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 1). 

Figure 1. Conductors and insulators.

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 Figure 1). 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 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 2).

Figure 2. The electricity which we use in
our homes is current electricity.

we will discuss only current electricity in detail. So, when we talk of electricity in these discussions, it will actually mean current electricity.

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