Free Charges and Bound Charges Inside a Conductor

Electrons bound to the nucleus inside an element are known as bound charges. For example, non-metals have a large number of bound electrons. To put it more articulately, “the portion of the electrical charge on a conductor that because of the inductive action of a neighbouring charge will not escape to the earth when a conductor is grounded”. The bound charge definition is also a significant construct for calculating the electrostatic field of the polarised material and a concentrated charge. Bound charges are bound to something, whereas free charges move freely in an isolated conductor.

Free Charge: Definition

Free charges are when electrons are not bound to any nucleus and move freely. Any electric charge placed within a dielectric or on a conductor or that can move completely free in space constitutes a free charge by definition.

For example, metals have a large number of free electrons.

Free Charge in a Conductor

The free charge in a conductor is redistributed and attains a speedy electrostatic equilibrium in response to an electric field. Additionally, Gauss’s law and electric potential can be used to analyse the resulting distribution charge and its electric field parameters. If an electric field exists inside a conductor, it exerts a force on free electrons. The electrons, in this case, are called conduction electrons. Conduction electrons do not bind to an atom, but instead, accelerate. However, the significance of free charge constitutes non-static conditions. Therefore, while attaining an electrostatic equilibrium, charges are distributed in a way that the electric field inside the conductor vanishes.

Free and Bound Charges inside a Conductor

Let’s understand the concept with the help of an example. Let’s assume that a conductor is made out of copper, with an atomic number 29 and an electronic configuration as [Ar] 3d104s1. 

It is visible that the electron is located in the fourth shell, which is extremely far away from the nucleus, and therefore this is loosely bound to the atom. In the presence of an external electric field, these electrons experience force much greater than the nucleus’ force, thereby making this electron move freely to an electropositive end. On the other hand, other electrons in the innermost shells are bound and hence are not free for conduction.

Do Bound Charges have an Impact on Materials like Rubber to not Conduct Electricity?

Bound charges are charges in a solid that cannot move around and conduct current. For example, rubber molecules do not possess free electrons, so if an electric field or voltage is applied to the rubber, it will not conduct electricity. A rubber electron cannot drift to conduct a current because it is bound to the rubber molecules.

However, even bound charges can move from their equilibrium positions. If you apply an electric field to rubber, the electrons will slightly move in the opposite direction from the electric field, dislodging them from their equilibrium positions. The positive charges won’t be displaced since the molecules’ nuclei don’t move a great deal.

The rubber, therefore, starts to exhibit some electrical polarity. In the rubber, the positive nuclei are in their original positions. The electrons are displaced a short distance in a direction opposite the applied electric field, creating a polarisation field between the nuclei and the electrons, also in the opposite direction of the applied electric field.

An effect of bound charges is that the net field inside rubber is less than the applied electric field. This is due to the sum of the applied electric field and the much weaker polarisation field in the opposite direction.

A dielectric-like rubber has a relative permittivity more significant than 1 since it requires the more applied electric field to produce the same net electric field in the rubber as in a vacuum, due to the effect of polarisation of bound charges cancelling some of the applied electric fields. Various electrical devices rely on the ability of these given free charges to move freely in response to an external voltage to generate or provide a path for current to flow.

Conclusion

The bound charge implies that the charged particle is not free to move under an external electrical field or potential as there is another force binding the particle to some other element. For example: Inside a stable isotope, protons bounded to other protons inside and are therefore unable to break off under an external electric field. Similarly, electrons in the inner orbits of metals cannot leave their associated nuclei to move freely. Free charges are the particles that have a non-existent binding force or are negligible compared to the exerted force by an external electric field. For example, a solution of ions, a gaseous plasma of ions, or a metal’s conduction band.