The following points highlight the four main types of electrical quantities. The types are: 1. Electric Charge 2. Electric Current 3. Electromotive Force and Potential Difference 4. Electric Field Strength.
Type # 1. Electric Charge:
Electrical energy transfer occurs because of the action of electrical charges. Electric charge is an indication of the quantity of electricity and is usually denoted by Q or q and is measured in coulombs (abbreviated C).
In electrostatics, charge is amount of electricity present upon any substance which has accumulated electrical energy.
While we can perceive of a wire or a similar object carrying a charge, it is difficult to visualize the charge itself being divorced from the object. The nature of electric charges is best understood in terms of the effects they produce.
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One of the first phenomenon observed in the study of electric charges was that there are two kinds- positive and negative Protons are considered positive charges, electrons negative.
A most significant effect of an electric charge is that it can produce a force specifically, a charge will repel other charges of the same sign; it will attract other charges of the opposite sign. The magnitude of the force between two charged bodies is proportional to the product of the charges and inversely proportional to the square of the distance between them.
That is, the force F between two charged bodies having charges Q1 and Q2 is given by Coulomb’s law as:
Where d is the distance between the charges and K is a constant depending on the medium surrounding the charges?
Type # 2. Electric Current:
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Electric current may be defined as the time rate of net motion of an electric charge across a cross-sectional boundary.
A random motion of electrons in a metal does not constitute a current unless there is a net transfer of charge with time:
Coulomb is the practical as well as SI unit for measurement of electric charge. One coulomb is approximately equal to 624 × 1016 electrons.
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Since current is the rate of flow of electric charge through a conductor and coulomb is the unit of electric charge, the current may be specified in coulombs per second. In practice the term coulomb per second is seldom used, a shorter term, ampere is used instead.
Type # 3. Electromotive Force and Potential Difference:
Electromotive force (emf) is the force that causes an electric current to flow in an electric circuit while the potential difference (pd) between two points in an electric circuit is that difference in their electrical state which tends to cause flow of electric current between them.
Voltage may be defined as the electric pressure available to cause flow of current when a circuit is closed. That is, voltage is a term that may be used in place of electromotive force (emf).
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Volt is a unit of electromotive force as well as potential difference in practical as well as in SI system of units.
The volt is defined as that potential difference between two points of a conductor carrying a current of one ampere when the power dissipated between these points is equal to one watt.
Type # 4. Electric Field Strength:
The situation defined in Eq. (1.1) can be described by saying that there is a region of influence of the neighbor of an electric charge wherein a force will be exerted when another charge is introduced. The force will grow progressively weaker as the new charge is placed in more remote positions. Such a region of influence is called an electric field.
The electric field is defined at a point as the force per unit positive charge. That is, the electric field at any point is the force in magnitude and direction, which would act on a unit positive charge. Contributions to the total field at any point are made by all charges that are close enough to have any influence.
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The force experienced on a unit positive charge placed at a distance of d from a charge Q is obtained by substituting Q1 = Q and Q-, = 1 in Eq. (1.1).
Thus,
In SI system, the force is measured in newton’s, distance in metres, charge in coulombs and the force experienced on a unit positive charge placed at a distance of d metres from a charge of Q coulombs is given by the expression:
Where ε0 is the permittivity of evacuated space or air and = 8.854 × 10-12: F/m and εr is relative permittivity of the surrounding medium with respect to evacuated space or air.
The quantity on the right hand side of the Eq. (1.4) is a function only of Q and the directed line segment from Q to the position of unit charge. This describes a vector field and is called the electric field strength or electric intensity. Thus electric field strength (or electric intensity) is defined as the vector force on a unit charge.
This is represented by E and the magnitude of electric intensity is given as:
A plot of vector E about a single concentrated charge Q is shown in Fig. 1.3. Figure 1.3 (a) shows the plot for positive charge and Fig. 1.3 (b) for a negative. The respective lengths of vectors represent the magnitude to a scale.
If a field is due to several charges (Q1, Q2, Q3, . . . ), the electric intensity will be the vector sum of electric intensities due to each charge taken separately i.e.:
E = E1 + E2 + E3 + … …(1.6)
Example:
Point charges in air are located as follows:
+ 5 × 10-8 C at (0, 0) metres, + 4 × 10-8 C at (3, 0) metres and – 6 × 10-8 C at (0, 4) metres. Find electric field intensity at (3, 4) metres.
Solution:
The electric intensity at point D (3, 4) due to positive charge of 5 × 10-8 C at point A,
E1 acts along AD, as shown in Fig. 1.4.
Similarly electric intensity at point D due to positive charge of 4 × 10-8 C at point B,
The resultant electric intensity at point D may be determined by resolving E1, E2 and E3 into their X- and Y-components we get:
