ELECTRICITY – 3 Mark Questions

Q1.  Draw a circuit diagram of an electric circuit containing a cell, a key, an ammeter, a resistor of 2 Ω in series with a combination of two resistor (4 Ω each) in parallel and voltmeter across the parallel combination. Will the potential difference across the 2 Ω resistor be the same as that across the parallel combination of 4 Ω resistors? Give reasons.  

Q2. You have two electric lamps having rating 40 W, 220 V and 60 W, 220 V. Which of the two has a higher resistance? Give reason for your answer. If these two lamps are connected to a source of 220 V, which will glow brighter?

Q3. An electrical bulb is rated 40 W, 220 V. How many bulbs can be connected in parallel with each other across the two wires of 220 V line, if the maximum allowable current is 6 A?        

Q4. An electric heater is rated at   2 kW. Calculate the cost of using it for 2 h daily for the month of September, if each unit costs Rs. 4.         

Q5. (a) List the factors on which the resistance of a conductor in the shape of a wire depends.

         (b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.

         (c) Why are alloys commonly used in electrical heating devices? Give reason.      

Q6. (a) Nichrome wire of length L and radius ‘R’ has resistance of 10Ω. How would the resistance of the wire change when:

(i)   Only length of the wire is doubled?

(ii)  Only diameter of the wire is doubled? Justify your answer.

(b) Why element of electrical heating devices is made up of alloys?

Q7. Three resistors of 5 Ω, 10 Ω and 15 Ω are connected in series and the combination is connected to the battery of 30 V. Ammeter and voltmeter are connected in the circuit.  Draw a circuit diagram to connect all the devices in proper correct order.  What is the current flowing and potential difference across 10 Ω resistance?

Q8. Draw schematic diagram of a circuit consisting of a battery of five 2V cells, a 5 ohm, a 10 ohm and a 15-ohm resistor and a plug key, all connected in series.  Calculate the electric current passing through the above circuit when the key is closed.

Q9. (i) Define electric power.  A device of resistance R is connected across a source of V voltage and draws a current I. Derive an expression for power in terms of voltage and resistance.

         (ii) An electric bulb is connected to a 220 V generator. The current is 0.5 A. What is the power of the blub?

Q10. (i) State one difference between Kilowatt and Kilowatt hour. Express 1kWh in joules.

           (ii) A bulb is rated 5V, 500 mA. Calculate the rated power and resistance of the bulb when it glows.

Q11.What is an electric fuse? Why it is used in electric circuits? Should it be placed on neutral, wire or on live wire? Justify your answer.

Q12.Define electric power. An electric motor is rated at 2kW. Calculate the cost of using it for 2 hours daily for the month of September if each unit costs 6.00.

Q13. Give reason for the following:

(i) Why are copper and aluminium wires used connecting wires?

(ii) Why is tungsten used for filament of electric lamps?

(iii) Why is lead-tin alloy used for fuse wires?

Q14. Electrical resistivities of some substances, in ohm­meter, at 20°C are given as follows:

(i) Out of the silver and copper, which is a  better conductor of electric current and why?

(ii) Which substance is preferred to be used for electrical transmission lines? Give reason.

(iii) Name the material that you would advise to be used in the heater elements of electric heating device and why?   

Q15. Explain the following:

(i) The elements of electric heating devices such as bread-toasters and electric iron are made of an alloy rather than of a pure metal.

(ii) Series arrangement is not used for domestic circuits.

(iii) Copper and aluminium wires are usually employed for electricity transmission.

Q16.Three resistors of 10 Ω,15 Ω and 20 Ω are connected in series in a circuit. If the potential drop across the 15 Ω resistor is 3 V, find the current in the circuit and potential drop across the 10 Ω resistor.

Q17.  A hot plate connected to a 220 V line as two resistance coils A and B, each of 22 Ω resistance. Calculate the amount of electric current flowing when these coils are:

(i) used individually.

(ii) connected in series.

(iii) connected in parallel.

Q18. A lamp rated 60W and an electric iron rated 800W are used for 6 hours everyday.  Calculate the total energy consumed in 30 days.

Q19. Five resistors are connected in a circuit as shown. Find the ammeter reading when circuit is closed.


Q20. A 400 W refrigerator operates for 16 h per day. Calculate the cost to operate it for 30 days at 3.40 kWh.

Q21. A heater coil is rated 100 W, 200 V. It is cut in two identical parts and both parts are connected together in parallel to the same source of 200 V. Calculate the energy liberated per second in the new combination.

Q22.  If a 12 V battery is connected to the arrangement of resistances as shown below:



(i) the total effective resistance of the arrangement,

(ii) the total current flowing in the circuit.

Q23. (i) A current of 0.5A is drawn by a filament of electric bulb for 10 min. Find the amount of electric charge that flows through the circuit.

           (ii) Should the resistance of an ammeter be low or high? Give reason.

Q24.   Aluminium wire has radius 0.25 mm and length of 75 m. If the resistance of the wire is l0Ω. Calculate the resistivity of aluminium.         

Q24.       Find the current drawn from the battery by network of four resistors shown in the figure.




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