Systems of units

   Electric Circuit: Electrical circuit is a system of interconnected components such as resistors, capacitors, inductors, voltage sources and so on.

   Circuit Theory:The concepts, mathematical relationships, methods of analysis which have evolved from these circuits are known as circuit theory.

   Electric Charge:Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).

The following points should be noted about electric charge:

1. The coulomb is a large unit for charges. In 1 C of charge, there 1/1.602X10 ^-19 =    6.24X10 ¹⁸ are electrons. Thus realistic or laboratory values of charges are on the order of pC, nC, or uC .
2. According to experimental observations, the only charges that occur in nature are integral multiples of the electronic charge e= -1.602X10 ^-19
3. The law of conservation of charge states that charge can neither be created nor destroyed, only transferred. Thus the algebraic sum of the electric charges in a system does not change.

System of Units:

System International:

The system of units is Known as System international.

Length:   The distance travelled by light in a vacuum in 1/299 792 458 of a second

Second:   The second is defined in terms of the period of a cesium-based atomic clock.

Kilogram:  The definition of the kilogram is the mass of a specific platinum-iridium cylinder (the international prototype), preserved at the International Bureau of Weights and Measures in France.

Electric Current: Electric current is the time rate of change of charge, measured in amperes (A).

Temperature: The degree of hotness of a body is called Temperature

Basic units are shown in Table 1.1

Table 1.1

Force: Force is a physical quantity, which changes or tend to changes the position of a body. Force is measured in Newton. The force,which produces an acceleration of 1-meter-second square in a body of mass 1 kg is 1 Newton. It is denoted by N.
Energy: The capacity of a body to do work is called Energy.The unit of the energy is joule, if a force of 1 N acts on a body and displaces it through a distance of 1 m in its direction, then the work done or energy will be 1 joule. It is denoted by j.
Power: Work done per unit time is called power. or The rate of doing work by an agency is called power.  The unit of the power is Watt,  if an agency performs 1 joule of work in 1 sec, then the power will be 1 watt.It is denoted by W.
Voltage(Potential difference): The potential energy per unit charge is called potential of that point.The difference of the potential energy per unit charge between two points is called Potential Difference or voltage.                                                                                OR
Work done in moving a unit positive charge from one point to other point against the electric field keeping the charge in electrostatics equilibrium is called Potential Difference or voltage.The unit of the voltage is volt, The voltage between two points will be one Volt if one joule of work is done to move a positive charge of one coulomb from one point to other point keeping the charge in equilibrium.
Charge: Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).
Resistance:The opposition offered by the atoms of the conductor to the flow of electric current is called Resistance.It is measured in ohms, the resistance of a conductor is said to be one ohm if a potential difference of 1 volt produces a current of 1 ampere in it.
Capacitance:The ability or capability of capacitor to store charge is called Capacitance. The unit of capacitance is Farad, The capacitance of the capacitor is one Farad if a charge of one coulomb given to one of the plates of a parallel plate capacitor, produces a voltage of one volt between them.
Inductance:  The ratio of the average emf to the change of current with time in the coil. The unit of  inductance is Henry. One henry is the inductance of the coil in which a rate of change of current of one ampere per second causes an induced emf of one volt in the coil itself. It is denoted by H. 
Frequency: The number of waves passing through a point is called Frequency.It is measured in Hertz denoted by Hz.
Magnetic Flux: The total number of magnetic lines of forces passing through any surface placed perpendicular to magnetic field is called magnetic flux. The unit of Magnetic flux is weber. It is denoted by Wb.
Magnetic Flux density: The flux per unit area held perpendicular to magnetic field is called Magnetic flux density. The unit of the flux density is Tesla denoted by T.
 Derived units are shown in Table 1.2 

Table 1.2

 Conversions of Units is shown in the Table 1.3

Table 1.3

 The System International prefixes are shown in Table 1.4

Table 1.4