and denoted by the symbol V.  The voltage drives electric current around the circuit and this current does the useful work (heating an element in a light bulb for example).  Electric current is  typically described as the flow of electrons around a circuit, is measured in Amperes (A) and denoted by the symbol I.   Resistance is the opposition an electric circuit provides to the flow of current;  it limits the amount of current flowing when a voltage is applied.  Resistance is measured in Ohms (Ω) and denoted by the symbol R.

 

0243_OhmsLawR

 

The illustration shows a simple circuit consisting of a supply voltage V and resistance R around which flows a current I.  Ohm's law provides the relationship between these quantities:

b9feb13037d99819754b63a3a1b3bad7

That’s it - really straight forward and simple.   In words we can say the relationship between voltage and current in a circuit is a constant - the resistance.

The law can of course to be rearranged to give each parameter:

8b8cd7af2635ae91937229848104fe40

A little more complexity

The above is strictly true for direct current (dc) circuits.  In a dc circuit the current flows in one direction (positive to  negative) and is a constant value.  This changes in alternating current (ac) circuits - see the note article on Alternating Current Theory.  Because the current is constantly changing in an ac circuit, the magnetic fields are in motion and introduce additional constraints on the flow of current.  In addition to resistance we have a new parameter called reactance.  These combine into a quantity called impedance  which opposes the flow of electric current when a voltage is applied.

Like resistance the ac impedance is measured in Ohms (Ω) and is denoted by the symbol Z.   The good news is that impedance behaves like resistance and Ohm's Law still applies:

b64bd55d9e01dd318d456c53b349cebe

Note: when talking about dc circuits the quantities  (V, I and R) are scalar values.  In ac circuits these are vector quantities and the mathematics can be more complicated.

Even more complexity

To fully round off the discussion on Ohm's Law, it should be noted that it can be reformulated to apply in a more general sense to many physics problems, other than it’s pure application to dc or ac circuits.  This reformulation was carried out by Gustav Kirchhoff and is:

36e51a153d82a4f925daf6cd1a6a1aaa

In this representation J is the current density at some location (in a material or free space) and is measured in Ampere per metre squared.  E is the electric field in Volts per metre and c71da7f85beafa4c1806da1df216bda2 is the conductivity of the material in Siemens per meter.  Conductivity is the inverse of resistivity (Ohm meter).

There are other representations of Ohm's Law, but these are starting to transgress into the realm of theoretical interest only.  If you are interested in an application of the above formulae, you can look at the Earth Electrode Resistance note.

 

Being such an important and fundamental electrical law, most of us have had the experience learning and applying Ohm’s law.  If you have any tips or other Ohm’s law goodies to share, please add these below.

See Also



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Power Factor

Power factor is the ratio between the real power (P in kW) and apparent power (S in kVA) drawn by an electrical load. The reactive power (Q in kVAr)...

Why a Sine Wave?

I received this question by email a few weeks. First thoughts was that it is a product of the mathematics of rotating a straight conductor in a magnetic...

New Mail Chimp

We've been sending out Newsletters on a regular basis for a few weeks now. To do this we have been using Google's Feedburner service. While Feedburner...

Earth Electrode Resistance

Earthing of electrical systems is essential for the correct functioning and the protecting of life and equipment in the event of faults.  The earth electrode...

Generator Sizing & Operation Limits

When selecting a generator, there are inherent limits on the active and reactive power which can be delivered. Generators are normally sized for a certain...

International System of Units (SI System)

The International System of Units (abbreviated SI) is the world's most widely used system of units.  The system consists of a set of units and prefixes...

Batteries

A battery consists of one or more cells, each of which use stored chemical energy to produce electrical energy, There are many types of cells and these...

Dielectric loss in cables

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes...

Three Phase Current - Simple Calculation

The calculation of current in a three phase system has been brought up on our site feedback and is a discussion I seem to get involved in every now and...

DC Component of Asymmetrical Faults

The image (reproduced from IEC 60909) shows a typical fault in an ac system.  From the illustration it can seen that there is an initial dc component ...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note