The electrical polarization occurs when, because of the electric field, the negative electrons are pushed towards the positive atom nuclei surrounding it. This distortion of charges results in one side of the atom becoming a little negative and the opposite side becoming a little positive. However, in some chemically bound molecules like water molecules, polarization partially takes place due to the rotation of molecules into the same line under the influence of the electric field.
Now we will define electric polarization and the effects of the application of electric fields in molecules. There are polar and nonpolar molecules. Now, the induced dipole moment is directly proportional to the strength of the electric field applied F.
Thus, this is the induced polarizability constant of the polarizing molecules,. Thus, the induced polarization of dielectric material in chemistry means the amount of induced moment in the polarized molecule when the unit electric field of the current strength is applied. The electric polarization constant has the dimension of volume and is derived from the definition and polarizing formula.
As the atom size, ionization energy, and atomic number increase, the polarizability of the atom increases. Dipolar polarization can be achieved by inducing an electric field in the molecules which can exhibit uneven distortion of the nuclei distortion polarization. The polarization density P of the material is then given as:. Since the dipole moment shows variation at different points within the dielectric, the electric polarization for an infinitesimal volume dV of dielectric is:.
This represents the ability of a material to concentrate electrostatic lines of flux, or, the ability of a material to store electrical energy in the presence of an electric field.
Permittivity impacts the Coulomb force that exists between two point charges in a material. Piezoelectric materials such as quartz experience polarization not because of the electric field but due to mechanical stress. Similarly, the mechanical strain, or strain gradient, is the cause of polarization in many solid dielectric materials, and this phenomenon is known as the flexoelectric effect.
There are mainly four categories in which dielectric polarization is classified:. Also known as atomic polarization, electric polarization occurs due to the separation of the center of positive charge and the center of negative charge in the atoms of a material in the presence of an electric field.
The induced dipole moment, in this case, is found directly proportional to the applied field. There are some dielectric materials such as NaCl and LiBr in which atoms are held together by ionic bonds. Induced polarization occurs in such materials when an external electric field is applied and the cations and anions are displaced in opposite directions, giving rise to a net dipole moment.
However, in the absence of an electric field, net polarization inside the dielectric material comes out to be zero because positive and negative ions cancel out each other and no net dipole moment is produced.
Orientational polarization arises when there is a permanent dipole moment in the material. Materials such as HCl and H 2 O will have a net permanent dipole moment because the charge distributions of these molecules are skewed.
So, when an electric field is applied, the molecules start to rotate in order to align in the direction of the field, and polarization results. Orientational polarization is also affected by temperature, thermal agitation occurring in dipolar molecules results in the cancellation of the net dipole moment when no electric field is applied.
This is also the reason why the selection of dielectric material for electronic or optical applications is sometimes made on the basis of their temperature dependency. It is very common to observe this polarization within molecules. In molecules that have long chains of atoms bonded together, there are often several locations along the chain or near the ends of the chain that have polar bonds.
This polarization leaves the molecule with areas that have a concentration of positive charges and other areas with a concentration of negative charges. This principle is utilized in the manufacture of certain commercial products that are used to reduce static cling. The centers of positive and negative charge within the product are drawn to excess charge residing on the clothes.
There is a neutralization of the static charge buildup on the clothes, thus reducing their tendency to be attracted to each other.
Other products actually use a different principle. During manufacturing, a thin sheet is soaked in a solution containing positively charged ions. The sheet is tossed into the dryer with the clothes. Being saturated with positive charges, the sheet is capable of attracting excess electrons that are scuffed off of clothes during the drying cycle. A complete discussion of the world of atoms, molecules and chemical bonds is beyond the scope of The Physics Classroom.
Nonetheless, a model of the atom as a distortable cloud of negative electrons surrounding a positive nucleus becomes essential to understanding how an insulating material can be polarized. If a charged object is brought near an insulator, the charges on that object are capable of distorting the electron clouds of the insulator atoms. There is a polarization of the neutral atoms. As shown in the diagrams below, the neutral atoms of the insulator will orient themselves in such a manner as to place the more attractive charge nearest the charged object.
Once polarized in this manner, opposites can now attract. A common demonstration performed in class involved bringing a negatively charged balloon near a wooden door or wooden cabinet.
The molecules of wood will reorient themselves in such a way as to place their positive charges towards the negatively charged balloon. The distortion of their electron clouds will result in an alignment of the wood molecules in a manner that makes the wooden cabinet attracted to the negatively charged balloon.
In human terms, one might say that the wood does some quick grooming and then places its most attractive side towards the balloon and its most repulsive side away from the balloon. In the world of static electricity, closeness counts. The negative balloon is closer to the positive portion of the wood molecules and further from the more repulsive negative portion.
The balloon and the wall attract with sufficient force to cause the balloon to stick to the wall. From a mechanics standpoint, we would say that the balloon and the wall are pressed together with a large force. The large normal force on the balloon results in a large static friction force.
This friction force balances the downward force of gravity and the balloon remains at rest. Another common physics and chemistry demonstration involves using a charged object to deflect a stream of water from its path. Most often, a comb is charged negatively by combing one's hair or a rubber balloon is charged in a similar manner. The negatively charged object is then brought near to a falling stream of water, causing the stream to be attracted to the comb or balloon and alter its direction of fall.
The demonstration illustrates the polar nature of water molecules. The hydrogen atoms serve as the positive poles within a water molecule; oxygen serves as the negative pole. Molecules of a liquid are free to rotate and move about; the water molecules realign themselves in order to put their positive poles towards the negatively charged object.
Once polarized, the stream and the balloon or comb are attracted. As the water molecules within the stream fall past the balloon, this realignment of individual molecules happens quickly and the entire stream is deflected from its original downward direction.
Examples of the attraction between charged objects and neutral objects are numerous and often demonstrated by physics teachers. Paper bits become polarized and are attracted to a charged piece of acetate. Small penguins cut from a sheet of paper are attracted to a charged plastic golf tube and demonstrate their happy feet. A long wooden 2x4 is placed on a pivot and becomes polarized and attracted to a charged golf tube. To the astonishment of students, the force of attraction on the wood is large enough to rotate it about the pivot point.
Perhaps the biggest misconception that pertains to polarization is the belief that polarization involves the charging of an object.
Polarization is not charging! When an object becomes polarized, there is simply a redistribution of the centers of positive and negative charges within the object.
Either by the movement of electrons across the surface of the object as is the case in conductors or through the distortion of electron clouds as is the case in insulators , the centers of positive and negative charges become separated from each other. The atoms at one location on the object possess more protons than electrons and the atoms at another location have more electrons than protons.
While there are the same number of protons and electrons within the object, these protons and electrons are not distributed in the same proportion across the object's surface. Yet, there are still equal numbers of positive charges protons and negative charges electrons within the object. While there is a separation of charge, there is NOT an imbalance of charge. When neutral objects become polarized, they are still neutral objects.
The process of polarization is often used in many charging methods. In one section of Lesson 2, the charging by induction process will be discussed. This charging process depends upon a charged object to induce polarization within a neutral object. While charging by induction includes polarization as one of its steps, polarization is still NOT a charging process. Details about the induction charging method can be read about in Lesson 2 of this unit.
Use your understanding of charge to answer the following questions. When finished, click the button to view the answers. A rubber balloon possesses a positive charge.
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