Electricity

IMPORTANT NOTES

1. There are two kinds of electric charges i.e., positive and negative. The opposite charges attract each other and the similar charges repel each other. Coulomb (C) is the standard unit of charge.

2. Conductors: The substances through which electricity can flow are called conductors. Silver, copper, gold and aluminium are examples of conductors.

3. Insulators: The substances through which electricity cannot flow are called insulators. Glass, wood, porcelain and rubber are examples of insulators.

4. One coulomb: A body is said to have one coulomb charge if it has 6.25 × 1018 electrons on it or is deficit as compared to the normal number of electrons.

5. Electric current: The rate of flow of charge from a body at higher potential to a body at lower potential is called electric current. Ampere (A) is the standard unit of current.

6. Electric potential: The amount of work done in moving a unit positive charge from infinity to a given point in an electric field is called the electric potential at that point. Thus, electric potential is a condition which determines the direction of flow of charges. The unit of electric potential is volt (V).

7. Potential difference: The amount of work done in moving a unit positive charge from one point to another in an electric field is called potential difference.

8. Closed electric circuit: An electric circuit in which all the components of the circuit are joined to one another, such that continuous current flows through them, is called closed electric circuit.

9. Open electric circuit: An electric circuit in which electric contact is broken at some point (say by a switch), such that no current flows through the components of the circuit is called an open circuit.

10. Electric resistance: The opposition or obstruction offered by a conductor to the flow of the electrons is called electric resistance. In SI system unit of resistance is ohm (Ω).

11. Resistivity: It is the amount of resistance offered by a conductor of unit length and unit area of cross-section, such that current enters and leaves from its opposite faces is called its resistivity or specific resistance.

12. Series circuit of resistors: When a number of resistors are connected end to end such that tail end of one resistor is connected to the initial end of the other resistor so as to form a closed circuit, then such a circuit is called the series circuit.

13. Parallel circuit of resistors: When a number of resistors are connected in such a way that they have common positive terminal and a common negative terminal, then the resistors are said to be connected in parallel circuit.

14. Ohm’s law: All physical conditions of a conductor remaining the same, the current flowing through it is directly proportional to the potential difference at its ends. If I is the current flowing through a conductor, such that V is the potential difference at its ends, then V ∝ I ⇒ V = I R Where R is the constant of proportionality and commonly called the resistance of a conductor.

15. Electric work: Electric work is said to be done when a charge flows through a conductor at some potential difference. If W is the amount of workdone in carrying Q charge from one point to another in an electric field, such that, V is the potential difference, then V = W Q ⇒ W = VQ

16. Electric power: The rate of doing electric work is called the electric power. The SI unit of power is watt (W). If W is the amount of electric work done in time t, such that P is the power, then P = W t But, W = I2Rt Thus, P = = I2R.

17. Overloading: Overloading of circuit means, passing more current through the circuit than it can tolerate without damage.

18. Short circuit: It means that live and neutral wires come in contact with each other, thereby bypassing the electrical device. It is caused due to melting of insulation of connecting wires or the live wire getting connected to earth.

19. Fuse: It is a safety device in an electric circuit. It is the weakest point in an electric circuit, which melts and breaks the electric circuit, when the circuit gets overloaded.