Electric Fields Chemistry

11 Key excerpts on "Electric Fields Chemistry"

• 

  eBook - ePub

  Physics for Chemists

  • Ruslan P. Ozerov, Anatoli A. Vorobyev(Authors)
  • 2007(Publication Date)
  • Elsevier Science

    (Publisher)

  From the point of view of electric properties, all substances can be divided into two main classes—conducting and nonconducting an electric current. Metals, their alloys, and a small number of chemical compounds with metal character of interatomic interactions relate to the class of conductors. The second class includes other substances and represents the overwhelming majority. Conductivity is defined by the presence of free charge carriers in a substance; their absence determines dielectric properties. So, dielectrics are substances in which there are no free charges capable of covering long distances in the substance. Depending on their molecular structure, all dielectrics can, in turn, be divided into two large groups—polar and nonpolar. In polar dielectrics, molecules themselves represent the electric dipoles with the electric moment; it appears due to displacements of electric charges from positions of their equilibrium in free atoms as a result of chemical bonding. The molecular dipoles of polar dielectrics participate in thermal motion; this can be translational motion (in gases and liquids), oscillation about equilibrium positions (solids and liquids) and rotation around the center of mass. As a result, the dipole electric moments are chaotically distributed along directions.

  Electric and magnetic interactions play an enormous role in chemistry and chemical technology; they govern the processes taking place in atoms and molecules, crystals, electrolysis, surfaces of solids electrolyzing of dielectric polymer materials and others. Because the electric field in molecular systems has a very complex structure, for the convenience of the reader, we will give the nomenclature of electric fields at the beginning of the chapter.

  Let us start by briefly presenting the main information on electric charges and the characteristics of the fields they create.

  4.1 ELECTROSTATIC FIELD

  4.1.1 General laws of electrostatics

  The main electric charge carriers are charged particles—electrons and protons. They carry a charge |e | = 1.6 + 10–19 C, electrons being negative and protons positive. In neutral atoms and molecules, negative electron charge is compensated for by a positive nuclear charge. By removing one or more electrons from an atom, one can make a positive (monoor multicharged) ion and, conversely, by adding electrons, it is possible to create a negative ion from an originally neutral particle. In such a way, positive and negative carriers of charge are created (in an electrolyte, for instance).

  When an excess or lack of electrons is created in a body, the body becomes charged, carrying a charge Q . The value Q is always proportional to ± |e |. However, at large Q (in comparison with |e |), this discontinuity is not exhibited, so it is possible to consider the charge changing continuously.

  The distribution of charge in a body can be described by the function ρ (r ) such that ρ (r )dV = dq , where dq is the charge, comprised in the volume dV . Function ρ (r ) is called the volume charge density. Charge distribution on a surface is described by surface charge density σ (r )dS = dq . Charge distribution along a line gives a linear charge density τ (l ) so that dq = τ (l )dl . (In the first two cases, r is a radius vector of the elements dV and dS , and l

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Electric Field Analysis

  • Sivaji Chakravorti(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  –19 C. Other particles (e.g. positrons) also carry charge in multiples of the electronic charge magnitude. However, these are not going to be discussed for the sake of simplicity.

  Electric charge is also conserved; that is, in any isolated system or in any chemical or nuclear reaction, the net electric charge is constant. The algebraic sum of the elementary charges remains the same. In physical terms, it implies that if a given amount of negative charge appears in one part of an isolated system, then it is always accompanied by the appearance of an equal amount of positive charge in another part of the system. In modern atomic theory, it has been proved that although fundamental particles of matter continually and spontaneously appear, disappear and change into one another, they always obey the constraint that the net quantity of charge is preserved.

  1.3 Electric Fieldlines

  From logical reasoning, it can be stated that if a charge is present in space, then another charge will experience a force when brought into that space. In other words, the effect of the first charge, which may also be called the source charge , extends into the space around it. This is known as the electric field caused by the source charge. If there is no charge in space, then there will be no electric field. If the source charge is of positive polarity and the test charge is of negative polarity, then the test charge will experience an attractive force. On the other hand, if the test charge is also of positive polarity, then it will experience a repulsive force. If the test charge is free to move, then it will move in accordance with the direction of the force. The loci of the movement of the test charge within the electric field are known as electric lines of force or electric fieldlines .

  Behaviour of an electric field is conventionally analyzed considering the test charge to be a unit positive charge. Hence, the test charge will experience repulsive force from a positive source charge and attractive force from a negative source charge. Therefore, the test charge will move away from the positive source charge and move towards the negative source charge. Accordingly, the directions of electric fieldlines are such that they originate from a positive charge and terminate on a negative charge, as shown in Figure 1.1 . Figure 1.1 a shows the electric fieldlines originating from a positive source charge whose magnitude is integer (N ) multiple of +e , whereas Figure 1.1 b shows the electric fieldlines terminating on a negative source charge of the same magnitude. The electric fieldlines are the directions of the force experienced by a unit positive charge +e , as shown in Figure 1.1

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Introductory Physics for the Life Sciences: Mechanics (Volume One)

  • David V. Guerra(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  5 Electric Forces and Fields

  DOI: 10.1201/9781003308065-5

  5.1 Introduction

  The electric force is another force of nature that is associated with electric charge, which is another property of matter. This chapter begins with a discussion of electric charge and then the format of the electric force is presented and examples are provided. Similar to the previous chapter, the concept of the electric field is discussed along with several examples.

  • Chapter question: Gel electrophoresis is a lab technique employed to separate and identify biological molecules, based on size. It is used to study proteins and DNA for medical and forensic investigations. The question is, what is the role of the electric field in the lab technique known as gel electrophoresis? This question will be answered at the end of this chapter after the concept of the electric field is developed throughout this chapter.

  5.2 Charge

  Like mass, charge is a property of matter and it is the starting place for the study of all of electricity and magnetism. Unlike mass that has only one type under normal conditions here on Earth (antimatter is possible but not common), there are two types of charge in common matter that balance each other. These two types are called positive and negative and, in most matter, there are equal amounts of both, so in most objects the net amount of charge is zero. So, it is the imbalance of charge that is measured and referred to in our analysis as charge. It is important to remember that the term charge on an object is not the total charge, but just the imbalance of excess positive or negative charge. The symbol for charge is either a capital or lower case (Q or q). Both symbols are used to represent the imbalance of charge of an object, and it is common to use the upper-case Q for larger charges in a problem and the lower-case q for the smaller charges in a problem. It is also acceptable to use only upper- or lower-case q

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Electrochemical Science and Technology

  Fundamentals and Applications

  • Keith Oldham, Jan Myland, Alan Bond(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  dipolar water molecule. The red and blue surface regions are charged positively and negatively respectively.

  Ions and electrons are the actors in the drama of electrochemistry, as are molecules. Most often these charged particles share the stage and interact with each other, but in this chapter we mostly consider them in isolation. The electrical force f between two charges Q 1 and Q 2 is independent of the nature of the particles on which the charges reside. With r 12 as the distance between the two charges, the force103 obeys a law

  1:2

  attributed to Coulomb104 . The SI unit of force is the newton 105 , N. Here ε is the permittivity of the medium, a quantity that will be discussed further on page 13 and which takes the value

  1:3

  when the medium is free space106 . The force is repulsive if the Q ’s have the same sign, attractive otherwise. To give you an idea of the strong forces involved, imagine that all the Na+ cations from 100 grams of sodium chloride were sent to the moon, then their attractive force towards the earthbound chloride anions probably exceeds your weight107 .

  A consequence of the mutual repulsion of two or more similar charges is that they try to get as far from each other as possible. For this reason, the interior of a phase108 is usually free of net charge. Any excess charge present will be found on the surface of the phase, or very close to it. This is one expression of the principle of electroneutrality .

  Charges at Rest: electric field and electrical potential 109

  Coulomb’s law tells us that an electric charge can make its presence felt at points remote from its site. An electric field is said to exist around each charge. The electric field is a vector; that is, it has both direction and strength. Figure 1-3

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Electromagnetism

  • I. S. Grant, W. R. Phillips(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  Figure 1.5 shows the lines of force near a pair of point charges of equal magnitude, one positive and one negative. Such a pair of equal and opposite charges is called an electric dipole. Very close to each charge, the field is almost the same as for isolated point charges, but the field lines starting off at the positive charge curve round to finish at the negative charge. The diagram gives an indication of the strength of the field as well as its direction, because lines of force are always densely packed in regions where the field is strong. The total number of lines on the diagram is not significant; if twice as many lines were drawn terminating on each charge, regions of relatively high or low density of lines of force would still correspond to regions of high or low field strength. The example of the electric dipole is an important one and later on we shall obtain a mathematical expression for the field at some distance from a dipole.

  1.3 ELECTRIC FIELDS IN MATTER

  1.3.1 The atomic charge density

  So far electric fields have been dealt with in terms of idealized particles which are stationary and which carry point charges. This is not satisfactory if we want to discuss the electric field within an actual atom, since electrons are certainly not stationary point charges. Indeed, according to quantum mechanics the position of an electron cannot even be sharply defined. Instead of imagining the electron as a point, it is more realistic to regard its charge as being smeared out in a cloud around the nucleus. The electric field in and around a hydrogen atom, for example, behaves as though only part of the electronic charge is contained by any section of the electron cloud. Let us write ρ el (r)δ τ for the charge contained within a small volume δ τ situated at the point with position vector r. The quantity ρ el (r) is the charge density of the electron cloud, i.e. its charge per unit volume at r. The charge density is another function of position, but unlike the electric field it is a scalar quantity, fully specified by its magnitude at each point. Scalar functions of position such as the charge density are called scalar fields . As with the vector fields, we shall often omit the argument of scalar fields, writing the electron charge density just as ρ el , for example. The total charge (– e ) carried by the electron in a hydrogen atom is equal to the sum of the charges contained in all the small volumes δτ making up the electron cloud. In the limit as the volumes δτ become infinitesimal, the sum becomes an integral; integrating over a volume V

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Human Exposure to Electromagnetic Fields

  From Extremely Low Frequency (ELF) to Radiofrequency

  • Patrick Staebler(Author)
  • 2017(Publication Date)
  • Wiley-ISTE

    (Publisher)

  Chapter 3 .

  Table 1.1

  . Summary of electric fields

  Key points of electric fields

  The level of electric fields is expressed in volts per meter (V·m–1 or V/m).

  An electric field appears in the presence of electric charges. These charges can be the result of a difference in potential (voltage). That means a field exists as soon as an electric device is connected to the power supply, even if it is not working.

  The field strength decreases with the distance from the source.

  An electric field can make electric charges move.

  The electric field alters the distribution of charges on the surface of a conducting object. Reciprocally, an object that is slightly or highly conductive alters the field.

  A distinction should be made between the ambient electric field and the electric field induced within an object during exposure.

  Most construction materials protect against electric fields to some extent.

  1.1.3. Magnetic fields

  A magnetic field is created when electric charges move to form a current. There can be no magnetic field without an electric current.

  The force lines of the magnetic field form concentric circles along a conductor in which a current is flowing. These circles (forces) are perpendicular to the conductor, as shown in Figure 1.4 .

  Figure 1.4.

  Magnetic field line around a wire

  The magnetic field is characterized by its strength, represented by the symbol H and measured in amperes per meter (A·m–1 ). This quantity is proportional to the intensity of the electric current and decreases with distance. For a spatially isolated, straight electric wire, the field is expressed by (Biot–Savart law):

  [1.4 ]

  where I represents the electrical intensity in amperes (A) and d is the distance to the wire in meters. A current of 100 A intensity creates a magnetic field of 15.9 A·m–1 at a distance of 1 m.

  A continuous current creates a static magnetic field, like a permanent magnet. An alternating current creates an alternating magnetic field.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Static Fields and Potentials

  • Joy Manners(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)
• Charges exert a force on each other. The properties of charge are summarized in Box 1.1 of Section 3.2.
• Coulomb's law of force between charges situated in a vacuum may be summarized in the vector equation
  F el

  =

  1

  4 π

  ε 0

  (

  q 1

  q 2

  r 2

  )

  r ^

  .

  (1.13)
  The constant ɛ0 is called the permittivity of free space.
• The electric field
  ℰ

  ( r )
  (r) is a vector quantity, defined at all points in space. At any particular point r its value is given by the electrostatic force per unit charge at that point. So
  ℰ

  ( r )

  =

  F el

  (

  on   q   at   r

  )

  q

  .

  (1.14)
• The electrostatic force experienced by a point charge q at r is given by
  F el

  (

  on   q   at   r

  )

  = q ℰ

  ( r )

  .

  (1.15)
• The electric field due to a point charge Q at the origin is given by
  ℰ

  ( r )

  =

  (

  Q

  4 π

  ε 0

  r 2

  )

  r ^

  .

  (1.16)
• Fields of a given type, due to different sources, may be added vectorially using the triangle or parallelogram rule.
• The similarities between the interaction of masses via gravitational fields and the interaction of charges via electric fields are summarized in Box 1.2 below.
• Gravitational and electric fields can be represented pictorially by means of field lines.
• Applications and occurrences of electric fields are widespread. Discussed in this chapter are Millikan's oil drop experiment, ink-jet and electrostatic printing, and liquid crystal displays. The Earth has a naturally occuring electric field.