CBSE Class 10 Physics Electricity MCQs Set I

Question. What is the maximum resistance which can be made using five resistors each of 1/5 Ω?
(a) 1/5 Ω
(b) 10 Ω
(c) 5 Ω
(d) 1 Ω 

Answer: d

Question. Two resistors of resistance 2 Ω and 4 Ω when connected to a battery will have
(a) Same current flowing through them when connected in parallel
(b) Same current flowing through them when connected in series
(c) Same potential difference across them when connected in series
(d) Different potential difference across them when connected in parallel

Answer: b

Question. How many electrons constitute a current of 1 ampere?
(a) 6*10¹⁵
(b) 6*10¹⁸
(c) 1.6*10^-19 
(d) 6*10²⁰

Answer: b

Question. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be −
(a) 100 W
(b) 75 W
(c) 50 W
(d) 25 W 

Answer: d

Question. 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 the ratio R/R' is −
(a) 1/25 
(b) 1/ 5
(c) 5
(d) 25 

Answer: d

Question. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be −
(a) 1:2
(b) 2:1
(c) 1:4
(d) 4:1 

Answer: c

Question. The resistivity does not change if
(a) The material is changed
(b) The temperature is changed
(c) The shape of the resistor is changed
(d) Both material and temperature are changed 

Answer: c

Question. Which of the following terms does not represent electrical power in a circuit?
(a) I²R
(b) IR²
(c) VI
(d) V²/ R 

Answer: b

Question. Electric current is measured by
(a) A voltmeter
(b) An ammeter
(c) A rheostat
(d) A potentiometer

Answer: b

Question. Assertion (A): A cell converts chemical energy into electrical energy.
Reason(R): A cell maintains a potential difference across its terminals due to chemical reactions. 
(a) Both A and R are true and R is the correct explanation for A
(b) Both A and R are true and R is not the correct explanation for A
(c) A is true but R is false
(d) A is false but R is true.

Answer: b

Question. Assertion (A): If a graph is plotted between potential difference and current a linear graph is obtained.
Reason(R): current is directly proportional to the potential difference.
(a) Both A and R are true and R is the correct explanation for A
(b) Both A and R are true and R is not the correct explanation for A
(c) A is true but R is false
(d) A is false but R is true.

Answer: a

Very Short Answer Type Questions :

Question. Calculate the amount of charge flowing in a wire if it draws a current of 2A in 10 minutes.
Answer: 2 x 10 x 60
= 1200 C

Question. Which device helps to maintain a potential difference across a conductor?
Answer: Cell or battery

Question. Why are heating elements made of alloys rather than metals?
Answer: High resistivity, does not oxidise at high temperatures

Question. Mention one reason why tungsten is used for making filament of electric lamp.
Answer:  Tungsten is used for making filament because of its high melting point and low resistivity.

Question. Why is tungsten metal selected for making filaments of incandescent lamps?
Answer:  Tungsten has high resistance and high melting point.

Question. What happens to resistance of a conductor if area of cross-section is doubled?
Answer: It halves

Question. a)What is the function of fuse in an electric circuit?
b) What would be the rating of the fuse for an electric kettle which is operated at 220V and consumes 500 W power?
c) How is the SI unit of electric energy related to its commercial unit? 
Answer: b) 2.2A flows through the circuit, fuse should be rated 3A.
c) 1 KWh = 3.6 X 106 J

Question. What do we mean when we say that potential difference between two points is 1volt?
Answer: Definition

Question. What are the advantages of connecting electrical devices in parallel with the battery rather than in series?
Answer: (1) The current required by each device is different which is possible only in parallel.
(2)Potential difference is constant for all devices.
(3) Total resistance in the circuit is decreased

Question. If three resistors of 6Ω, 9Ω and 21Ω are connected in series to a 12V battery, find
a) The total resistance of the circuit.
b) The current flowing through the circuit.
c) The potential difference across the 21 Ω resistor.
Answer: a) 36 Ω
b) 0.33A
c) 6.93V

Question. a) State Ohms law. Give the graphical relation between V & I.
b) An electric oven rated at 500W is connected to a 220V line and used for 2 hours daily. Calculate the cost of electric energy per month at the rate of Rs.5 per KWh.
Answer: a) Ohms law state that current flowing in aconductoris directly proportional tothe applied potentialprovided that temperature and physical conditions remainssame.
b) Energy consumed per day = 1 KWh (P x t) cost for 30 days = 1 X 5 X 30 = Rs.150.00

Question. A wire of resistance 5 W is bent in the form of a closed circle. What is the resistance between two points at the ends of any diameter of the circle?
Answer:  R = 5 W
It can be assumed as two semi circular elements are connected across diameter AB.
Total resistances of couple wire 5 W . Resistance of two semicircular elements = 5/2 Ω 
Both the elements are connected in parallel. 
1/R , = /R1 + 1/R2 + = 2/5 + 2/5 = 4/5
or R' = 5/4 Ω

Question is much less, heat generated in long electric cables than in filaments of electric bulbs?
Answer:  As heat produced = I2Rt
Electric cables are made of thick good conductor where as filaments are made of thin tungsten wire whose resistances is high. The resistance of electric cables is very less. So heat produced in cable is much lesser than filaments.

Question. Define resistivity of a material.
Answer:  We have R =P,l/A
Resistivity, P = R , A/l
If
A = 1, l = 1 then r = R i.e. resistivity of a material is the resistance of a conductor of this material whose length and area of cross- section both are unity.
or
Resistivity of a material is the resistance of a conductor of this material whose volume is unity.

Question. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Answer: The resistivity of an alloy is higher than the pure metal. Moreover, at high temperatures, the alloys do not melt readily. Hence, the coils of heating appliances such as electric toasters and electric irons are made of an alloy rather than a pure metal. 

Question. What does an electric circuit mean?
Answer: An electric circuit consists of electric devices, switching devices, source of electricity, etc. that are connected by conducting wires.

Question. A cylinder of a material is 10 cm long and has a crosssection of 2 cm². If its resistance along the length be 20 ohm, what will be its resistivity in number and units?
Answer:  l = 10 cm, A = 2 cm²
R = 20 ohm
R = p,L/A
or P = R,A/l = 20 x 2/10 Ω cm
r = 4 W - cm

Question. (a) Name the instrument/device used to measure electric current in a circuit.
(b) How is an ammeter connected in a circuit to measure current flowing through it?
Answer: 
a. Ammeter is used to measure electric current.
b. Ammeter is connected in series in an electric circuit.

Question. In an electric circuit, state the relationship between the direction of conventional current and the direction of flow of electrons.
Answer:  Electrons flows from negative terminal to positive terminal where as current flows from +ve terminal to -ve terminal in external circuit i.e. Conventional current and electrons flow are opposite to each other.

Question. Calculate the number of electrons that would flow per second through the cross- section of a wire when 1 A current flows in it.
Answer:  Given: I = 1A, t = 1 s
I = Q/t ⇒ Q = I x t
Q = 1A#1 s (Q = 1 coulomb)
But Q = ne
n = Q/e = 1C/1.6 X 10^-19
= 6.25 X 10¹⁸ electrons

Question. (a) What material is used in making the filament of an electric bulb?
(b) Name the characteristics which make it suitable for this.
Answer:
a. Tungsten is used in making filament.
b. Its high resistivity and high melting point. 

Question. Tungsten is used almost exclusively for filaments of electric bulb. List two reasons.
Answer:  (1) high resistivity (2) high melting point and does not oxidise at very high temperature.

Question. How does the resistivity of alloys compare with those of pure metals from which they may have been formed?
Answer: The resistivity of pure metals is lesser than resistivity of alloys with which these alloys are made.

Question. Write SI unit of resistivity.
Answer:  Ohm-m

Short Answer Type Questions :

Question.(a) How is the direction of electric current related to the direction of flow of electrons in a wire?
(b) Calculate the current in a circuit if 500 C of charge passes through it in 10 minutes.
Answer:
(a) Conventional direction of electric current is opposite to the direction of flow of electrons in a wire.
(b) q = 500 C, t = 10 X 60 = 600 s
I = 500/600 = 5/6A

Question. Resistivity of two elements A and B are = 1.62X10-8 Wm and 520#10-8 Wm respectively.
Out of these two, name the element that can be used to make:
a. filament of electric bulb.
b. wires for electrical transmission lines. Justify your answer in each case.
Answer:  Given: r A = 1.62X10-8 Ωm
r B = 520X10-8 Ωm
a. For filament of electric bulb resistivity must be higher i.e. 520X10-8 Ωm . So element B is used for filament.
b. For electric transmission resistivity must be lower which is of element A.

Question. Name the physical quantity which is (i) same (ii) different in all the bulbs when three bulbs of:
(a) same wattage are connected in series.
(b) same wattage are connected in parallel.
(c) different wattage are connected in series.
(d) different wattage are connected in parallel.
Answer: (a) For identical bulbs in series- same current, same potential difference.
(b) For identical bulbs in parallel- same potential difference, different current.
(c) For unidentical bulbs in series- same current, different potential difference.
(d) For unidentical bulbs in parallel- different current, same potential difference.

Question. (a) Define the term ‘volt’.
(b) State the relation between work, charge and potential difference for an electric circuit.
Calculate the potential difference between the two terminals of a battery if 100 J of work is required to transfer 20 C of charge from one terminal of the battery to the other.
Answer:
(a) When 1 joule of work is done in carrying 1 coulomb of charge, from infinity to a point in the electric field, then potential at that point is called 1 volt.
(b) Potential difference, V = Work done on unit charge =W/q Work is 100 J,q=20C
Potential difference,V=W/q=100/20=5V

Question. Out of the two wires X and Y shown below, which one has greater resistance? img
Answer: Wire ‘Y’ has greater resistance as it has more length than wire ‘X’. It is because resistance of wire is directly proportional to the length of wire.

Question. A nichrome wire has a resistance of 10 W Find the resistance of another nichrome wire, whose length is three times and area of cross-section four times the first wire.
Answer:  We have R = 10 Ω
l1 = l
A1 = A
R = P,l/A = 10 Ω
or l/A = 10/P
For new wire,
L2 = 3l
A2 = 4A
R2 = P,3l/4A = 3/4 P,l/pΩ
R2 = 3/4 R = 3/4 X 10 = 15/2 Ω

Question. Define electric current and state its SI unit. With the help of Ohm’s law explain the meaning of 1 Ohm resistance.
Answer:  Electric current may be defined as the rate of flow of charge through a circuit. Its SI unit is Ampere.
According to Ohm’s law V = IR
R = V/I
If V = 1 Volt, I = 1A then R = 1 ohm.
A conductor has a resistance of 1 Ohm if a current of one amp flows through it when a potential difference of 1 volt is applied across it.

Question. State Ohm’s law. Calculate the resistance of a conductor, if the current flowing through it is 0.2 A when the applied potential difference is 0.8 V.
Answer:  If the physical conditions of a conductor are kept same then current is directly proportional to the potential difference across the ends of the conductor V ? I .
V = RI
I = 0.2 A
V = 0.8 Volt.
R = ?
R = V/I = 0.8/0.2 Ω
R = 4 Ω

Question. (a) How is the direction of electric current related to the direction of flow of electrons in a wire?
(b) Calculate the current in a circuit, if 500 C of charge passes through it in 10 minutes.
Answer: a. The direction of flow of electrons is opposite to the direction of conventional current.
b. Q = 500 C, t = 10 mts = 10 X 60 = 600 s 
I = Q/t = 500/600 A
I = 0.83 A

Question. (a) Why are copper or aluminium wires generally used for electrical transmission and distribution purposes?
(b) Two wires, one of copper and other of manganin, have equal lengths and equal resistances. Which wire is thicker? Given that resistivity of copper is lower than that of manganin.
Answer:
a. Copper or aluminium wires are used for transmission and distribution of electricity due to their low resistivity and high conductivity.
b. We know that R = P,l/A
r ∝ A
Thicker the wire, more the resistivity. The resistivity of manganin is more than copper. So manganin wire is thicker than copper.

Question. (a) Define the term ‘volt’.
(b) State the relation between work, charge and potential difference for an electric circuit.
Calculate the potential difference between the two terminals of a battery, if 100 joules of work is required to transfer 20 coulombs of charge from one terminal of the battery to the other.
Answer: 
a. Potential difference b/w two points in an electric field is said to be 1 volt if the amount of work done in bringing a unit positive charge from one point to another point is 1 J.
b. Given: W = 100 J, Q = 20 C, V = ?
As V = W/Q ⇒ V = 100/20 JC-I
V = 5 JC-1
V = 5 Volt.

Question. Two conducting wires of same material, equal length and equal diameter are connected in series. How does the heat produced by the combination of resistance change?
Answer: Let the resistances of two wires are R each.
Heat produced by individual resistor 87

Question. Series arrangements are not used for domestic circuits. List any three reasons.
Answer: Series arrangements are not used for domestic circuit because
1. The electrical appliances need current of widely different values to operate properly.
2. In series arrangement, when one component fails, the circuit is broken and none of the components works.
3. All electrical appliances work at a constant voltage. But in series circuit, the current is constant throughout the electric circuit and potential is different across the different components. So, series arrangement is not suitable for domestic circuits.

Long Answer Type Questions :

Question. (a) Define potential difference between two points in a conductor.
(b) Name the instrument used to measure the potential difference in a circuit. How is it connected?
(c) A current of 2 A passes through a circuit for 1 minute. If potential difference between the terminals of the circuit is 3 V, what is the work done in transferring the charges?
Answer:
a. Electric potential is the amount of work done in bringing a unit positive charge from one point to another.
b. Voltmeter. It is connected in parallel in the circuit.
c. I = 2A, t = 1 min. = 60 s, V = 3 V
W = VQ = V^Ith
W = 3X2X60 J
W = 360 J

Question. (a) Why is the series arrangement not used for domestic circuits?
(b) Why is the tungsten used almost exclusively for filament of electric lamps?
(c) Why are the conductors of electric heating devices such as bread toasters and electric irons made of an alloy rather than a pure metal?
(d) Why are copper and aluminium wires usually employed for electricity transmission?
(e) Why does the cord of an electric heater not glow while the heating element does?
Answer: 
a. In the series arrangement if any device fails to work, circuit will be broken and other devices stop working. In series combination current remains same. Where as different devices requires different current.
b. Tungsten is used due to its high resistivity and high melting point.
c. Alloys have higher resistivity than their pure constituent and hence produces more heat.
d. Copper and aluminium are good conductors and have low values of resistivity so these are used for electric transmission.
e. Cords core made up of good conductor with low resistivity where as heating elements are made up of alloys of higher resistivity to produce more heat.

Question. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
Answer: Energy consumed by an electrical appliance is given by the expression,
H = Pt
Where,
Power of the appliance = P
Time = t
Energy consumed by a TV set of power 250 W in 1 h = 250 × 3600 = 9 × 10⁵ J
Energy consumed by a toaster of power 1200 W in 10 minutes = 1200 × 600 = 7.2× 10⁵ J
Therefore, the energy consumed by a 250 W TV set in 1 h is more than the energy consumed by a toaster of power 1200 W in 10 minutes.

Question. An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.
Answer: Rate of heat produced by a device is given by the expression for power as
P = I²R
Where,
Resistance of the electric heater, R = 8 Ω
Current drawn, I = 15 A
P = (15 )² X 8 = 1800 J / s
Therefore, heat is produced by the heater at the rate of 1800 J/s.

Question. A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of electric charge that flows through the circuit.
Answer:  Given: I = 0.5 A
t = 10X60 s
I = Q/t or Q = It
Q = 0.5X10x60 C
Q = 300 C

Question. An electric iron has a rating of 750 W; 200 V. Calculate:
a. the current required.
b. the resistance of its heating element,
c. energy consumed by the iron in 2 hours.
Answer: Rating of iron is 750 W–200V.
P = 750W, V = 200 Volt.
a. P = VI or I = P/V
I = 750/200 = 3.75 A
b. Resistance = V/I = 200/3.75 = 53.3 Volt.
c. Energy consumed in 2 hr = Pxt
E = 750X2Wh = 1500Wh

Question. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 ohm?
(b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 ohm?
Answer: (a) Given:V = 220 V
R = 1200 ohm
As V = IR
220 = Ix1200 or I = 220/1200A
I = 0.18 A
(b) V = 220 V
R = 100 ohm
I = V/R = 220/100 A
I = 2.2 A

Question. Define power. State the difference between 1 watt and 1 watt hour. Establish the relationship between unit of electric energy and SI unit of energy. An electric heater rated 1000 W/220 V operates 2 hours daily.
Calculate the cost of energy to operate for 30 days at the rate of Rs 5.00 per kWh.
Answer:  Electric Power: P is defined as the rate at which electric energy is consumed in an electric circuit.
1 watt is the power consumed by an electric appliance that carries 1A current when a potential difference of 1 volt is applied across it. Whereas 1 Wh. is the unit of electric energy, which is consumed by an electric appliance of 1 W power in one hour.
Commercial unit of electric energy is kWh.
1 kWh = 1000Wx60x60 s
1 kWh = 3.6x106 J
Energy consumed by heater in one month
= 1000Wx2hx30 days = 60 kWh
= 60 units
Total cost @₹5/-kWh = 5x60 = ₹300

Question. Compare the power used in the 2 Ω resistor in each of the following circuits:
(i) a 6 V battery in series with 1 G and 2 Ω resistors, and
(ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.
Answer: (i) Potential difference, V = 6 V
1 Ω and 2 Ω resistors are connected in series. Therefore, equivalent resistance of the circuit, R = 1 + 2 = 3 Ω
According to Ohm’s law,
V = IR
Where,
I is the current through the circuit
I = 6 / 3 = 2A
This current will flow through each component of the circuit because there is no division of current in series circuits. Hence, current flowing through the 2 Ω resistor is . Power is given by the expression,
P = (I)² R = (2)² X 2 = 8 W
(ii) Potential difference, V = 4 V
12 Ω and 2 Ω resistors are connected in parallel. The voltage across each component of a parallel circuit remains the same. Hence, the voltage across 2 Ω resistor will be 4 V. 
Power consumed by 2 Ω resistor is given by 
P = V² / R = 4² / 2 = 8W
Therefore, the power used by 2 Ω resistor is 8 W.

Question. When two resistors of resistances R1 and R2 are connected in parallel, the net resistance is 3 W . When connected in series, its value is 16 W . Calculate the values of R1 and R2.
Answer:  R1 and R2 are in parallel combination.
1/RP = 1/R1 + 1/R2
R1 X R2 /R1 + R2 = 3
When R1 and R2 are in series combination.
Rs = R1+ R2 = 16 W
R1+ R2 = 16 ...(2)
Solving eq. (1) and eq. (2)
R12 - 16R2 + 48 = 0
(R1- 4)(R1- 12) = 0
R1 = 4 Ω, 12 W
R2 = 12 Ω or 4 Ω