Conductor and Insulators – Definition, Examples and Potential Difference

Physics Topics can help us understand the behavior of the natural world around us.

Introduction to Electricity and Types of Electric Charges

Electricity is an important source of energy in the modern times. Electricity is used in our homes, in industry and in transport. For example, electricity is used in our homes for lighting, operating fans and heating purposes (see Figure). In industry, electricity is used to run various types of machines, and in transport sector electricity is being used to pull electric trains. In this chapter, we will discuss electric potential, electric current, electric power and the heating effect of electric current. In order to understand electricity, we should first know something about the electric charges. These are discussed below.

If we bring a plastic comb near some very tiny pieces of paper, it will not have any effect on them. If, however, the comb is first rubbed with dry hair and then brought near the tiny pieces of paper, we find that the comb now attracts the pieces of paper towards itself. These observations are explained by saying that initially the comb is electrically neutral so it has no effect on the tiny pieces of paper.

When the comb is rubbed with dry hair, then it gets electric charge. This electrically charged comb exerts an electric force on the tiny pieces of paper and attracts them. Similarly, a glass rod rubbed with silk cloth; and an ebonite rod rubbed with woollen cloth also acquire the ability to attract small pieces of paper and are said to have electric charge.

Types of Electric Charges

It has been found by experiments that there are two types of electric charges : positive charges and negative charges. By convention, the charge acquired by a glass rod (rubbed with a silk cloth) is called positive charge and the charge acquired by an ebonite rod (rubbed with a woollen cloth) is called negative charge. An important property of electric charges is that:

1. Opposite charges (or Unlike charges) attract each other. For example, a positive charge attracts a negative charge.
2. Similar charges (or Like charges) repel each other. For example, a positive charge repels a positive charge; and a negative charge repels a negative charge.

The SI unit of electric charge is coulomb which is denoted by the letter C. We can define this unit of charge as follows : One coulomb is that quantity of electric charge which exerts a force of 9 × 10⁹ newtons on an equal charge placed at a distance of 1 metre from it. We now know that all the matter contains positively charged particles called protons and negatively charged particles called electrons. A proton possesses a positive charge of 1.6 × 10^-19 C whereas an electron possesses a negative charge of 1.6 × 10¹⁹ C. It is obvious that the unit of electric charge called ‘coulomb’ is much bigger than the charge of a proton or an electron. This point will become more clear from the following example.

Example Problem.
Calculate the number of electrons constituting one coulomb of charge.
Solution:
We know that the charge of an electron is 1.6 × 10^-19 coulomb (or 1.6 × 10^-19 C).
Now, If charge is 1.6 × 10^-19 C, No. of electrons = 1
So, If charge is 1 C, then No. of electrons = \(\frac{1}{1.6 \times 10^{-19}}\) × 1
= \(\frac{10^{19}}{1.6}\)
= \(\frac{10}{1.6}\) × 10¹⁸
= 6.25 × 10¹⁸
Thus, 6.25 × 10¹⁸ electrons taken together constitute 1 coulomb of charge.

The above example tells us that the SI unit of electric charge ‘coulomb’ (C) is equivalent to the charge contained in 6.25 × 10¹⁸ electrons. Thus, coulomb is a very big unit of electric charge.

Conductors and Insulators

In some substances, the electric charges can flow easily while in others they cannot. So, all the substances can be divided mainly into two electrical categories : conductors and insulators.

Those substances through which electric charges can flow, are called conductors. But the flow of electric charges is called electricity, so we can also say that: Those substances through which electricity can flow are called conductors. All the metals like silver, copper and aluminium, etc., are conductors (see Figure below figure). The metal alloys such as nichrome, manganin and constantan (which are used for making heating elements of electrical appliances) are also conductors but their electrical conductivity is much less than that of pure metals. Carbon, in the form of graphite, is also a conductor. The human body is a fairly good conductor.

Those substances through which electric charges cannot flow, are called insulators. In other words : Those substances through which electricity cannot flow are called insulators. Glass, ebonite, rubber, most plastics, paper, dry wood, cotton, mica, bakelite, porcelain, and dry air, are all insulators because they do not allow electric charges (or electricity) to flow through them (see above Figure). In the case of charged insulators like glass, ebonite, etc., the electric charges remain bound to them, and do not move away.

We have just seen that some of the substances are conductors whereas others are insulators. We will now explain the reason for this difference in their behaviour.

All the conductors (like metals) have some electrons which are loosely held by the nuclei of their atoms. These electrons are called “free electrons” and can move from one atom to another atom throughout the conductor. The presence of “free electrons” in a substance makes it a conductor (of electricity).

The electrons present in insulators are strongly held by the nuclei of their atoms. Since there are “no free electrons” in an insulator which can move from one atom to another, an insulator does not allow electric charges (or electricity) to flow through it.

Electricity can be classified into two parts :

1. Static electricity, and
2. Current electricity.

In static electricity, the electric charges remain at rest (or stationary), they do not move. The charge acquired by a glass rod rubbed with a silk cloth and the charge acquired by an ebonite rod rubbed with a woollen cloth are the examples of static electricity.

The lightning which we see in the sky during the rainy season also involves static electricity. In current electricity, the electric charges are in motion (and produce an electric current). The electricity which we use in our homes is the current electricity (see Figure). In this chapter, we will discuss only current electricity in detail. So, when we talk of electricity in these discussions, it will actually mean current electricity.

Electric Potential

When a small positive test charge is placed in the electric field due to another charge, it experiences a force. So, work has to be done on the positive test charge to move it against this force of repulsion. The electric potential (or potential) at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point. Potential is denoted by the symbol V and its unit is volt.

A potential of 1 volt at a point means that 1 joule of work is done in moving 1 unit positive charge from infinity to that point, Since the unit of charge is coulomb, so we can also say that: A potential of 1 volt at a point means that 1 joule of work is done in moving 1 coulomb of positive charge from infinity to that point. A more common term used in electricity is, however, potential difference which we will discuss now.

Potential Difference

The difference in electric potential between two points is known as potential difference, The potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit charge from one point to the other point. That is :
Potential difference = \(\frac{\text { Work done }}{\text { Quantity of charge moved }}\)

If W joules of work has to be done to move Q coulombs of charge from one point to the other point, then the potential difference V between the two points is given by the formula :
Potential difference, V = \(\frac{W}{Q}\)
where W = work done
and Q = quantity of charge moved
The SI unit of potential difference is volt which is denoted by the letter V. The potential difference is also sometimes written in symbols as p.d.

The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to the other.
Thus, 1 volt = \(\frac{1 \text { joule }}{1 \text { coulomb }}\)
or 1 V = \(\frac{1 \mathrm{~J}}{1 \mathrm{C}}\)
1 V = 1 J C^-1
The potential difference is measured by means of an instrument called voltmeter (see Figure). The voltmeter is always connected in parallel across the two points where the potential difference is to be measured. For example, in Figure we have a conductor AB such as a resistance wire (which is the part of a circuit), and we want to measure the potential difference across its ends.

So, one end of the voltmeter V is connected to the point A and the other end to the point B. We can read the value of the potential difference in volts on the dial of the voltmeter. A voltmeter has a high resistance so that it takes a negligible current from the circuit. The term “volt” gives rise to the word “voltage”. Voltage is the other name for potential difference. We will now solve some problems based on potential difference.

Example Problem 1.
How much work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts ?
Solution:
We know that :
Potential difference = \(\frac{\text { Work done }}{\text { Charge moved }}\)
or V = \(\frac{W}{Q}\)
Here, Potential difference, V = 128 – 118
= 10 volts
Work done, W = ? (To be calculated)
And, Charge moved, Q = 2 coulumbs
Putting these values in the above formula, we get :
10 = \(\frac{W}{2}\)
or W = 10 × 2
Thus, Work done, W = 20 joules

Example Problem 2.
How much energy is given to each coulomb of charge passing through a 6 V battery ?
Solution:
The term ‘each coulomb’ means ‘every 1 coulomb’, so the charge here is 1 coulomb. The potential difference is 6 volts. We have to find out the energy. This energy will be equal to the work done.
Now,
Potential difference = \(\frac{\text { Work done }}{\text { Charge moved }}\)
or V = \(\frac{W}{Q}\)
6 = \(\frac{W}{1}\)
So, Work done, W = 6 × 1 joules
= 6 J
Since the work done on each coulomb of charge is 6 joules, therefore, the energy given to each coulomb of charge is also 6 joules.