# What is resistance? Electrical Resistance

### Electrical resistance is one of the key attributes in an electrical circuit – it determines the current flowing for a given voltage.

Resistance Tutorial Includes:
What is resistance     Ohms Law     Resistivity     Series & parallel resistors

There are three basic measurements which can be made in an electrical circuit. Voltage and current are the first two, and the third is resistance.

As electrical resistance is such a basic concept in electrical and electronic circuits it is necessary to answer some questions: what is resistance, what are resistors, and how resistance affects circuits.

## What is resistance?

Before looking at what resistance is, it is necessary to understand a little about current and what it is. Essentially a flow of current in a material consists of a movement of electrons in one direction. In many materials there are free electrons moving about randomly within the structure. While these move randomly there is no current flow, because the number moving in one direction will be equal to the number moving in the other. Only when a potential causes a drift in a particular direction can a current be said to flow.

#### What is resistance

Resistance is the hindrance to the flow of electrons in material. While a potential difference across the conductor encourages the flow of electrons, resistance discourages it. The rate at which charge flows between two terminals is a combination of these two factors.

If two different conductors are placed in a circuit, then the amount of current found to flow in each may not be the same. There are a number of reasons for this:

1. The first is that the ease with which electrons are able to move within the structure of the material. If the electrons are bound tightly to the crystal lattice, then it will not be easy to pull them free so that there can be a drift of electrons in a particular direction. In other materials there are very many free electrons drifting randomly around the lattice. It is these materials that allow a current to flow more easily.
2. Another factor that affects the electrical resistance of an item is its length. The shorter the length of material, the lower its overall resistance. From the analogy with a pipe the longer the pipe the more resistance to the water flow there will be.
3. The third is the cross sectional area. The wider the cross sectional area the lower the resistance. Again comparing it to a water pipe, the wider the bore, the easier it is for water to flow through it.

In most cases conductors are required to carry current with as little resistance as possible. As a result copper is widely used because current flows easily within its structure. Also its cross sectional area is made wide enough to carry the current without any undue resistance.

In some instances it is necessary to have elements which resist the flow of current. These items are called resistors and they are made out of materials which do not conduct electricity as well as materials like copper or other metals.

## Resistance notation

The basic unit of electrical resistance is the Ohm as already mentioned. This is often denoted by the Greek symbol Ω.

In addition to this the basic unit can be prefixed by multipliers. This is because the range of values for electrical resistance can span many decades and it is necessary to have an easy notation that does not rely on counting the numbers of zeros in a number as this would easily lead to mistakes.

Multiplier Meaning Name
R units Ohms, Ω
k thousands kilohms, kΩ
M millions Megohms, MΩ

Occasionally resistances less than an ohm are encountered, these are measured in milliohms (m ) thousandths of an ohm.

Normally when resistances are indicated on an electronic circuit diagram they are denoted as 10R for a ten ohm resistor, 10k for a ten thousand ohm resistor, and 10M for a ten MΩ resistor. The reason for this is that the Greek letter omega is not as easy to use as the prefixes R, k, and M.

## What are resistors?

In order that the current can be limited in a particular circuit, a component known as a resistor may be used. Resistors come in a variety of forms from large wired components, or even some using terminals to the very small surface mount components used in many electronics circuits today.

#### Note on Resistor Types:

There are many different types of resistor. Even though they all resist current flow, their other parameters mean that some types are more suitable for particular applications than others.