Electric Dipole
An electric dipole is a pair of equal and opposite electric charges separated by a small distance. It is characterized by a dipole moment, which is the product of the charge magnitude and the separation distance. Electric dipoles are important in understanding the behavior of electric fields and are commonly used in various applications, such as antennas and molecular interactions.
6 Key excerpts on "Electric Dipole"
eBook - ePub- Orjan G. Martinsen, Sverre Grimnes(Authors)
- 2014(Publication Date)
- Academic Press(Publisher)
...However, a local disturbance of the distribution of bound charges will occur in an imposed E-field. Suppose that two charges are equal but of opposite sign and kept at a small distance and thus hindered to recombine. Such an electric doublet is called a dipole (see Figure 6.4). An atom with the electrons at a distance from its positive nucleus does not necessarily form a net dipole: the center of the electron cloud may coincide with that of the nucleus. However, every such atom is polarizable because the electrical centers of the charges will be displaced by an external electric field. As positive and negative charges move in opposite directions, dipoles are formed (induced) and the material is polarized. Also, bound ions of the dielectric can move but only locally (translate, rotate) and under strong confinement. No electric charges are exchanged between the dielectric and the metal electrode plates. Electrostatic Dipole Moments In electrostatic theory, a dipole is characterized by its electrical dipole moment, the space vector p : p = q L [ Cm ] (3.4) The unit for p is coulomb meter [Cm], or outside the SI system: the Debye unit (D = 3.34 × 10 − 30 [Cm]). A pair of elementary charges +e and − e held at a distance of 0.1 nm has a dipole moment of 4.8 D, a water molecule has a permanent dipole moment of about 1.8 Debye. Figure 3.1 The basic capacitor experiment. The dipole moment p may be the resultant dipole moment of a molecule, many molecules, or a whole region. Polarization P [Cm/m 3 = C/m 2 ] is the electrical dipole moment per unit volume (dipole moment volume density)...
eBook - ePubElectromagnetics Explained
A Handbook for Wireless/ RF, EMC, and High-Speed Electronics
- Ron Schmitt(Author)
- 2002(Publication Date)
- Newnes(Publisher)
...However, the electron’s magnetic field does play an important role when the electron is bound in the atomic structure of materials. EFFECTS OF THE MAGNETIC FIELD The Dipole Now that you understand how magnetic fields are created, you need to understand how magnetic objects are affected by an external magnetic field. The situation is more complex than the electric field, where charges just follow the electric field lines. The effect of the magnetic field is rotational. To analyze how the magnetic field operates, you need some form of fundamental test particle. For the electric field, we use a point charge (i.e., a charged, infinitesimally small particle). Since magnetic charges do not exist, some alternative must be used. One such test particle is an infinitesimally small magnetic dipole. A magnetic dipole test particle can be thought of as a compass needle made exceedingly small. A magnetic dipole has a north pole and a south pole, implying that it has direction in addition to magnitude. In other words, it is a vector quantity. The property of direction highlights a fundamental characteristic of the magnetic field that makes it different from the electric field. You know from experience that a compass needle always rotates so that the marked end (north pole) of the needle points north. If we place our conceptual compass in a magnetic field, the needle will likewise rotate until it points along the field lines. Its orientation will be such that its field lines up with those lines of the field in which it is immersed. So instead of a force being transmitted to the test dipole, torque is transmitted. A torque is the rotational analogy to a force. In this instance, the magnetic field acts as a “torque field” in comparison to the electric force field...
eBook - ePub- Jacob N. Israelachvili(Author)
- 2010(Publication Date)
- Academic Press(Publisher)
...We shall make use of this property again when we consider other types of interactions. Problems and Discussion Topics 4.1 A free, positively charged ion A is placed close to a fixed dipolar molecule in a liquid medium. By considering the electric field lines emanating from the dipole (see Figure 4.1 b and c), in what direction will A move if it is placed (i) somewhere along the long axis (along θ = 0), and (ii) somewhere along the perpendicular bisector (at θ = 90°) of the dipolar molecule? In another situation, A is a free cigar-shaped dipolar molecule. (iii) How will A orient and then move with respect to the fixed dipole when it is placed in the same two positions as the preceding? Under natural or laboratory-controlled conditions, an ion or molecule is not “placed” somewhere but nevertheless “gets there” due to random or directed motion. Describe some of these motions. 4.2 Certain linear molecules such as O=C—C=C—C=C—NH 2 containing conjugated bonds are easily polarizable by an electric field that causes intramolecular charge separation resulting in a highly dipolar molecule such as − O—C=C—C=C—C=N + H 2. A nonpolar but easily polarizable molecule of length l = 1 nm acquires a dipole moment u = el due to a potential of ψ = 1 V acting along its length. The resulting unit charges ± e at either end of the molecule are pulled by the field E = ψ / l in opposite directions, which acts to increase the length of the molecule. However, these same charges attract each other with a Coulomb force that acts to decrease the length of the molecule. Does the molecule expand or contract? [ Hint: In this example consider whether the charge separation and induced dipole has occurred due to an internal Harpooning effect.] 4.3 Look at Figure 3.3. Assume glycine to be a molecule with a dipole of unit charges ± e at a distance l apart. Estimate l from the data of Figure 3.3...
eBook - ePubLiquid Crystal Displays
Fundamental Physics and Technology
- Robert H. Chen(Author)
- 2011(Publication Date)
- Wiley(Publisher)
...To differentiate, the term “charge polarization” will indicate the charge separation in molecules, and the single word “polarization” will be reserved for light whenever there is danger of confusion. As the story of liquid crystals unfolds, understanding of the two phenomena will increase, and the polarization in question will be clear in context, rendering particularization unnecessary. As described above, a liquid crystal’s molecules have a dipolar intrinsic charge polarization structure that can be described by a dipole, which can be described mathematically as just the separation of positive and negative charge (q) times the distance and direction between the charges expressed by a vector lever arm (l); this is the intrinsic Electric Dipole moment p = q l, as shown schematically in Figures 3.1 and 3.2 for between the charges and farther away from the charges respectively. Figure 3.1 Dipole field between the charges. Figure 3.2 Dipole field away from the charges. When no external electric field is applied, the intrinsic Electric Dipole moments in the liquid crystal are randomly oriented with no particular orientational order, and thus cancel each other out to have no net gross effect (the liquid crystal is non-polar). But applying an electric field will cause the Electric Dipole moments to tend to align with the field to produce a gross Electric Dipole field, that from a large distance (“large” meaning compared with the length of the dipole’s lever arm) manifests the response of the liquid crystal to the external electric field. In truth, since the human eye cannot perceive the microscopic individual dipoles, the gross average effect is the phenomenon that is observed anyway. So the molecules’ dipole moments produce an average dipole moment field that manifests the liquid crystal’s structural anisotropy...
eBook - ePubEnergy Medicine - E-Book
The Scientific Basis
- James L. Oschman(Author)
- 2015(Publication Date)
- Churchill Livingstone(Publisher)
...This electric field influences other electrically charged objects. There are two ways of describing the influences of fields and the ways they interact with each other. One perspective is that objects have properties that modify the space around them such that another object entering that space will have a force exerted upon it. A second perspective does not require the concept of force: Objects have properties that modify the space around them such that another object entering that space will experience a change in its motion. In the case of the electron, shown in Figure 2.2, the lines of force reveal the direction of motion a positive test charge would experience when brought into the space around the electron. Specifically, since opposite charges attract, the positive test charge will be drawn towards the center of charge of the electron. Figure 2.2 The electric field of a stationary electric charge. Note that the charge is imaged as a point in space. This is a simplification that has made it easier to calculate charge interactions. However, there are other valuable perspectives on the nature of the electric charge that will be discussed in this chapter. With regard to the image of the electron shown in Figure 2.2, recognize that the view of the electron as a point in space is but one of several models of the electron and other charged particles. What is an Electron? Much of the discussion that follows will concern the behaviour of electrons and other charged particles. We shall see that when a charge moves, magnetic fields are produced. And we will also see that the opposite is true: Magnetic fields alter the motions of nearby charges. These principles are profoundly important for energy medicine. Many of the techniques used in energy medicine look like New-Age hocus-pocus until they are viewed through the discerning eyes of the physicist and biophysicist...
eBook - ePubElectrical Engineering
Fundamentals
- Viktor Hacker, Christof Sumereder(Authors)
- 2020(Publication Date)
- De Gruyter Oldenbourg(Publisher)
...Through charge separation another electric field forms within the conductor, which counteracts the outer field. The charge redistribution is completed when the opposing field is equal to the outer field. The inside of the conductor is thus field free meaning that this space is shielded against the outer field (Faraday cage – charges on the metal surface). Figure 4.8: Electrostatic induction is the redistribution of electric charges in a conductor under the influence of an electric field. 4.4 Polarisation The redistribution of charges in the electric field does not only take place in electric conductors but also in electrically insulating materials. Due to the charge carriers being stationary in insulators, the electric charges can only be moved or twisted within their associated atom or molecule. Insulating materials have hardly any free electrons; charges are only moved slightly (weak induced voltage). Therefore, due to the impact of the electric field small, Electric Dipoles form on the previously neutral insulating material atoms. If charges are electrostatically separated from an originally neutral object, we talk about polarisation. There are two types of polarisation: Dielectric polarisation occurs with certain insulating materials. Within the molecules, charges are aligned. These molecules are called dipoles. Paraelectric polarisation : Insulating materials are already organised as dipoles. However, without an outer electric field, the electric effects cancel each other out due to the disordered thermal movement (a). If an outer field is applied, the dipoles align themselves according to their polarities (b). 4.5 The electric displacement flux Ψ Electric fields are the sum of all field lines. The fields occur due to the redistribution of charges; the sum of all field lines therefore is called displacement flux Ψ, which corresponds to the amount of separated charges...
