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What is resistance?

- a tutorial about what is resistance and how resistance affects electrical circuits, resistors and Ohm's law.

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. In fact so basic are these measurements that multimeters measuring them were named after them. In the UK the name AVO standing for Amps Volts and Ohms became a trade name for a multimeter, and in North America the term VOA meter was sometimes used as a generic name for a multimeter which could make these three measurements.

As electrical resistance is such a basic concept in electrical and electronics circuits it is necessary to know 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.

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 resistance of an item is its length. The shorter the length of material, the lower its overall resiatnce. 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.


Ohm's Law

There is a mathematical relationship which links current, voltage and resistance. A German scientist named Geog Ohm performed many experiments in an effort to show a link between the three. In the days when he was performing his experiments there were no meters as we know them today. Only after considerable effort and at the second attempt did he manage to devise what we know today as Ohm's Law. Simply stated this says that the current flowing in a circuit is directly proportional to the applied potential difference and inversely proportional to the resistance in the circuit. In other words by doubling the voltage across a circuit the current will also double. However if the resistance is doubled the current will fall by half.

In this mathematical relationship the unit of resistance is measured in ohms.

Ohm's law can be expressed in a mathematical form:

V (potential in volts)     =    I (current in amps)     x     R (resistance in ohms)

The formula can be manipulated so that if any two quantities are known the third can be calculated. To help remember the formula it is possible to use a triangle with one side horizontal and the peak at the top like a pyramid. In the top corner is the letter V, in the left hand corner, the letter I, and in the right hand bottom corner, R.

To use it cover up the unknown quantity and then and then calculate it from the other two. If they are in line they are multiplied, but if one is on top of the other then they should be divided. In other words if current has to be calculated the voltage is divided by the resistance i.e. V/R and so forth. If the voltage has to be calculated then it is found by multiplying the current by the resistance i.e. I x R.

To give an example: if a voltage of 10 volts is placed across a 500 ohm resistor it is possible to calculate the current which will flow. Looking at the triangle the current is the unknown leaving the voltage and resistance as the known values. The current is found by dividing the voltage by the resistance i.e. V/R = 10 / 500. This is 0.02 Amps or 20 milliamps.


Resistance notation

The basic unit if resistance is the Ohm as already mentioned. This is often denoted by the Greek symbol omega - In addition to this the basic unit can be prefixed by multipliers. This is because the range of values for 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
k thousands kilohms
M millions Megohms

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 megohm 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.


Resistance summary

When working with any electrical and electronics circuits, it is necessary to know what is resistance and how resistance affects the circuit. In view of the importance of resistance in circuits, resistors are widely used, possibly the most commonly used components in electronics circuits. These components are very easy to use, and the calculations associated with them are normally simple.

 


Practical Electronics Handbook

Ian Sinclair, John Dunton


Practical Electronics Handbook

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