This chapter deals with Electric Charge, Electric Current, Electric Circuits and Electric Potential and Potential Difference, Ohms Law, Concepts of Resistance and Resistivity, Combination of Resistors, Heating Effect of current and Electric Power.

Full attempt is made to explain every concept so that it is very easy for the students and apply in appropriate situations (reasoning, solving numericals etc)

Quick Revision
• Electric charge: this is the fundamental property of protons and electrons which gives rise to electric force between them.
• Electric current: the time rate of flow of charge through any cross section of a conductor is the measure of current
Electric current = total charge flowing/time taken
• Unit of electric current = ampere
• One ampere: the current through a wire is said to be one ampere if one coulomb of charge flows through it in one second.
• Direction of electric current is taken to be the direction of flow of positive charge or opposite to the direction of flow of negative charge (electron)
• Charge of an electron is = -1.6 x 10-19 C and charge of proton is : +1.6 x 10-19C (Q)
• Net charge on any body is given by
Q = ± ne where
e = electric charge
n = number of charges

Qn: 1. Define one Ampere?
2. Calculate the number of electrons constituting one coulomb of charge?
we know, Q = ±ne,
here Q=1C
e= 1.6 x 10-19 C
n= 1/1.6 x 10-19 = 6.25 x 1018

• To set the electrons in motion in an electric circuit, we use a cell or battery. Current flows in a circuit from positive terminal to negative terminal of the cell.
• Current in the circuit is measured by an ammeter
• Define Electric Potential Difference
Electric potential difference between any two points in an electric circuit is the amount of work done (W) in moving a unit charge from one point to another.
• Potential difference V = work done (W) / charge (Q)
• Unit of potential difference = volt
• Define 1 Volt:
The potential difference between any two points in an electric circuit is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to another.
• Potential Difference (p.d) is measured by a volt meter
• Ammeter is connected in series in the circuit and voltmeter is connected parallel across the points between which p.d is to be measured.
QN :
How much energy is given to each coulomb of charge passing through a 6V battery?
Ans: V= W/Q Therefore , W= VQ V=6V Q=1C
Therefore, W = 1J

• Ohm’s law: The current through the conductor is directly proportional to the potential difference across the ends of the conductor provided the physical conditions such as temperature, pressure etc. are kept constant.
V ∝ I V = IR Where R= resistance of the conductor
• Resistance: It is the opposition offered by the conductor to the flow of charge trough it.
• Unit of resistance – ohm (Ω).
• Define one ohm :
We know V=IR When V=1V, I=IA Then R=1Ω.
The resistance of a conductor is said to be one ohm if 1ampere of current flows through it when the p.d across it is 1volt.
• Factors on which the resistance of the conductor depends:
(i) R is directly proportional to the length of conductor (l).
(ii) R is inversely proportional to the area of cross section (A).
R ∝ l /A R =ρl / A.
When l= 1m A=1m2
Then R= ρ
Resistivity of the material of conductor is defined as the resistance of conductor of unit length and unit area of cross section
• Unit of Resistivity: Ωm.
• Both resistance and resisitivity of conductor depend on temperature (they increase with increase in temperature). It is because as the temperature is increased the electrons in the conductor undergo so much of collision with ions which increases the resistance.
• Resistivity depends on temperature, material of the conductor and not on the dimension of the conductor.