Conductor, Insulator (dielectric) and Semi-conductor

Conductor, Insulator and Semi-conductor:

An atom consists of a central positively charged core known as nucleus. The nucleus is surrounded by a number of electrons revolving around it in practically circular orbits. In some substances, the electrons in the outermost orbits are loosely bound to the nucleus. These electrons may leave the atom and become 'free electrons'. The motion of free electrons is random. Due to the motion of free electrons, charge is carried from one end of the substance to the other. In certain substances, the orbital electrons are strongly bound to the nucleus. In these substances, free electrons are available only in small number. So, charge cannot flow easily from one end of the substance to the other. Depending upon the capacity to allow the passage of charge, the substances are generally classified into three categories. 

1. Conductor

2. Insulator

3. Semi-conductor

1. Conductor. Conductor are those substance through which electric charge can pass easily. 

Example- Silver, Iron, Copper, Aluminium etc. 

Among metter silver is the best conductor of electricity. Conductor contain large number of free electron, due to the repulsion between free electron they get evenly scattered through out the conductor. Therefore, no portion of the conductor has accumulation of electrons (i.e., charges). 

2. Insulator. Insulator are those substance through which electric charge cannot pass easily. 

Example- Glass, Wood, Mica, Plastic, Rubber, Umber, Sulphur etc. 

Insulator contain a negligible number of free electron. 

If any region of the insulator happens to have an accumulation of electron they will remain localised in that region. 

But conductivity of any substance is effected by temperature on heating the insulator tends to become conductor.

3. Semi-conductor. The substance whose conductivity lies between conductor and insulator are called semi conductor. 

Example- Silicon (si), Germanium (G) are known as semi conductor. 

Semi conductor is covalent bond.

No substance is a perfect conductor or a perfect insulators. The difference between conductors and insulators is only of the degree. The insulating ability of fused quartz is about 10²⁵ times as great as that of copper. Thus, these materials behave as perfect insulators for many practical purposes. Under suitable conditions, both conductors and insulators can be electrified.

Conductors in an Electric Field:

(i) Field inside a conductor in an electric field. When an electron leaves an atom, the electron is known as free electron and the reminder atom is known as positive ion. In a conductor the number of free electrons is equal to the number of positive ions. 

Consider a conductor (of any shape) placed in a uniform electric field (strictly speaking, it is an electrostatic field). This field is produced by two oppositely charged plates A and B. Under the influence of this external field, the positive ions (in the conductor) will being to move in the direction of the field, i.e., along the electric lines of the force. The free electrons (in the conductor) are driven in the opposite direction, i.e., in a direction opposite to the electric lines of force. Both the positive ions and electrons cannot go beyond the surface of the conductor. This results in concentration of opposite charges at the two ends of the conductor. These charges set up their own electric field inside the conductor as shown in    [Fig. 1(i)]. This electric field tends to oppose that external electric field. The strength of the opposing field goes on increasing with the increase in concentration of charges increases at the surface of the conductor. The concentration of charges increases to such a value that their electric field exactly balances the external electric field. At this stage there will be an electric lines of force inside the conductor as shown in [Fig. 1(ii)]. This indicates that the electric field inside a conductor is zero. Because of the absence of electric field inside the conductor, all the points just below the surface of the conductor are at the same potential. 

Fig. 1. A conductor in uniform electric field. 

(ii) Electrostatic shielding. Consider an electric field produced by pair of two oppositely charged parallel plates P and Q [in Fig. 2]. The lines of force starts from the conductor at Higher potential and terminate on a conductor at lower potential. Therefore, there cannot be any lines of force between two conductors having same potential. 

Let a hollow conductor ABCD be placed inside the field. Since the surface of a conductor, whatever is shape may be, is always equipotential, therefore, there cannot be any line of force inside the conductor. Because in the event of a line of force being there, it would have started and then ended on two points at same potentials which is not possible.

Fig. 2. Electrostatic shielding. 

 The absence of lines of force inside a conductor indicates that no force is experience by buy any charge placed inside it, i.e., the charge inside a hollow conductor remains unaffected by a field outside it. In other words, the charge is shielded from the outside field. 

Similarly a charge placed inside the hollow conducting enclosure will not produce any effect outside it. 

The phenomenon of electrostatic shielding is employed extensively to protect very delicate instruments from any external charged particles. 

(iii) Charge resided inly on the surface. It can be proved, using Gauss's theorem, that a conductor having a static charge has whole of the charge residing on the surface of conductor. Consider a Gaussian surface as shown dotted in [Fig. 3], drawn in such a way that it lies very close to it and below it. 

Fig. 3. A conductor with static charges. 

Since E(r) everywhere is zero the flux through, this surface is zero. Applying Gauss's theorem it can be concluded that there is no charge inside the surface. This is possible only if whole of the charge is distributed over the surface. 

(iv) Field is always perpendicular to the surface. It can further be shown that the electric field, just outside the surface must be perpendicular to the surface at all points. Let 'E' be the direction of field, in general, at a point P, just outside a conductor C with static charges [Fig. 4]. Resolving E into two components, we get, 

• E cos θ along PX, acting perpendicular to the surface. 

• E sin θ along PY, acting tangential to the surface. 

Fig. 4. Electric field always perpendicular to surface of conductor. 

The tangential component 'E cos θ' can make the free electrons to drift continuously this, causing electric currents flowing in closed circuits. This is not possible in case if a conductor with static charge. Hence there cannot be any existence of 'E cos θ' meaning that the field should always be perpendicular to the surface. 

(v) Field strength at a point just outside the surface. According to Gauss's law, 

           ∫ E. ds = 1/ε₀ q 

E(r) being always perpendicular to the area, E and ds have same direction (θ = 0°) 

     ∫ E(r). dS = ∫ EdS = ∫ E dS = ES

Using Gauss's theorem, we get                                                 ES = 1/ε₀ q

It σ us surface charge density,   q = σS

∴      ES = 1/ε₀ (σS)    or    E = σ/ε₀

 

This gives magnitude of the field strength. 

(vi) Conductor containing a charge in a cavity inside it. Consider an uncharged conductor having a cavity in it containing a charge +q at O [Fig.5].

Fig. 5. Charge located in a cavity inside a conductor. 

Let there be a Guassian surfaces as shown dotted. 

According to the Gauss's theorem, 

     ∫ E(r). dS = 1/ε₀ Σq

Where  'Σq' is the total charge located inside the surface. 

Since E in the conductor is zero, 

∴      Σq = 0

Since we know that a charge +q is located at O, Σq = 0 only if there is charge -q on the inside of the cavity. For the conductor to be neutral, a charge +q will have to be there on the surface. This is again an evidence in the support of fact that the charge resides on the outer surface. 

Insulator in an Electric Field:

An insulator has no free electrons and thus no motion of charge carries takes place when it is placed in an electric field. However, when it is placed in a strong electric field, the orbits of the atoms of the insulator and stretched which results in the separation of centers of negative and positive charges. The atom is said to be polarised. The polarised charges in the surface of insulator produce an electric field which decreases the resultant electric field inside the insulator.