ELECTRICITY– 2020 Important Questions

1 MARK QUESTIONS

Q1. Why closed path is required for the flow of current?

Q2. What causes the potential difference between the two terminals of a cell?

Q3.  A wire of resistivity is pulled to double its length. What will be its new resistivity?

Q4. In how much time will a bulb of 100 W consume an energy of 2 kWh?

Q5. How does the resistance of an ohmic conductor depend on the applied voltage?

Q6. What happens to the other bulbs in a series circuit, if one bulb blows off?

Q7. What is meant by potential difference between two points?

Q8. Name the device that helps to maintain a potential difference across a conductor.

Q9. State the relationship between 1 ampere and 1 coulomb.

Q10. A charge of 150 coulomb flows through a wire in one minute. Find the electric current flowing through it.

Q11. A given length of a wire is doubled and this process is repeated once again. By what factor does the resistance of the wire changes?

Q12. The resistance of a resistor is kept constant and the potential difference across its two ends is decreased to half of its former value.  State the change that will occur in the current through it.

Q13. Calculate the number of electrons constituting one coulomb of charge. (Charge on 1 electron = 1·6 × 10-19 C)  

Q14. Write SI unit of resistivity.

Q15. Name the instrument used for measuring

(i) Potential difference

(ii) Current 

Q16. Which is having more resistance – a 220 V, 100 W bulb or a 220 V, 60 W bulb.

Q17. Nichrome is used to make the element of electric heater. Why?

Q18.  Electric fuse is an important component of all domestic circuits. Why? 

Q19.   You are given three bulbs of 40 W, 60 Wand 100 W. Which of them has lower resistance?

Direction (Q. No. 20- 24) In each of the following questions, a statement of Assertion is given by the corresponding statement of Reason. Of the statements, mark the correct answer as

(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

(c) If Assertion is true, but Reason is false.

(d) If Assertion is false, but Reason is true.

(e) If Assertion and Reason both are false.

Q20. Assertion: The connecting wires are made of copper.

            Reason: The electrical conductivity of copper is high.

Q21. Assertion: Resistors connected in series are more effective than those connected in parallel.

            Reason: Resistance of resistors increase when connected in series.

Q22. Assertion: Domestic circuits are connected in Parallel.

            Reason: Parallel circuits have same current in every part of the circuit.

Q23. Assertion: Two wires of same length and material have different areas of the cross-section.

            Reason: The wire with the lower area of cross-section has the lower resistivity.

Q24. Assertion: Nichrome is used in making of heater rods.

            Reason: Nichrome has high resistivity.

Q25. How much work is done in moving a charge of 2 C from a point of 118V to a point of 128V?

(a)20 J   

(b)30 J   

(c)40 J   

(d)   10 J

Q26.  Which of the following terms does not represent electric power in a circuit?

(a) I2R   

(b) IR2   

(c) VI    

(d) V2 /R

Q26.  What is the minimum resistance which can be made using five resistors each of 1/5Q?

(a) 1/5 Q              

(b) 1/25 Q            

(c) 1/10 Q            

(d) 25 Q

Q27.  When a conducting wire is stretched to double of its initial length, then which property of conducting wire increases?

(a) Resistivity     

(b) Resistance     

(c) Volume         

(d) Density

3 MARKS QUESTION

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.

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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:

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Calculate

(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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5 MARKS ANSWER

Q1.         (i)  Distinguish between the terms electrical resistance and resistivity of conductor.

(ii)   A    copper wire of resistivity 1.63 × 10-8 Ω-m has cross-section area of 10.3× 10-4 cm2. Calculate the, length of the wire required to make a 20Ω coil.

Q2. Two resistances when connected in parallel give resultant value of 2Ω, when connected in series the value becomes 9 Ω. Calculate the value of each resistance.

Q3.         (i)  Find the value of current I in the circuit given as below:

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           (ii)  You have four resistors of 8 Ω each. Show how would you connect these resistors to have effective resistance of 8 Ω?

Q4.  Three incandescent bulbs of 100 W each are connected in series in an electric circuit. In another set of three bulbs of the same wattage are, connected in parallel to the source.     

(i)  Will the bulb in the two circuits glow with the -same brightness? Justify your answer.

(ii)   Now, let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit?

Give reason.

Q5. A household uses the following electric appliances;

(i)  Refrigerator of rating 400 W for 10 h each day.

(ii)   Two electric fans of rating 80 W each for 6 h daily.           .              .

(iii)   Six electric tubes of rating 18 W each for 6 h daily.

Calculate the electricity bill, for the household   for month   of June, if cost   of electrical energy is Rs.3 per unit

Q6. How will you conclude that the same potential difference (voltage) exists across three resistors connected in a parallel arrangement to a battery?

Q7.  With the help of a circuit diagram prove that when a number of resistors are connected in parallel, the reciprocal of equivalent resistance of the combination is equal to the sum of the reciprocals of the individual resistances of the resistors.

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Find the resistance between  A  and   B  in the following network.

Q8. (a) Two identical resistors each of resistance 10 ohm are connected in:

(i) Series, (ii) Parallel

in turn to a battery of 6V. Calculate the ratio of Power consumed by the combination of resistors In the two cases.

        (b) List two factors on which the resistance of a conductor depends.

       (c) Write a   difference between an ammeter and voltmeter.

Q9. (a)  Two students perform experiments on given resistors R1 and R2 and plot the following V-I graphs. If R1 > R2, which of the two diagrams correctly represent the situation on the plotted curves? Justify your answer.

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        (b) An electric lamp of 24 Ω and a conductor of 6 Ω are connected in parallel to a 12 V battery. Calculate:

(i)  Total resistance,

(ii)  Total current in the circuit,

(iii) Potential difference across the conductor.

Q10. Draw a circuit diagram for a circuit consisting of a battery of five cells of 2 volts each, a 5 Ω resistor, a 10 Ω resistor and  a 15 Ω resistor,  an ammeter and a plug  key; all connected in series. Also connect a voltmeter to record the potential difference across the 15 resistor and calculate:

(i) The electric current passing through the above circuit and

(ii) Potential difference across 50 resistors when the key is closed.

Q11. What is meant by electric current? Name and define S.I. unit in a conductor electron are flowing from B to A. What is the direction of conventional current? Give justification for your answer.

A steady current of 1 Ampere flows through a conductor. Calculate the number of electrons that flow through any section of conductor in 1 second. (Charge on electron = 1.6 × 10-19 C)

Q12. (i) A wire of resistivity p is stretched to double its length which is its new resistivity. Give reason for your answer.

           (ii) Draw a schematic diagram of a circuit consisting of a battery of three cells of 2V each, a 50 Ω resistor, 80 Ω resistor and 120 Ω resistor and a plug key all connected in series.

           (iii) Two wires, one of copper and other of manganese have equal lengths and equal resistances. Which is thicker? (Given that resistivity of manganese is lower than that of copper.)

Q13. (i)  What is meant by the statement that the resistance of a wire is 1 Q?

           (ii) Two identical resistors each of resistance 12 Q are connected

(a) in series 

(b) in parallel, in tum to a battery of 6 V.

Calculate the ratio of power consumed in the combination of resistors in the two cases.

           (iii)   What combination is used for connecting the circuit to measure the potential difference across two points?

Q14. Two conductors A and B of resistances 5Ω and 10 Ω respectively are first joined in parallel and then in series. In each case the voltage applied is 20 V.

(i) Draw the circuit diagram to show the combination of these conductors in each case.

(ii) In which combination will the voltage across the conductors A and 8 be the same?

(iii) In which arrangement will the current through A and B be the same?

(iv) Calculate the equivalent resistance for each arrangement.

Q15. (i) The potential difference between two points in an electric circuit is 1V. What does it mean? Name a device that helps to measure the potential difference across a conductor.

           (ii) Why does the connecting cord of an electric heater does not glow while the heating element does?

           (iii) Electrical resistivities. of some substances at 20°C  are as given below:

Silver                    1. 60 ×10-8 Q-m

Copper                1.62 ×10-8 Q-m

Tungsten              5.2 × 10-8 Q-m

Iron                       10.0 ×10-8 Q-m

Mercury               94.0 ×10-8 Q-m

Nichrome              100 ×10-8 Q-m

Answer the following questions using following data.

(a) Among silver and copper, which one is a better conductor and why?

(b) Which material would you advise to be used in electrical heating devices and why?

Q16. Three incandescent bulbs of 100 W each are connected in series in an electric circuit. In another set, three bulbs of the same voltage are connected in parallel to the same source.

(i) Will the bulbs in the two circuits glow with the same brightness? Justify your answer.

(ii) Now let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit? Give reasons.

Q17.   A wire is cut into three equal parts and then connected in parallel with the same source. How will its

(i)  resistance and resistivity gets affected?

(ii)  How would the total current and the current through the parts change?

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