**IMPORTANT CONCEPTS **

1. Like charges repel each other and unlike charges attract each other.

2. **Coulombs Law** – The electrostatic force of attraction or repulsion between a pair of charges is directly proportional to the product of the charges and inversely proportional to the square of distance between them.

3. **electric current**– The rate of flow of electric charges is called electric current.

4. **electric potential difference**– Work done to move a unit positive charge from one point to another.

V=W/Q

5. **ohms law**– At constant temperature , the current passing through a conductor is directly proportional to the potential difference across its ends.

V= I R

6. Factors affecting resistance

Resistance of a conducting wire depends on

1) Nature of the material.

2) Length of the wire

3) The cross sectional area of the wire.

7. Resistance and resistivity of a material vary with temperature.

8. Alloys do not oxidise at high temperature and their resistivity is higher than that of the pure metals.

9.Resistances in series –

1) Equivalent resistance is the sum of the resistances.

Rs =R1+R2+R3

2) Total potential difference is the sum of the potential difference across the

Individual resistors.

V= V1+V2+V3

3) Same current passes through them.

10.Resistances in parallel-

1) Equivalent resistance = +

2) Potential difference across the ends of the resistors is the same.

3) Total current is the sum of the currents across the individual resistors.

I= I1+I2+I3

11. Joules law of heating- The heat produced in a resistor is 1) directly proportional to the resistance for a given current. 2) Directly proportional to the square of current

for a given resistance. 3) Directly proportional to the time for which current flows through the resistor.

H= I2Rt

12.Power – is the rate at which electric energy is dissipated or consumed.

P= VI = =I2 R

ASSIGNMENT

1. Define electric potential difference?

2. Why is an ammeter connected in series in a circuit?

3. Why is a voltmeter connected in parallel in a circuit?

4. State the law that governs the strength of the current passing through a metallic conductor when a p.d is applied across its end. Illustrate this law graphically?

5. State the law which governs the amount of heat produced in a metallic conductor when current is passed through it for a given time. Express this law mathematically?

6. Define resistance. What are the factors on which it depends?

7. A copper wire of resitivity P is stretched to reduce its diameter to half its previous value. What is the new resistivity?

8. Define the S I unit of electric current and potential difference.

9. Derive expressions for equivalent resistance of resistors connected in

a. series combination

b. parallel combination

10. What is an electric fuse? Explain its function.

11. Explain, how the heating effect of electric current is made use in the functioning of an electric bulb?

12. State the relation between the commercial and SI unit of electric energy.

13. Write the advantages of connecting electrical appliances in parallel and disadvantages of connecting them in series in a household circuit.

14. Why is tungsten used almost exclusively for making the filaments of electric lamps?

15. A piece of wire is redrawn by pulling it until its length is doubled .Compare the new resistance with the original value.

16. An electric geyser has rating 2000 w, 220 v marked on it. What should be the minimum rating in whole number of a fuse wire that may be required for safe use with the geyser?

17. Three resistors each of resistance 10 ohm are connected, in turn, to obtain a. minimum resistance b. Maximum resistance. Compute

a) The effective resistance in each case

b) The ratio of minimum to maximum resistance so obtained.

18. Two resistors of resistance 2 ohms and 4 ohms are, in turn connected

a) In series

b) In parallel to a given battery for same interval.

Compute the ratio of the total quantity of heat produced in the combination in the two cases.

19. Two metallic wires A and B are connected in parallel. Wire A has length L and radius R wire B has a length 2L and radius 2r. Compute the ratio of the total resistance of parallel combination and resistance of wire A.

20. In a house three bulb of 100w each are lighted for 4 hours daily and six tube lights of 20w each are lighted for 5 hours daily and a refrigerator of 400w is worked for 10 hours daily for a month of 30 days. Calculate the electricity bill if the cost of one unit is Rs 4.00.

21. Three resistors of 4ohms, 6ohms and 12 ohms are connected in parallel. The combination of above resistors is connected in series to a resistance of 2 ohms and then to a battery of 6 volts. Draw a circuit diagram and calculate

a) Current in main circuit

b) Current flowing through each of the resistors in parallel

c) P.d and the power used by the 2 ohm resistor.

22. Two lamps, one rated 100 W at 220 V and the other 60W at 220 V, are connected in Parallel to a 220 volt supply. What current is drawn from the supply line?

# Electricity Class X Science and tech notes

**30**
*Sunday*
Aug 2009