 # Electicity | Important Questions

## Numericals

1. What is the commercial unit of electrical energy? Represent it in terms of joules.
2. A current of 5 A is flowing through a resistor of 15 Ω. Calculate the potential difference between the ends of the resistor. [Ans: 75 V ]
3. A piece of wire of resistance 20 Ω is drawn out so that its length is increased to twice its original length. Calculate the resistance of the wire in the new situation. [Ans: 80 Ω]
4. Resistance of a metal wire of length 1 m is 26 Ω at 20°C. If the diameter of the wire is 0.3 mm, what will be the resistivity of the metal at that temperature? [Ans: 1.84 × 10–6 Ω m.]
5. A 4 Ω resistance wire is doubled on it. Calculate the new resistance of the wire.[Ans: 1 Ω]
6. A current of 1 ampere flows in a series circuit having an electric lamp and a conductor of 5 Ω when connected to a 10 V battery. Calculate the resistance of the electric lamp.
Now if a resistance of 10 Ω is connected in parallel with this series combination, what change (if any) in current flowing through 5 Ω conductor and potential difference across the lamp will take place? Give reason. [Ans: 5 Ω, no change]
7. Two resistors, with resistances 5 Ω and 10 Ω respectively are to be connected to a battery of emf 6V so as to obtain: (i) minimum current (ii) maximum current
1. How will you connect the resistances in each case? [Ans: (i) series (ii) parallel ]
2. Calculate the strength of the total current in the circuit in the two cases.[Ans: (i) 0.4 A (ii) 1.8 A]
8. Two resistances when connected in parallel give resultant value of 2 ohm, when connected in series the value becomes 9 ohm. Calculate the value of each resistance. [Ans 3 Ω and 6 Ω]
9. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R′ then find the ratio R/R′.
10. A wire of resistance 5 ohms is bent in the form of a closed circle. What is the effective resistance between the two points at ends of any diameter of the circle?
11. An electric iron has a rating of 750 W, 220 V. Calculate (i) the current passing through it, and (ii) its resistance, when in use.
12. A geyser is rated 1500 W, 250 V. It is connected to 250 V mains. Calculate (i) the current drawn, (ii) the energy consumed in 50 hours, and (iii) the cost of energy consumed at ₹ 2.20 per kWh.
13. Two resistors of 4 Ω and 6 Ω are connected in parallel. The combination is connected across a 6 volt battery of negligible resistance. Calculate (i) the power supplied by the battery, (ii) the power dissipated in each resistor.
14. An electric bulb is rated 220 V and 100 W. Calculate the power consumed when it is operated on 110 V.
15. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then in parallel in a circuit across the same potential difference. Find the ratio of heat produced in series and parallel combination.
16. Which has a higher resistance: a 50 W – 220 V lamp or a 25 W – 220 V lamp? Calculate the ratio of their resistances.
17. An electric iron consumes energy at a rate of 840 W when heating it at the maximum rate and 360 W when the heating it at the minimum. The applied voltage is 220 V. What is the value of current and the resistance in each case?
18. A heater coil is rated 100 W, 200 V. It is cut into two identical parts. Both parts are connected together in parallel to the same source of 200 V. Calculate the energy liberated per second in the new combination.
19. Two lamps, one rated at 40 W – 220 V and the other at 60 W – 220 V, are connected in parallel to the electric supply at 220 V.
1. Draw a circuit diagram to show the connections.
2. Calculate the current drawn from the electric supply source.
3. Calculate the total energy consumed by the two lamps together when they operate for one hour.
20. A wire carries a current of 0.1 A and has a resistance of 120 milliohm per metre. What is the p.d. across 1 m of this wire?
21. An electric iron draws a current of 4 A from 220 V supply line. When voltage of supply is changed, then iron draws a current of 2 A, what is the new voltage applied?
22. A copper wire has a diameter 0.5 mm and resistivity 1.6 × 10–8 Ω m.
1. What will be the length of this wire to make the resistance of 12 Ω?
2. How much will be the resistance of another copper wire of same length but half the diameter?
23. How can three resistances of 2, 3 and 6 ohms be connected so as to give a total resistance of 1 ohm? Justify your answer.
24. Three resistances of 5, 10 and 15 ohms are connected in series and a potential difference of 15 volts is applied across the combination. Calculate the current flowing through the circuit.
25. A voltmeter reads 24 V across a resistor and an ammeter reads a current of 3 A through it. Calculate R. What will be the current through the resistor if the voltage changes to 12 V?
26. A wire of uniform cross-section and length l has a resistance of 4 Ω. The wire is cut into four equal pieces. Each piece is then stretched to length ‘l’. Thereafter, the four wires are joined in parallel. Calculate the net resistance.
27. A cable of resistance 12 Ω carries electric power from a generator producing 250 kW at 10,000V. Calculate (i) the current in the cable, (ii) the power lost in the cable during transmission, and (iii) the p.d. across the ends of the cable.
28. Calculate the electrical energy produced in 5 minutes when a current of 2 A is sent through a conductor by a potential difference of 500 volts.
29. Two bulbs have the following ratings: (i) 40 W at 220 V, and (ii) 40 W at 110 V. The filament of which bulb has a higher resistance? What is the ratio of their resistances?
30. A potential difference of 40V is applied across a resistance of 8 Ω for a period of 2 minutes.
1. What current is drawn?
2. Calculate the charge which passes through the resistor.
31. An electric iron has a rating of 750 W, 220 V. Calculate
1. the current passing through it.
2. its resistance, when in use.
32. An electric heater draws a current of 10 A from a 220 V supply. What is the cost of using the heater 5 hours per day for 30 days if the cost of 1 unit is ₹ 2.50?
33. A torch bulb is rated 3V and 600 mA. Calculate its resistance if it is lighted for 4 hours.
34. In a household circuit, an electric bulb of 100 W is used for 10 hours and an electric heater of 1000 W for 2 hours everyday. Calculate the cost of using the bulb and the heater of 30 days if the cost of one unit of electrical energy is two rupees.
35. An electric bulb draws a current of 0.2 A when it operates at 220 V. Calculate the amount of electric charge flowing through it in 1 h.