Free Charges and Bound Charges Inside a Conductor

The amount of work done per unit positive test charge or in moving the unit positive test charge from infinity to that point, against the electrostatic force without acceleration, defines the electrostatic potential at any position in an electric field.

Electrostatic Potential = Work done (W) / charge (q)

The positive charge causes potential at a point to be positive, whereas negative charges cause it to be negative. A positive charge is moved from areas of higher potential to places of lower potential when it is placed in an electric field. A negative charge, on the other hand, experiences a force that moves it from a lower potential to a higher potential.

What are Free Charges?

When electrons are not attached to any nucleus and are free to move, a free charge is present. A free charge is by definition any electric charge that is deposited inside a dielectric, on a conductor, or that is entirely free to travel in space.

In reaction to an electric field, the free charge in a conductor redistributes and quickly reaches electrostatic equilibrium. The distribution charge that results and its electric field parameters can also be examined using Gauss’s equation and electric potential. Free electrons are subject to a force if an electric field is present inside a conductor. Conduction electrons are the proper name for the electrons in this scenario. Conduction electrons speed up rather than attach to an atom. However, the importance of a free charge implies non-static circumstances. Therefore, as charges are diffused to reach electrostatic equilibrium, the electric field inside the conductor disappears.

What are Bound Charges?

The term “bound charge” refers to a charged particle that is tied to another element, indicating that the charged particle is not free to move in the presence of an external electric field or potential. For instance: Protons inside a stable isotope are bonded to one another and are unable to separate under the influence of an external electric field. Similarly, electrons in the inner orbits of metals are unable to separate from their corresponding nuclei and flow freely.

Bound Charge and Free Charge Diagram

The image given below tells us about the free charge and bound charge inside an atom.

Free and Bound Charges inside a Conductor

Assume that for copper conductor which has an atomic number of 29 and its electronic configuration is [Ar] 3d¹⁰4s¹.

It can be seen that the electron is loosely linked to the atom since it is in the fourth shell, which is very far from the nucleus. These electrons encounter a force that is substantially stronger than the force of the nucleus in the presence of an external electric field, allowing the electron to freely migrate to an electropositive end. However, some of the electrons in the innermost shells are bonded and are just not available for conduction because of this.

Effect of Bound Charges on Different Materials

Charges that are bound to a solid are immobile and unable to conduct current. For example, because rubber molecules lack free electrons, they will not transmit electricity when an electric field or voltage is provided to them, an electron in rubber cannot travel in order to conduct a current because it is linked to the rubber molecules.

Even bound charges have the ability to migrate from their equilibrium positions. If an electric field is applied to rubber, the electrons will be somewhat displaced from their equilibrium positions and migrate slightly in the opposite direction of the electric field. Since the nuclei of the molecules don’t move around much, the positive charges won’t be displaced.

The net electric field within a material is less than the applied electric field due to the bound charges. This is caused by the combination of the applied electric field and the opposite, considerably weaker polarization field.

Dielectric Materials

Dielectric materials are a particular class of insulators that polarize when exposed to an electric field. It can support an electrostatic field readily while being a poor conductor of electricity. They exist as gases, liquids, and solids. Components like capacitors and radios use dielectrics. Dielectric materials can also be utilized to store energy when they are set up properly. Although some of these substances are fluids and gases, the majority of these substances are solids by nature. 

Types of dielectric materials include,

• Dielectric Gas: Dry Air
• Solid Dielectrics: Mica, Ceramic, Plastic, and Glass
• Dielectric Liquid: Distilled Water

Polar Molecules

Dipolar Polarization, also known as Orientation Polarization, occurs in polar molecules. On the other hand, a polar molecule is endowed with electric dipoles and these dipoles are not induced. Because of the bonds and structure of a polar molecule, this dipole is present. But we are unable to immediately make use of this dipole moment that already exists. A polar material’s dipoles are randomly orientated as a result of thermal agitation. As a result, the material’s molecules’ individual dipole moments cancel out to produce a net dipole moment of zero. However, for various reasons, we also need to apply an electric field in this situation.

Non-Polar Molecules

A molecule without a dipole is referred to as nonpolar. A nonpolar molecule has an even distribution of charges. A dielectric nonpolar material placed into an electric field will be impacted despite the absence of a dipole. Because of their opposing polarities, the positive and negative charges in a nonpolar molecule feel forces in opposite directions in an electric field. A nonpolar molecule’s electron cloud is moved in the direction of attraction by this force. This displacement continues until the internal forces of the molecule balance the attraction of the electric field. Thus, even a nonpolar molecule experiences an induced dipole moment in the presence of an electric field.

Characteristics of Polar and Non-Polar Molecules

A dielectric is an insulator that becomes electrically polarized when an electric field is applied. The dielectric polarization is caused by the charges in a dielectric substance shifting slightly from their equilibrium location without actually moving. We looked at the type of bonds that determine the polarity of molecules in our post on Polar and Non-Polar Material: Dielectric Material and Dipole Moment. The molecules’ electronegativity and atomic configuration determine whether they are polar or nonpolar molecules.

Read, More

• Electric Charge
• Electrostatics of Conductors
• Flow of Electric Charges in a Metallic Conductor

FAQs on Capacitors

Question 1: What is a free charge inside a conductor?

Answer:

The electrons feel force that is larger than the force of the nucleus when an electric field is applied, which enables them to travel to the electropositive end.

Question 2: What is the meaning of bound charge?

Answer:

The bound charge means that the charged particle is tied to another element, preventing it from moving freely in the presence of an external electrical field or potential.

Question 3: What are the different types of capacitors?

Answer: 

1. Parallel plate capacitors
2. Spherical capacitors
3. Cylindrical capacitors

Question 4: Explain the term Capacitance, and what is the SI unit?

Answer: 

The capacitor’s capacity to store charge is known as its capacitance.

C = Q / V

where, 
C is capacitance
Q is charge
V is potential across capacitor

SI unit of capacitance is Farad (F).

Question 5: What is the distinguishing factor between free charges and bound charges? 

Answer: 

Free charges can have zero energy since they are not connected to the nucleus. On the other hand, a bound charge can only have quantized energy since it is bound to the nucleus.

Question 6: What are dielectric materials?

Answer: 

Dielectric materials are a particular class of insulator that polarize when exposed to an electric field.