Electric Polarization

Claimed Edit by Eleanor Thomas Fall 2016

The Main Idea

Polarization, used broadly, is the act of dividing into opposites. Electric polarization is the process of separating opposite charges inside an object. This occurs when an electric field, let's say created by a charged object A, induces the electrons to move in object B. This electron movement causes one portion of object B to have an excess negative charge and the other to have an excess positive charge. Object B could be a neutral object with a net charge of zero, but can still be polarized and attracted to object A. If A were positively charged, the electrons in object B would be attracted to the side closest to A (since opposite charges attract) which would create an induced dipole. This dipole is not permanent; if object A were to be removed, B would return to its neutral state.

Cause of Polarization

The cause of polarization can be found when the structure of an atom is closely examined. Atoms have a positively charged nucleus consisting of protons and neutrons tightly clumped together. Surrounding this nucleus is a negatively charged electron cloud which is not rigidly connected to the nucleus. Since the negative electron cloud does not have to be centered at its corresponding positive nucleus, if an electric field is applied on the atom, the two opposite charges can move relative to one another. This applied electric field induced the polarization of atoms.

Conductors and Insulators

Conductors are materials that allow electric charge to pass through it since it contains freely moving charged particles. When an electric field is applied to a conductor, electrons are transferred across the surface of the object and it becomes polarized. An insulator is a material that does not conduct very much electric charge because all the electrons are rigidly bound to the atoms or molecules; they do not permit the free flow of electrons. When an insulator is subjected to an electric field, individual atoms or molecules are polarized rather than the whole object.

An example of a polarized insulator:

Misconception

Neutral objects (with a net charge of zero) CAN be attracted to charged ones because induced dipoles are formed and create an electric field at the location of the object. However, repulsion of an induced object cannot happen as it always brings unlike signs closer.

Mathematical Model

The amount of induced polarization is directly proportional to the magnitude of the applied electric field. Induced Polarization : [math]\displaystyle{ \vec{P} = \alpha \vec{E} }[/math] "P" is the dipole moment of polarized atoms, "α" is the polarizability of a particular material, and "E" is the applied electric field. Applied electric field is equal to:

Examples

Simple

Question: A positively charged object is placed near a neutral atom. Draw the polarization of the atom.

Middling

Question: A negatively charged object is placed above a metal sphere in equilibrium. Draw the polarization of the sphere.

Difficult

Question: A negatively charged metal object is placed in a region with an electric field going in the +y direction. Draw the polarization of the object.

Connectedness

Electric polarization is a key component of insulators and conductors. These materials and their uses are incredibly important for every day life. For example, lightbulb filaments are conductors which carry electrons from a negatively charged area to a positively charged area. Insulators, which don't conduct electric charges, can be used to insulate buildings. This is related to my major of study, environmental engineering, because we work to make buildings more sustainable by reducing the amount of energy required to keep them running.

History

Benjamin Franklin was the first person to use "positive" and "negative" in describing charges and came up with principle of conservation of charges. In 1897, J.J. Thomson experimented with cathode rays and found out that electrons exist in the rays. In 1911, Ernest Rutherford discovered that atoms have a concentrated positive center with protons fixed inside nucleus.

See Also

• Charge

• Electric Field

• Benjamin Franklin

• J.J. Thomson

• Ernest Rutherford

Further Reading

Physics of Dielectrics for the Engineer by Roland Coelho

Application

http://www.nrcresearchpress.com/doi/abs/10.1139/t89-067?journalCode=cgj#.VmFE97mFPIU

External Links

https://courses.cit.cornell.edu/ece303/Lectures/lecture7.pdf

Reference

http://www.physicsclassroom.com/class/estatics/Lesson-1/Polarization

https://www.youtube.com/watch?v=3xSIA5UVAo8

http://scienceline.ucsb.edu/getkey.php?key=408

https://www.aip.org/history/gap/Franklin/Franklin.html

Matter and Interactions Fourth Edition by Ruth W. Chabay, Bruce A. Sherwood