Electromagnetism Lecture#2 Instructor: Engr. Muhammad Mateen Yaqoob.

ElectromagnetismLecture#2

Instructor:

Engr. Muhammad Mateen Yaqoob

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Electric Charge There are two types of observed electric charge, which we designate as positive and negative.

The convention was derived from Benjamin Franklin’s experiments: ◦ He rubbed a glass rod with silk and called the charges on the glass rod positive. He rubbed sealing wax

with fur and called the charge on the sealing wax negative. Like charges repel and opposite charges attract each other. The unit of charge is called the Coulomb (C).

The smallest unit of “free” charge known in nature is the charge of an electron or proton, which has a magnitude of e = 1.602x10-19 C

Charge of any ordinary matter is quantized in integral multiples of e. An electron carries one unit of negative charge, -e, while a proton carries one unit of positive charge, +e.

In a closed system, the total amount of charge is conserved since charge can neither be created nor destroyed. A charge can, however, be transferred from one body to another.


Electric Charge Using the convention suggested by Franklin, the electric charge on the glass rod is called positive and that on the rubber rod is called negative.

Therefore, any charged object attracted to a charged rubber rod (or repelled by a charged glass rod) must have a positive charge, and any charged object repelled by a charged rubber rod (or attracted to a charged glass rod) must have a negative charge.

In 1909, Robert Millikan (1868–1953) discovered that electric charge always occurs as integral multiples of a fundamental amount of charge e. In modern terms, the electric charge q is said to be quantized, where q is the standard symbol used for charge as a variable.

That is, electric charge exists as discrete “packets,” and we can write

Where N is some integer. Other experiments in the same period showed that the electron has a charge -e and the proton has a charge of equal magnitude but opposite sign +e. Some particles, such as the neutron, have no charge.


Charging Objects by Induction It is convenient to classify materials in terms of the ability of electrons to move through the material:

Electrical conductors are materials in which some of the electrons are free electrons that are not bound to atoms and can move relatively freely through the material; electrical insulators are materials in which all electrons are bound to atoms and cannot move freely through the material.

Materials such as glass, rubber, and dry wood fall into the category of electrical insulators. And materials such as copper, aluminum, and silver are good electrical conductors.

Semiconductors are a third class of materials, and their electrical properties are somewhere between those of insulators and those of conductors. Silicon and germanium are well-known examples of semiconductors commonly used in the fabrication of a variety of electronic chips used in computers, cellular telephones, and home theater systems. The electrical properties of semiconductors can be changed over many orders of magnitude by the addition of controlled amounts of certain atoms to the materials.


Charging Objects by Induction (Conductors)


Charging Objects by Induction (Insulators)

A process similar to induction in conductors takes place in insulators. In most neutral molecules, the center of positive charge coincides with the center of negative charge.

In the presence of a charged object, however, these centers inside each molecule in an insulator may shift slightly, resulting in more positive charge on one side of the molecule than on the other. This realignment of charge within individual molecules produces a layer of charge on the surface of the insulator.


Coulomb’s Law We use the term point charge to refer to a charged particle of zero size. The electrical behavior of electrons and protons is very well described by modeling them as point charges. From experimental observations, we find that the magnitude of the electric force (sometimes called the Coulomb force) between two point charges is given by Coulomb’s law.

Where ke is a constant called the Coulomb constant. In his experiments, Coulomb was able to show that the value of the exponent of r was 2 to within an uncertainty of a few percent. Modern experiments have shown that the exponent is 2 to within an uncertainty of a few parts.


Coulomb’s Law The value of the Coulomb constant depends on the choice of units. The SI unit of charge is the coulomb (C). The Coulomb constant ke in SI units has the value

1 C of charge is approximately equal to the charge of 6.24x1018 electrons or protons


The Hydrogen Atom


Coulomb’s Law When dealing with Coulomb’s law, remember that force is a vector quantity and must be treated accordingly. Coulomb’s law expressed in vector form for the electric force exerted by a charge q1 on a second charge q2, written

The Electric Field The concept of a field was developed by Michael Faraday (1791–1867) in the context of electric forces and is of such practical value.

An electric field is said to exist in the region of space around a charged object, the source charge.

When another charged object—the test charge—enters this electric field, an electric force acts on it


The Electric Field We define the electric field due to the source charge at the location of the test charge

It has the SI units of newton per coulomb (N/C).

The test charge serves as a detector of the electric field: an electric field exists at a point if a test charge at that point experiences an electric force.

This equation gives us the force on a charged particle q placed in an electric field. If q is positive, the force is in the same direction as the field. If q is negative, the force and the field are in opposite directions.



The Electric Field We imagine using the test charge to determine the direction of the electric force and therefore that of the electric field. According to Coulomb’s law, the force exerted by q on the test charge is

Electric Field Lines Let’s now explore a means of visualizing the electric field in a pictorial representation. A convenient way of visualizing electric field patterns is to draw lines, called electric field lines and first introduced by Faraday, that are related to the electric field in a region of space in the following manner:


Electric Field Lines The lines are actually directed radially outward from the charge in all directions; therefore, instead of the flat “wheel” of lines shown, you should picture an entire spherical distribution of lines.

Because a positive test charge placed in this field would be repelled by the positive source charge, the lines are directed radially away from the source charge.

The electric field lines representing the field due to a single negative point charge are directed toward the charge.

In either case, the lines are along the radial direction and extend all the way to infinity.

Notice that the lines become closer together as they approach the charge, indicating that the strength of the field increases as we move toward the source charge.


Rules for drawing electric field lines

The lines must begin on a positive charge and terminate on a negative charge. In the case of an excess of one type of charge, some lines will begin or end infinitely far away.

The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge.

No two field lines can cross.


Electric Potential When a test charge q0 is placed in an electric field E created by some source charge, electric force acting on test charge is q0.E

Force is conservative because the force between charges described by Coulomb’s law is conservative.

When test charge is moved in field by some external agent ◦ Work done by field on charge is equal to negative of the work done by the external agent causing

displacement

This is analogous to situation of lifting an object with mass in a gravitational field—work done by external agent is mgh and the work done by the gravitational force is -mgh


Electric Potential The potential energy per unit charge is independent of the value of q0 and has a value at every point in an electric field

This quantity is called electric potential (or simply the potential)



Potential Difference Potential difference between two points A and B in an electric field is defined as

“Change in potential energy of system when a test charge is moved between the points divided by the test charge q0”

⌂V = V2-V1

Electric potential is a scalar characteristic of an electric field, independent of any charges that may be placed in the field

Potential Difference and Electric Potential

Electric potential is a measure of potential energy per unit charge, SI unit of both electric potential and potential difference is joules per coulomb, which is defined as a volt (V)

That is, 1J of work must be done to move a 1C charge through a potential difference of 1V



Potential Difference and Electric Potential

A unit of energy commonly used in atomic and nuclear physics is electron volt (eV), which is defined as the energy a charge–field system gains or loses when a charge of magnitude e (that is, an electron or a proton) is moved through a potential difference of 1V


Applications of Electrostatics Practical application of electrostatics is represented by such devices as lightning rods and electrostatic precipitators and by such processes as xerography and painting of automobiles.

Scientific devices based on principles of electrostatics include electrostatic generators, field-ion microscope, and ion-drive rocket engines.


Xerography or Photocopy


Laser Printing

Electromagnetism Lecture#2 Instructor: Engr. Muhammad Mateen Yaqoob.

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