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Monday, 15 July 2019

Introduction to DC Circuits

Introduction to DC Circuits:

In the study of electric circuits we are interesting in the flow of electricity from one device to another. A simplest of electric devices will have a pair of terminals. Electricity enters at one terminals and leaves from another.

The energy associated with an electric system is measured by quantities like electric charge, potential and current. Historically electric charges were first observed by rubbing certain dry substances together. Atomic physicists now picture the electric charges as one of the building blocks of the universe. The negatively charged electron is one of the constituents of the atom Its mass is 9.107 ×10-28 gm. As the electron is very small unit, the practical unit of charge is the coulomb.

1 coulomb = 9.107 ×10-28 × Qe

Where Qe is the electronic charge


All matter consists of minute particles called molecules. Molecules are made up of more minute particles called atoms. Inside the atom there a central nucleus and a number of electrons around the nucleus. The nucleus contains protons and neutrons. Protons are positively charged particles and neutrons do not carry any charge. The electron carries a negative charge.

In a normal atom, the positive charge on the nucleus is exactly equal to the total negative charge due to all the electrons surrounding the nucleus.

Every atom in which the positive and negative charges are equal has no net positive and negative charge.

Electrons revolve around the nucleus in elliptical orbits. The first orbits has a maximum of 2 electrons, second 8 electrons , third 18 electrons and so on. Number of electrons present in the orbit is 2n2 , where ‘n’ is the orbit number.


1. Current

Flow of electrons in any conductor is called current. It is represented by the letter ‘I’
The unit of the current is ampere
Current , I = Charge/ Time
I = Q/T
Where ‘Q’ is in coulomb and ‘t’ is in second.

2. One Ampere

One ampere can be defined as one coulomb of charge is transferred in one second.

3. Voltage

The difference of potentials between two points is called voltage or potential difference.
The voltage represented by the letter ‘V’
The unit of the voltage is volt.

4. One volt

One volt is defined as the potential difference across a resistance of one ohm carrying a current of one ampere

5. Resistance

The opposition offered by a substance to the flow of current is called resistance.
The resistance represented by the letter ‘R’
The unit of the resistance is Ohm (Ω)

Laws of Resistance

The resistance (R) of a conductor
is directly proportional to its length (l)
is inversely proportional to its area of cross section (a)
depends up on the nature of material
depends on it temperature

R α l/a
R = ρl/a
Where ρ (Rho) is a constant for the material called resistivity (or) specific resistance

Conductance , G

Conductance is the reciprocal of resistance.
The conductance represented by the letter ‘G’
The unit of the conductance is mho
G = 1/R = a/ ρl = σa/l
Where σ = 1/ρ
where σ (sigma) is called the conductivity.
The reciprocal of resistivity is called conductivity.

6. Ohm’s Law

The current flowing in an electric circuit depends on two quantities, the applied emf and the resistance in the circuit. The connecting the three factors was established first by G.S. Ohm in 1826 and is popularly known as the Ohm’s law.

Statement of Ohm’s Law

At constant temperature, the current flowing through the conductor is directly proportional to the potential difference (voltage) between the two ends of the conductor
i.e., V α I
V = RI
V = IR
Where ‘R’ is the resistance of the conductor in ohms


Consider a simple closed circuit as shown in figure.

Let ‘V’ be the supply voltage (dc)
‘I’ be the current flowing through the circuit (amp)
‘R’ be the resistance of the circuit (ohm)
According to Ohm’s law, V α I
V = RI
V/I = R (constant)
The unit of resistance is Ohm

7. Power

Power is the rate of doing work.
Power = Work done/Time
The power represented by the letter ‘P’

The unit of the power is watt or joules / second

Power is the product of voltage and current

Power, P = voltage x current
P = VI watts (or)
P = (IR) I
P = I2R watts (or)
P = V (V/ R)
P = V2/R watts

The unit of power can also be expressed in kilowatts (Kw) , 1 Kw = 1000 watts

8. Energy:

Energy is the capacity to do work. Energy is the product of the power ant time

Energy = Power x time
= P x t
= VI x t (or)

Energy = I2R x t (or)
Energy = V2 x t/ R

The unit of energy is watt-sec. Watt-sec. is the smallest unit of energy. So kilo watt hour (Khr) is generally used.

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