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Quiz 8
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Q.1
If copper wire is stretched to make it 0.1 % longer, the the percentage change in resistance is approximately
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a)0.1%
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b) 0.2%
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c)0.4%
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d)0.8%
Explanation
When wire is stretched it length increases and area decreases. New area can be found using formula Af = AiLi/Lf Lf= Li (1.001) Thus Af =Ai/1.001 Now form formula for resistance Increase in resistance ΔR= Rf - Ri ΔR =R(1.002) -R= (0.002)R % Increase = (ΔR/R) × 100 = 0.002 × 100 = 0.2 % Answer: (b)
Q.2
In the circuit shown below, the key is pressed at time t =Which of the following statement(s) is(are) true?
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a) The voltmeter displays - 5 V as soon as the key is pressed, and displays + 5 V after a long time
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b) The voltmeter will display 0 V at time t = ln 2 seconds
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c) The current in the ammeter becomes 1/e of the initial value after 1 second
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d) The current in the ammeter becomes zero after a long time
Explanation
40µF and 20µF capacitor will conduct and offer no resistance. Thus Potential Q will be +5 Volts and at P= 0V. Thus voltmeter will display VP – VQ = 0-5V = -5V. After long time current will not conduct through both capacitor and are fully charged, Voltage at Q = 0 and Voltage at P = 5V Since voltmeter is ideal thus have infinite resistance. No current will flow through ammeter option d is correct Option a correct . Charging of capacitor is given by Voltage across capacitor after time t τ = RC called as RC time constant For 40µF capacitor τ = 40×10-6×25×103 = 1s For 20µF capacitor τ = 20×10-6×50×103 = 1s Voltage across 40µF capacitor after time t=ln2 Thus voltage at point Q = 5- 2.5 = 2.5V And voltage across 20µF capacitor after time t=ln2 V2 = 2.5 V VP – 2.5V = 0 thus VP =2.5V As VP = VQ voltmeter reading is zero (Option b correct) Option c Total current I1 + I2 after 1s Initial value of current at t=0 , I1 = 0.2×103 + 0.1×103 = 0.3×103 A Thus current after 1 sec = 1/e of initial current Option c is correct Answer:(a, b, c, d)
Q.3
When a potential difference V is applied across a conductor at temperature T, the drift velocity of electrons is proportional to
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a) √T
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b) V0
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c)√V
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d)V
Explanation
Answer:(d)
Q.4
You have three equal resistance's. How many different combinations can you have with these resistance's
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a) 2
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b) 3
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c) 4
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d) 6
Explanation
Answer: (c)
Q.5
All of the following statements are true, except:
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a)conductance is the reciprocal of resistance and is measured in sidemen
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b) Ohm's law is not applicable at very low and very high temperatures
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c)Ohm's law is applicable to semiconductors
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d)Ohm's law is not applicable to electron tubes, discharge tubes and electrolytes
Explanation
Answer: (c)
Q.6
The electric current in a discharge tube containing a gas is due to
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a) electrons only
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b) positive ions only
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c)negative ions and positive ions both
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d)electrons and positive ions both
Explanation
Answer: (d)
Q.7
The quantity in electricity analogous to temperature is
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a) resistance
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b) potential
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c)charge
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d)inductance
Explanation
Answer:(b)
Q.8
In figure potential difference between point x and y is
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a) 10 V
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b) 5 V
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c) 15 V
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d) 8.33 V
Explanation
Current I=(10-5) / (1+2)=(5/3) A Vx - Vy=E - Ir=10 -(5/2)×1=8.33V Answer: (d)
Q.9
Two wire of same length and material have their cross-sections in the ratio of 1:The two wires are joined so that their ends fuse into one another without any other change. The ratio of the resistance of the fused wire and the wire of smaller cross-section is
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a) 1:3
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b) 1:4
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c) 1:2
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d) 4:1
Explanation
R1 / R2=3/1 R1 /3=R2 When the wire are joined so their ends fuse into one another, they are parallel. Thus R=R1×R2 / (R1×R2) substituting value of R2 in above equation We get R/R1=1/4 Answer: (b)
Q.10
(1) The product of a volt and coulomb is a Joule (2) The product of a volt and an ampere is Joule/sec (3) the product of volt and watt is horse power (4) watt-hour can be measured in terms of electron -volts
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a) all the four are correct
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b) (1) and (2) and (4) are correct
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c)(1) and (3) are correct
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d)(3( and (4) are correct
Explanation
Answer:(b)
Q.11
Assume that each atom of copper contributes one free electron. If the current flowing through a copper wire of 1mm diameter is 1.1 ampere, the drift velocity of electrons will be [ density of copper=9 gm/cm3 and atomic weight of copper=63]
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a) 0.3 mm/sec
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b) 0.1 mm/sec
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c) 0.5 mm/sec
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d) 0.2 mm/sec
Explanation
Density of electrons=n Answer: (b)
Q.12
A beam of 16MeV deuterons ( charge=1.6×10-19 C) from a cyclotron falls on a copper block. The beam is equivalent to current of 15µA. At what rate do the deuterons strike the block?
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a)9.4 × 1013
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b) 9.4 × 1011
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c)9.4 × 109
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d)9.4 × 107
Explanation
charge on alpha particle=ei=ne / t For t=1 n=i /e=15×10-6/1.6×-19=9.375×1013 particle per secondAnswer: (a)
Q.13
Overloading of an electric circuit implies
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a) drawing of large current
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b) connecting high resistance device
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c)connecting a number of electric devices in series with the power supply
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d)none of the above
Explanation
Answer: (a)
Q.14
Two conductor are made of the same material and have same length. Conductor A is solid wire of diameter 1mm. Conductor B is hollow tube of outer diameter 2 mm and inner diameter 1mm. The ratio of resistance RA to RB
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a) 3:1
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b) 1:3
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c)1:1
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d)2:1
Explanation
Answer:(a)
Q.15
The temperature of metal wire rises when an electric current passes through it because
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a) collision of conduction electrons with each other releases heat energy
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b) collision of conduction electrons with atoms of the metal gives them energy which appears as heat
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c) when the conduction electrons fall from higher energy level to a lower energy level, heat energy is released
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d) heat energy is released when the conduction electrons are separated from their parent atoms
Explanation
Answer: (b)
Q.16
Which of the following can be used to express energy? Symbols have usual meaning for the units of physical quantities (1) kWh (2) VC (3) VAs2 (4) A2Ωs2
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a)All of above
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b) (1), (2), (3)
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c)(3), (4)
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d)(1), (2)
Explanation
Answer: (d)
Q.17
A current I flows in a wire of circular cross-section with the free electrons travelling with a drift velocity v. What is the drift velocity of electrons when a current 2I flows in another wire of twice the radius and of the same material
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a) v / 4
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b) v / 2
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c)v
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d)2v
Explanation
Answer: (b)
Q.18
Two wires of same material have same length. Their resistance's are proportional to
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a) 1 / masses of the wires
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b) square of masses of the wires
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c)masses of the wires
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d)cube of masses of the wires
Explanation
Answer:(a)
Q.19
A steady current i passes through a linear conductor of uniform cross-section. Any given segment of the conductor has
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a) A net negative charge
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b) a net positive charge
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c) a zero net charge of any kind
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d) a net charge proportional to the volume of the segment
Explanation
Answer: (c)
Q.20
Increase in which property of free electrons causes increase in the resistance of a conductor with rise in temperature?
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a)number density
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b) relaxation times
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c)mass
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d)none of above
Explanation
Answer: (d)
Q.21
A steady current is set up in a metallic wire of non-uniform cross-section. How is the rate flow of electrons (R) related to the area of cross-section A?
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a) R is independent of A
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b) R ∝ A-1
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c)R ∝ A
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d)R ∝ A2
Explanation
Answer: (a)
Q.22
Two unequal resistor are connected in series with a cell. Which of the following statement is true?
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a) Potential drop across either resistor is same
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b) Potential drop across smaller resistor is more
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c)Potential drop across larger resistor is more
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d)Any one of the above can be true depends on the emf of the cell
Explanation
Answer:(c)
Q.23
A potential difference of 10V is applied across a conductance 4 S. The current in the conductor is
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a) 40 A
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b) 2.5 A
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c) 2.4 A
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d) none of the above
Explanation
Answer: (a)
Q.24
The resistivity of a material is inversely proportional to
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a)number density of electrons as well as relaxation time
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b) number density of electrons and directly proportional to relaxation time
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c)relaxation time and directly proportional to the number density of electrons
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d)neither relaxation time nor number density of electrons
Explanation
Answer: (a)
Q.25
How will the reading in ammeter A affected if another identical bulb Q is connected in parallel to P as shown in figure . The voltage of the mains is maintained at a constant value
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a) the reading will be reduced to one half
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b) the reading will not affected
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c)the reading will be double of previous one
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d)the reading will be increased to four fold
Explanation
Answer: (c)
Q.26
Wire of resistance 0.5 Ω/m is bent into a circle of radius 1 m. The same wire is connected across a diameter AB as shown in figure. The equivalent resistance is
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a)π ohm
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b)π/ (π+2) ohm
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c)π/ (π+ 4) ohm
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d)(π+1) ohm
Explanation
R1=R2=(πr) ×0.5=0.5πΩ R3=(2r)× 0.5=1 Ω All three resistance are in parallel Answer:(c)
Q.27
A galvanometer together with an unknown resistance in series is connected across two identical cells each of 1.5V. When the batteries are connected in series, the galvanometer records a current of 1A and when the batteries are connected in parallel, the current is 0.6A. The internal resistance of the battery is
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a) 0.33 Ω
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b) 1 Ω
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c) 0.66Ω
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d) 0.5Ω
Explanation
Butteries connected in series Total potential 3V , R=Resistance of galvanometer , r is internal resistance then Current i=3 / (R+2r) ,as i=1 R+2r=3 --eq(1) Batteries sin parallel Potential 1.5 R resistance of galvanometer Total resistance R + r/2 Current 0.6=1.5 / (R +r/2) R + r/2=2.5 -- eq(2) From equation (1) and (2) we get r=0.33Ω Answer: (a)
Q.28
In figure point x and y represent the terminals of an unknown emf. On moving jockey J from A towards B it is observed that deflection of galvanometer remains in the same direction but increases. Which one of the following conclusions is correct
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a)the emf E is less than the unknown emf
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b) the emf E is more than the unknown emf
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c)the potential drop of potential across potentiometer wire is less than the unknown emf
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d)the unknown emf is wrongly connected
Explanation
Answer: (d)
Q.29
In figure the potentiometer wire AB has a resistance of 5Ω and length 10 m. The balancing length AJ for the emf of 0.4V is
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a) 8 m
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b) 0.8 m
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c)4 m
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d)0.4 m
Explanation
Total resistance in circuit=45+5=50Ω Current through wire=5/50=0.1 AResistance between AJ R=0.4/0.1=4ΩResistance per meter of wire=5/10=0.5 Ω Thus 4Ω resistance will have length=4/0.5=8 mAnswer: (a)
Q.30
Circuit whose resistance is R is connected to n similar cells. If the current in the circuit is the same whether the cells are connected in series or in parallel, then the internal resistance r of each cell is given by
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a) r=R/n
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b) r=nR
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c)r=R
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d)r=1/R
Explanation
Current in series connection of battery I=nE / (R +rn) Current in parallel connection of battery I'=E / ( R+ r/n) from above question r=R Answer:(c)
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