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Basic Physics Lesson - 18: Electrical Resistance

Umesh is a freelance writer contributing his creative writings on varied subjects in various knowledge and educational sites in internet.

Introduction

Electrical Resistance is the property of a material to impede the flow of the electrical current under the applied voltage.

In an earlier article Basic Physics Lesson-14: Voltage and Current, we had studied about electrical voltage and electrical current and also seen as how they were related to each other. We also studied that when an electrical voltage was applied across a material then depending on the electrical properties of the material either nothing happened or some electric current started to flow through the material. Those materials through which current can flow are known as conductors and those, which do not allow any current to flow through them, are known as insulators.

Iron, Copper, Aluminium, Silver etc are examples of good conductors while Wood, Granite, Stone, Marble, Cardboard etc do not allow any currents to flow through them and are known as insulators. This property affecting the flow of current is known as Resistance and we will learn about it in more details in this lesson.

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How does the current flow in a material

Based on the electrical properties of a material they are classified in two main categories - conductors and insulators. Mostly metals are good conductors of electricity. That is the reason why we use Copper or Aluminium wire in our house for laying the electrical line from room to room.

In metals, generally the atomic structure is such that there are some free electrons available in the metal material which are actually the carrier of electricity from one place to other within that material. As we all know that the atoms contain a positively charged nucleus and around which negatively charged electrons revolve. Some of the electrons in the outermost orbits get dislodged and freely move in the metal and start moving in the direction of electrical field whenever an external voltage is applied to it.

Let us take a simple example of a torch bulb and a small dry battery (called cell also). Let us also take two small copper wires and connect the bulb to the battery. The bulb will start glowing. This is an ordinary traditional torch bulb which has got a metal filament which heats up whenever a current is sent through it and gives light. Nowadays, these bulbs are replaced by LED bulbs which have different mechanism for producing light but they also require an electrical current to pass through them for this purpose. Incidentally, LED bulbs draw little current so torch battery will run for a longer time as compared to old filament type bulbs.

Every battery or cell has two terminals. One is called positive terminal while other is called negative terminal. There is a voltage or electromotive force present between these two terminals and that is the source of electric power that drives current in the circuit. In the above example the electrical circuit is created by connecting the bulb to the battery using the two wires.

The negative terminal of battery is negatively charged while positive terminal of battery is positively charged. As soon as we connect the wires coming from the bulb to the battery the circuit is completed and the free electrons being negatively charged get attracted to the positive terminal and move towards it. This movement is actually the current in the circuit and as per scientific convention the direction of current is taken as opposite to the flow of electron. You might have seen the arrow mark in the drawings of various electrical circuits for showing the current direction. It is the same thing as mentioned here.

One thing that is important at this juncture is that in case of a filament type bulb it does not matter as which wire we connect to which terminal of the battery but if it is a modern LED bulb then as the LED bulbs are basically solid state devices (semiconductors) known as diodes, they have a peculiar property of conducting in one direction only and we have to be careful in checking or glowing a LED bulb with a battery.

Relationship between applied voltage and produced current in a conductor

When a voltage is applied across a conductor then depending upon its electrical properties some current starts to flow through it. As different conducting materials have different properties the value of this current will change from one material to other for the same applied voltage. For example if we use an electrical wire made up of Copper then it will have different resistance than that of an Aluminium wire. Actually the resistance of Copper wire is less than the resistance of Aluminium wire of same size and shape and then we say that Copper is a better conductor of electricity than Aluminium.

The electrical resistance depends on the shape and size of the material also and for an example if we increase the length of a metal wire then its resistance increases. At the same time a thin wire will have more resistance as compared to the thick wire. This implies that thick wires have more current carrying capacity than that of the thin ones and that is the reason why we require thick wires (thick electrical wires) to be used for high electrical power consuming appliances in our house.

In our houses the electrical wires used are generally of Copper though due to cost considerations many people also opt for less costly Aluminium wires.

So, the resistance is an important property of a material which determines as how much current will flow through it when a voltage is applied across it. We will be going through these details in the coming paragraphs.

Ohm's Law

When we apply a voltage across a conductor then some current flows through it. The current depends on the resistance of the conductor. It is observed that the current is inversely proportional to the resistance of the conductor. More the resistance less the current and less the resistance more the current. If we increase the applied voltage then also this current increases for the same resistance. These observations give us a relationship as follows -

I (current) is inversely proportional to R (resistance)

and

I (current) is proportional to applied V (voltage)

So, combining the above two we get I = V/R

This is the famous Ohm's law and generally written as V = I R

Here V is in volt, I is in Ampere, and R is the Ohm.

The unit 'Ohm' was named in the honour of Georg Simon Ohm, a German physicist who was the first person to verify Ohm’s law experimentally.

Ohm's law is very useful for electrical calculations required to be made in various electrical circuits or in various industrial electrical configurations.

Unit of Resistance

The unit of Resistance is Ohm. The large resistances are represented by Kilo or Mega prefixes. For example 1 Kilo Ohm is equal to 1000 Ohm while 1 Mega Ohm is equal to 1000000 Ohm.

Types of Resistances

Resistances are manufactured for industrial usage in variety of designs. Long back, the resistances were designed and based on metal wire wound on a bobbin. By changing the area of cross-section or diameter of wires and by changing their lengths a desired resistance could be designed. Later other types of Resistances were manufactured by using carbon film techniques and after the invention of semiconductor chips, many resistances were designed within a semiconductor chip itself. The chip was then soldered on the relevant Printed Circuit Board (PCB) as per the circuit diagram. Still, we can find some of the resistances outside the chip mounted on the PCB here and there as required in the circuit.

More about Resistance

The electrical resistance or simply resistance of a conductor depends on its size and shape. This is a very important point and it is used for designing of the electrical resistances for industrial purposes.

The resistance of a metal wire slightly increases with the increase of temperature and accordingly the current flowing through it will be decreased to that extent.

Resistances in series and in parallel

When the resistances are electrically connected with each other in series then the effective resistance is equal to the sum of these resistances and is mathematically presented as (assuming there are three resistances R1, R2, and R3) -

R = R1 + R2 + R3

Let us understand it from an example of a car battery which is generally of 12 Volt and we can glow the car lights with it.

Now let us take an electric bulb which glows with 12 Volt and connect it to battery. It will simply glow. Let us now take 2 or 3 same bulbs and connect them in series and then connect to the battery. What happened? The bulbs will give faint glow. Why? The reason is that when we have connected so many bulbs in series the net resistance of the bulbs is increased significantly and due to that increased resistance, the 12 Volt battery will now be sending less current through them. So the bulbs will give feeble light or may be no light at all.

What happens when we connect each bulb to the battery directly using two wires? Then all the bulbs glow brightly. Please note that now they are not in series and actually they are now connected in parallel.

The effective resistance in parallel system is governed by the equation -

1/R = 1/R1 + 1/R2 + 1/R3

Let us understand this series and parallel arrangement in numerical terms. If each bulb is having a resistance of say 20 ohm then in the series arrangement the total resistance will be 3 x 20 = 60 ohms and the current through it would be 12/60 = 0.2 Ampere. On the other hand if they are in parallel then the total effective resistance will be 1 / (1/20 + 1/20 + 1/20) = 20/3 = 6.6 Ohm and the total current flowing out of the battery would be 12/6.6 = 1.8 Ampere.

Please note that in parallel arrangement each bulb is independent and takes current from the battery and if one bulb goes bad the other two glow while in the series arrangement if one of them goes bad the circuit is broken and no current flows and no bulb glows.

Resistivity

We have seen how the resistance of a conductor changes with its length, thickness etc. In order to compare the property called electrical resistance of different mediums or materials, the concept of resistivity is evolved. Resistivity of a medium or material is the characteristics of that material and is a fixed value for that material. The advantage of this is that we can compare two materials by knowing there resistivities.

Let us understand it in mathematical terms. For sake of simplicity we take a metal wire and observe that -

(1) The resistance (R) of this wire is proportional to its length (L).

(2) The resistance (R) of the wire is inversely proportional to its area of cross-section (A).

We can write the above as -

R is proportional to L

and R is proportional to 1/A

(Note: If the diameter/thickness of wire is d then A = πd2/4)

Combining the above two relations, we get -

R = k L / A

Where k is the constant of proportionality and is called resistivity. The unit of k is Ohm-meter.

DC / AC voltage and Resistance

So far we were talking of car battery voltage or torch battery voltage for explaining many concepts. These sources of electricity are known as direct current (DC) sources. The electricity which comes to our houses is not DC but is known as AC (alternating current) and is quite different than DC in the sense that while DC produces a current flowing in one direction only AC produces the current but changes its direction in the circuit as per its frequency. The frequency of this mains AC line is generally 50 or 60 cycles/second and if it produces a current in a resistance then obviously that current is also of AC nature.

AC current not only depends on the resistance but also on other properties like inductance and capacitance of the material. In AC current scenario the concept of impedance comes in picture in place of resistance which can be understood as the total of all the elements like resistance, inductance, and capacitance.

Practical applications

Resistance is the integral part of electronic and electrical circuits in the industry. Resistance (also called Resistor) is the vital component of these circuits. We cannot think of an electrical circuit without the presence of resistance. In the electronic circuits the resistances are depicted by R1, R2 etc or by writing their value in short form like 2.5 M for a resistance of 2.5 Mega Ohm.

Due to the advancements in solid state and semiconductor areas it has been possible to design and create resistors in the semiconductor chip itself. Not only resistor but also capacitors, diodes, transistors etc are also made inside the same chip. Still some of the components especially having high current flowing through them, including resistors, have to be kept outside of chips due to the technical reasons.

Measurement of the resistance is very much used in the industry for various purposes. Let us go through some of them. Before the advent of computerised line diagnostics and remote sensing technologies, resistance measurement was done in many areas for locating the faults in long cables or telegraph lines. It was used in the Post Offices to find the distance of the place where the telegraph line, may be due to wind or storm, broke and fell on the Earth. Once that is found one can go to that distance to repair the fault.

Measurement of resistivity of the subsurface rocks in Earth for water, oil or coal prospecting is one major area where resistance measurement becomes very important and is already being used in the industry in a big way. In a well drilled for locating or exploiting these natural resources some sophisticated electronics tools are lowered which measure the resistivity of the rocks around them and from that one can infer very much about the presence of oil, fresh water, saline water, coal etc and that helps in deciding for further designing of well for production. These wells are generally very small in their diameters mostly ranging in the size 6 to 16 inches and accordingly the slim electronic devices encapsulated in metal pipes are lowered in them using a wire line cable or some remote sensing pulsar mechanism to get the resistivity data in real time to take decisions for the further course of drilling as well as to exploit the promising rock layers.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Umesh Chandra Bhatt

Comments

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 10, 2021:

Flourish, I feel happy that it rekindled some old memories.

FlourishAnyway from USA on July 10, 2021:

When I started reading, you definitely refreshed my memory of my high school studies.

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