Ohms Law

What is Ohm's Law

Ohm's law, named after Georg Ohm, is a set of relationships between the fundamental electrical quantities: Voltage, Resistance and Current. As we will see shortly, it is a very straightforward mathematical relation ( nothing more complicated than multiplication and division), but an understanding of the law is the key to a general understanding of many topics in audio electronics, including input and output impedances, equalizers and shelf circuits, the high-frequency rolloff due to a guitar cable, etc.

Electrical Quantities

As mentioned previously, the three main electrical quantities are Voltage, Current and Resistance. These quantities are all related through Ohm's law. A solid conceptual understanding in Ohms law is an incredibly valuable tool for understanding much of what goes on in audio systems, as well as electrical circuits for audio.

Before we jump into the details, lets first take a look at a useful analogy in order to foster a general understandings of the concepts.

A Useful Analogy


Example showing relationship between voltage and current


Example showing relationship current and resistance

Relating this to Electricity

Analogy Created by: Electrical Concept Created By Measured in:
Water Force Water Tank Electrical Force Battery ( or generator) Volts
Water Current Flow of Water Electrical Current Flow of Charges Amperers (Amps)
Resistance to Flow Pipe Diameter Electrical Resistance Resistance to moving charges Ohms

Here is a summary …


Voltage, which is also known as electromotive force, is the force which drives the current (electrical charges) through an electrical circuit.


Current is the amount of charge which is pushed through a circuit by the Voltage.


Resistance, is just what it sounds like — the resistance to the flow of charge through the circuit.

Relationships Between Electrical Quantities

Current is Proportional to Voltage [ The higher the voltage, the more current that you are going to get our of a circuit. ] This is captured by the following relationship:

$V = IR$

By using simple algebra, you can derive 2 other relationships. ( by knowing 2 out of the 3 quantities, you can always determine the 3rd )

$I = { V \over R}$ and $R = {V \over I}$

Making Connections

Simple Circuit: Battery and Resistor


Simple Circuits: Examples


Electrical Power

The last of the relationships that you need to know is the expression for electrical power, $P$. In terms of voltage and current, power is given by.

$P = VI$

Hence, the higher the voltage the more power delivered. Similarly the more current in a circuit the greater the power level is.

There are 2 other expressions which can be derived by substituting the Ohm's law expressions for voltage and current into the above expression, yielding:

$P = I^2R$ and $P = {V^2 \over R}$

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