Electricity & Magnetism

Electric charge

The primordial stuff of electricity is electric charge. It is the essence of electrical phenomena. It is so basic that is difficult to describe except in the context of the effect that are ascribe to its existence. These effects are only manifest as forces of interaction.We experience something and consequently seek to “explain” our experience in term of something more elemental. It may be, however, that charge is simply a property of certain of nature’s elementary particles and does not exist “outside” of these particles (as in the case of electron, the m meson, ect). How-ever, we will talk of charge as if it possesses an independent existence.

So far as we know, electric charge has the following  characteristics:

  1. There are two kinds of electric charge, denoted arbitrariry as positive and negative charge. The magnitude of the charge is given by a positive real number, and its type is denoted by a plus (+) or minus (-) sign. All charge is equivalent, however, in the sense that charges may be added to each other algebraically just like real (positive or negative) numbers to give other charges. It is found that two charge of opposite sign attract each other.
  2. In nature, the total amount of positive charge just balances the total amount of negative charge; electrical neutrality of objects is the most common occurrence. Moreover, it is not possible to create (or annihilate) positive (or negative) charge without creating or annihilating an equal amount of negative (or positive) charge. This may be regarded as a principle of conservation of charge)
  3. Phisically, we also would like to belive in what may be called “charge symmetry.” This mean that two words that differ only in that all charges in one have opposite signs from the charges of the another would be physically indistinguishable. We see “approximate” manifestations of this effect in the realm of elementary particle physics. There, it has been found that for every elementary particle with a positive charge, there exists another “identical “ elementary particle that has a negative charge of equal magnitude. Thus we have electrons and positrons, protons and antiprotons.

Coulomb’s Law

His basic experimental law refers to the situation illustrated in figure 2-2 in which we have two point charge q and q’ separated by a distance R; we assume the charge to be fixed in position and that there is no other matter, that is, the charges are situated in a vacuum. The force on q due to q’ will be written as Fq’®q. Thus, q’ is being treated as the origin of the force, and we can refer to q’ as the “source” and its location given by r’ as the “source point” located by r. The R is relative position vector of q with respect to q’ and is seen to be directed from the source point to the field point as is its corresponding unit vector R. Thus, we have:

R = r- r’,

In term of all of all these quantities, Coulomb’s law says that:

so that the force is proportional to product of the charges and to the inverse square of the distance between them; it seen to be similar to gravitation in these respect.

Sistem of Point Charge

Now suppose that, in addition to q, there are a number N of point charge distributed at fixed positions throughout otherwise empty space. We designate each charge by q, and its position vector by ri, where i = 1, 2, 3 …, N. This situation is illustrated in figure 2-3; for clarity, the individual position vector are not shown, but the unit vectors R, corresponding to the relative position of q with respect to the qi are shown. Each of these charges can exert a force on q, Fqi®q’ which will be of the general form givem by (2-3). The experimental fact of superposition properties of force are already familiar to us from mechanics; hence Fq’ the total force on q, will be given by the vector sum of the individual forces so that



(1)    What does electric charge mean?

(2)    How do you make an object for getting electric charge?

(3)    How if are two electric objects in same electric charge approached each other?

(4)    How if are two electric objects in different electric charge approahed each other?

(5)    Give example of electric charged object

(6)    Why the electrical field strength within conductor ball surface equals to zero?

(7)    Type of coulomb’s is nearly same with Newton’s gravitation law. What is the difference of both laws? Also compare

mass of gravity with electric charge.

(8)   Two points in electric charge; each has charge a 6 mC and 12mC, and in 3m distance each other. Count the magnetitude

of electrostatic force encountered by those both charges in the air.

Problem 1.


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