Free Tutorial in Basic Physics - Understanding Electrical Resistance


Electrical Resistance is the property of a material to impede the flow of the electrical current under the applied voltage. We will try to understand in details about this unique property in this article.

Introduction

In some earlier basic Physics articles, we learned about Electrical Voltage and Electrical Current. Now it is time to learn about Electrical Resistance.

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

Conductors and Insulators

When an electrical voltage is applied across a material say an iron rod or wood piece or a stone or a plastic piece then depending on the electrical properties of that material either nothing happens or some electric current starts to flow through it. This provides us a clear demarcation between the materials with other materials considering whether electrical current can flow or cannot flow through them. The materials through which current can flow are known as conductors while those which do not allow any current to flow through them, are known as insulators.

Iron, Gold, Copper, Aluminium, Silver, etc are some examples of electrical conductors while Wood, Granite, Stone, Marble, Plastic, Cardboard, Paper, etc do not allow any electrical currents to flow through them and are known as insulators. For example in the electric wiring in our houses, each wire contains an Aluminium or Copper wire inside it covered from all the sides with an insulating material. The electrical current flows through the inside wire and the outside insulator material protects us from it.

From the above behaviour of the materials to the applied electrical voltage we can infer that there is a unique property of material resisting the flow of electrical current and this property is known as electrical resistance. We will denote it further simply by 'resistance' in this article.

Flow of electrical current in conductors

In some materials, especially metals, generally, the atomic structure is such that some of the electrons get dislodged from the atoms and can freely move in the metal. These are known as free electrons and they are the carrier of electricity in the metals. Whenever an external voltage is applied, negatively charged particles, start moving towards the positive terminal of the applied voltage.

Let us understand it from a simple electrical circuit consisting of a car battery and an electric bulb. Here car battery is the source of electrical power and generally has 12 Volt electric voltage (also known as electromotive force or EMF) between its positive and negative terminals. Now what we do is we take two electric wires and with the help of them connect the bulb to the battery. What happens? The bulb starts glowing. Why? Because as soon as we connect the bulb the free electrons start moving in the circuit and a current starts flowing through the metal filament in the bulb. The current heats the filament and light is generated. We should note that free electrons would be moving in a direction toward the positive terminal of the battery. As per the convention the current direction is taken opposite to this. So, if we see an electrical diagram in which the current direction is shown by arrows then we know what it means.

Resistance of a conductor

In the above example, the bulb has an electrical resistance, and current is flowing through it. Current is also flowing through the connecting wires but their electrical resistance is so small that we can neglect that. Let us put one more bulb in series with the first one and then connect them to the battery and we will find that now the bulbs are glowing with low intensity. Why does it happen because we have increased the electrical resistance in the circuit by adding one more bulb in series. From this it is very clear that increasing resistance in an electrical circuit will decrease the current and a decrease in the resistance will increase the current.

The resistance of the conducting medium or material changes as per its size. For example, a thick wire will have less resistance than a thin wire. Another thing is that a long wire will have more resistance as compared to a short wire. In our houses in the electrical lines, the thick wire is used for high-power appliances, while the thin wire is used for normal lighting purposes. The reason for doing so is because thick wires having less resistance will allow high current to flow through them which is required to run the power gadgets. For ordinary lighting purposes, thin wires would suffice as the current required is less.

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

Unit of Resistance

The unit of Resistance is Ohm. Bigger resistances are represented by Kilo or Mega prefixes. So, 1 Kilo Ohm is equal to 1000 Ohm while 1 Mega Ohm is equal to 1000000 Ohm or 1000 Kilo Ohm.

Resistivity of a material

We have now understood that conductors have a resistance to the flow of electrical current and their resistances depend on their sizes. Now how can we find out that a particular material is more resistive than the other? For example, the same-sized wire of Copper has less Resistance as compared to Aluminium. To understand this there is a concept of resistivity which is the intrinsic property of a material irrespective of its size and from this we can compare the electric resistance of various conductors.

Let us take a metal wire of length L and diameter d having resistance R.
The resistance R of the wire is proportional to its length L
and is inversely proportional to its area of cross-section A.
Mathematically, we can write the above as -
R is proportional to L, and R is proportional to 1/A, where A = (pd^2)/4
Combining the above we get - R = k L / A
Where k is the constant of proportionality and is called resistivity. Here the unit of R is Ohm, L is in meter, A is in meter square, and that results in the unit of k being Ohm-meter.

Resistivity is an intrinsic property of an electrically conducting medium. For conducting materials it is very low but for non-conducting materials, it is very high. Some representative values of resistivity (in Ohm-meter) for different materials are -
  • Copper - 1.7 x 10^(-8)
  • Aluminium - 2.65 x 19^(-8)
  • Steel - 9.7 x 10^(-8)
  • Glass - 10^(9) to 10^(13)
  • Hard Rubber - 10^(13) to 10^(15)
From the above, we observe that Copper has very low resistivity. That is why Copper is an item of choice for making electrical wires for carrying current from one place to another.

Glass and Rubber have very high resistivities. They do not allow currents to flow through them. They are known as insulators. They are used for insulation purposes in electrical items.

Relationship between applied voltage and produced current in a conductor

When we apply some voltage across a conductor then depending upon the resistance a current starts flowing in it. If we increase the voltage then the current also increases. At the same time if we keep the voltage constant then increasing the resistance will decrease the current while decreasing the resistance will increase it. It means the current is inversely proportional to the resistance.

From these observations, we find a very interesting formula and the relation between the current, applied voltage, and resistance. This formula is known as famous Ohm's Law and can be written like -
I = V/R
or commonly written as V = IR
Where I is the current, V is the voltage applied, and R is the resistance. The unit of current is in Ampere, voltage in Volt, and resistance in Ohm.

The unit 'Ohm' was named in honor of German Physicist Georg Simon Ohm, who was the first person to verify this law experimentally and to honour him this relationship is known as Ohm's Law.

Example:
A 1.5 Volt dry battery (used in torch or TV remotes or toys) is connected to a filament-type torch bulb having 10 Ohm resistance. How much current is flowing through the bulb?
To solve the above problem we can use Ohm's Law -
I = V/R = 1.5/10 = 0.15 Ampere = 150 milli ampere

Resistances in series and parallel

Resistances can be connected in series or parallel depending upon the need.
When the resistances are connected in series then the resultant resistance is the total of them. For example, if we connect three resistances R1, R2, and R3 in series then the resultant resistance R can be written as -
R = R1 + R2 + R3
On the other hand, if we connect them in parallel then the resultant R is given by -
1/R = 1/R1 + 1/R2 + 1/R3

Example:
Let us take a 12 Volt car battery and three bulbs each having a resistance of 24 Ohms. Let us connect them in series. The net resistance is equal to 24 + 24 + 24 = 72 Ohms. So, the current flowing will be equal to 12/72 = 0.16 Ampere = 160 milli amperes.
Now let us connect them in parallel. The net resistance will be calculated from -
1/R = 1/24 + 1/24 + 1/24
On solving we get R = 8 Ohms.
So, the current flowing will be equal to 12/8 = 1.5 Amperes = 1500 milli amperes

Behaviour of resistance for DC and AC voltages

So far we were talking of a car battery and a torch battery but both are DC voltages. DC means direct current which flows in one direction. On the other hand, the electricity which is supplied to our houses is an AC voltage. AC means alternating current which changes its direction as per its frequency. The electricity in our house has a frequency of 50 cycles/second which means that in one second it changes its direction 50 times.

When a resistor is used in a circuit through which AC is flowing it reduces the current but other factors like inductance and capacitance also play a part and then the current is controlled by the electrical impedance of the circuit where the resistor is also a part of that impedance and simple Ohm's Law will not be applicable.

Types of Resistors

Resistor is the technical name of resistance parts used in electronic and electrical circuits. Initially, resistors were made by winding metal wire on an insulated bobbin like wood, ceramic, or bakelite. By changing the thickness and length of the wire the requisite value of resistance was achieved. Later with advancements in technology other types of resistors like carbon film resistors were made. With the advent of semiconductor technology, it became possible to design the resistors in the semiconductor chip itself.

Conclusion

Understanding the property 'Electrical Resistance' helps us understand how the current in an electrical circuit flows depending upon the resistance present. Resistors have a great use in electronic and electrical industries for controlling the current in the electrical circuit.

Frequently Asked Questions

What is the meaning of zero electrical resistance?
There is nothing like zero resistance but it can be made minimum by taking some good conductors like gold, silver, copper, aluminium, iron, etc which have low electrical resistance and using them for making connecting wires or taking electricity from one place to another.
What is the power rating of a resistor?
Every resistor has a power rating. It tells us how much maximum current can pass through without damaging it by the heat generated. We have to use an appropriate power rating resistor in different electronic and electrical circuits depending on the current flow.
What is electrical short circuit?
Whether it is a car battery or the electricity coming to our houses there should be a load like bulb, motor, gadget, etc connected to it. If by mistake or some fault, the two ends of the electrical power get connected directly to each other then it is a short circuit and a very high current will flow which can damage the electrical line and that is why a 'fuse' or 'cut off' is used in our houses which blows off or disconnects the circuit and protects the electrical line from damage. In a car, too, there are fuses to protect the circuit and battery in such an eventuality.
Can we touch the terminal of a 12 Volt car battery?
The human body can withstand touching the electrical voltage only up to approximately 35-40 Volts so touching a car battery would not be harmful. But accidentally touching the main line which is 240 Volts in our houses will give a big shock.


Comments

Guest Author: Shyam Lal Bhatia09 Mar 2024

This is a very nicely explained article about the property of resistance of materials. No doubt the author Shri Umesh has done a very good job!

Guest Author: Suresh Deol09 Mar 2024

This is a very good article on electrical resistance which is a key aspect of the subject of Physics. Thank you, Sir.

Author: Umesh17 Mar 2024 Member Level: Diamond   Points : 0

Thanks Mr Bhatia and Mr Deol for sparing your time and showing interest in this article. Appreciate it.



  • Do not include your name, "with regards" etc in the comment. Write detailed comment, relevant to the topic.
  • No HTML formatting and links to other web sites are allowed.
  • This is a strictly moderated site. Absolutely no spam allowed.
  • Name:
    Email: