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Quiz 6
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Q.1
The resistance of carbon filament at 0°C is 104 Ω. It is connected in series to an iron wire. The temperature coefficient of resistivity of carbon and iron are -0.0003 and +0.0052/ °C respectively. What must be the resistance of iron wire so that the combined resistance does not change with temperature?
0%
a) 15Ω
0%
b) 6Ω
0%
c)1.5Ω
0%
d)156/104 Ω
Explanation
Decrease in resistance of carbon=Increase in resistance of ironR1α1t=R2α2tR1α1=R2α2104× 0.0003=R2 × 0.0052R2=6Ω Answer:(b)
Q.2
A copper wire of length 1 metre and radius 1mm is joined in series with an iron wire of length two metres and radius 3mm and current is passed through the wire. the ratio of the current density in the copper and iron wire is [ MPPMT 1994]
0%
a) 18/1
0%
b) 9/1
0%
c) 6/1
0%
d) 2/3
Explanation
J1 / J2=A2 / A1 Answer: (b)
Q.3
A ring of radius R and linear charge density λ on its surface is performing rotational motion about an axis perpendicular to its plane. If the angular velocity of the ring is ω, how much current is constituted by the ring [ GSEB]
0%
a)Rωλ
0%
b) R2ωλ
0%
c)Rω2λ
0%
d)Rωλ2
Explanation
Charge on ring Q=(2πR)λ ω=2πf f==ω / 2π Current I=Qf I=[(2πR)λ] [ω / 2π]I=RωλAnswer: (a)
Q.4
A cell supplies a current of 0.9A through a 2Ω resistance and current of 0.3A through a 7Ω resistor. What is the internal resistance of the cell?
0%
a) 1.5Ω
0%
b) 1.0Ω
0%
c)1.2Ω
0%
d)2.0Ω
Explanation
We know that E=V +Ir , here r is internal resistance of battery For case 1E=(0.9)(2) + (0.9)r E=1.8 + 0.9r --eq(1)For case 2E=(0.3)(7) + (0.7)rE=2.1 + 0.7r --eq(2)eq(1) - eq(2)-0.3 + 0.2r=0r=1.5 ΩAnswer: (a)
Q.5
The cross sectional area of the plane shown in figure is equal to 1cm2A current flows through a conductor. The current density at point P in the conductor will be ..
0%
a)
0%
b)
0%
c)
0%
d)
Explanation
J=I / Acosθθ is angle between direction of current and Area vector=30°J=2 / (10-4 ×cos30=2 / 10-4×(√3 /2)=(4/√3)104 Answer:(a)
Q.6
A current density of 2.5Am-2 is found to exist in conductor when an electric field of 5×10-8 Vm-1 is applied across it. The resistivity of a conductor is .. [ GSEB]
0%
a) 1×10-8Ωm
0%
b) 2× 10-8 Ωm
0%
c) 0.5×10-8Ωm
0%
d) 12.5×10-8 Ωm
Explanation
We know that J=σE Here σ is conductivity 1 /σ=E/J resitivity ρ=E/J ρ=5×10-8 / 2.5=2×10-8 Ωm Answer: (b)
Q.7
A resistive wire is stretched till its length is increased by 100%. Due to the consequent decrease in a diameter, the change in the resistance of a stretched wire will be ... [ GSEB]
0%
a)300%
0%
b) 200%
0%
c)100%
0%
d)50%
Explanation
Volume of resistance before and after stretching is samel1A1=l2A2 given l2=2l1 l1A1=2l1A2 A2=A1 / 2formula for R=ρ(l/A)Thus new resistance R2=4R1 Increase in resistance is 3R1Increase in percentage is 300%Answer: (a)
Q.8
At what temperature would the resistance of copper conductor be double its resistance at 0°C? Given α for copper=3.9×10-3 °C-1
0%
a) 256.4 °C
0%
b) 512.8°C
0%
c)100°C
0%
d)256.4K
Explanation
From formula R=Ro[ 1 + αt]Given R=2 Ro∴ 2 Ro=2 Ro [1 + 3.9×10-3 × t]2=1 + 3.9×10-3 × t t=1/3.9×10-3=265.4 °CAnswer: (a)
Q.9
P and Q are to points on a uniform ring of resistance R. O is the centre of the ring. If the part PQ of the ring subtends an angle θ at the centre O of the ring ( i.e. ∠POQ=θ), the equivalent resistance of the ring between the points P and Q will be .... [ Radius of ring=r and resistance per unit length of the ring=ρ] [GSEB]
0%
a)
0%
b)
0%
c)
0%
d)
Explanation
We may consider there are two resistance connected in parallelResistance per unit length=R/ 2πrR1 and R2 connected in parallel let R' be equivalent resistance Answer:(a)
Q.10
Figure shows a part of a closed circuit. If the current flowing through it is 2A, what will be the potential difference between point A and B
0%
a) +2V
0%
b) +1V
0%
c) -2V
0%
d) -1V
Explanation
Applying Kirchoff's law VA - 2(1/4) - 1 - 2(1/4)=VB VA - 2=VBVA - VB=2 Answer: (a)
Q.11
A voltmeter of resistance R is used to measure the potential difference across a resistance r. In which case the reading will be nearest to that in the absence of voltmeter
0%
a) R < r
0%
b) R > r
0%
c)R=r
0%
d)R=0
Explanation
To measure a potential difference across resistance Voltmeter is connected in parallel. Thus Resistance of the voltmeter should be greater than resistance of component r .Answer: (b)
Q.12
Four wires of equal length and of resistance 5Ω each are connected in the form of a square. The equivalent resistance between the diagonally opposite corners of the square is
0%
a) 5 Ω
0%
b)10 Ω
0%
c)20 Ω
0%
d)(5/4) Ω
Explanation
Answer:(a)
Q.13
The supply voltage to a room is 120V. The resistance of the lead wire is 6ohm. A 60 watt bulb is already giving light. What is the decrease in voltage across the bulb when a 240 watt heater is switched on
0%
a) zero volt
0%
b) 2.9 volt
0%
c) 13.3 volt
0%
d) 10.4 v
Explanation
Rbulb=V2 / P=(120)2 / 60=240 ΩRheater=(120)2 / 240=60 Ω In presence of bulb and 6Ω only resistance current I=120 / (240+6) Voltage across bulb=(I) ×240Ω Voltage across bulb=(120 / 246 )×240=117.7 Volts In presence of heater and bulb equivalent resistance R' Current through circuit I'=120/54 Current through the bulb Potential across bulb=I"× Resistance of bulb Potential across bulb=(24/54) × 240=105.66 volts Change in potential across bulb=117.7 - 106.66=10.4 voltAnswer: (d)
Q.14
A 60 watt incandescent lamp operates at 120V. The number of electrons passing through the filament per second
0%
a)1..6×1012
0%
b) 3.125×1018
0%
c)7200
0%
d)12.5 ×1018
Explanation
I=P/V=(1/2) ABut 1A=6.25×1018 e/sec∴ number of electrons passing in the bulb per second=3.125×1018Answer: (b)
Q.15
A wire 100 cm long and 2.0 mm diameter has a resistance of 0.7Ω. The electrical resistivity of the material is
0%
a) 4.4 ×10-6 Ω × m
0%
b) 2.2 ×10-6 Ω × m
0%
c)1.1 ×10-6 Ω × m
0%
d)0.22 ×10-6 Ω × m
Explanation
Answer: (b)
Q.16
When a current I is set up in wire of radius r, the drift speed is vd. If the same current is set up through a wire of radius 2r., the drift speed will be
0%
a) vd /4
0%
b) vd /2
0%
c)2vd
0%
d)4vd
Explanation
vd ∝ (1 /A) Area gets four times when radius gets doubled Answer:(a)
Q.17
In a region 1019 α-particles and 1019 protons move to the left while 1019 electrons move to the right per second./ The current is
0%
a) 3.2 amp towards left
0%
b) 6.4 amp towards left
0%
c) 9.6 amp towards left
0%
d) 6.4 amp towards right
Explanation
Direction of current is the direction of positive charge direction of electron is opposite but charge is also opposite thus charge of electrons should be added to charge of positive particle i=(1019 ×2e + 1019 ×e + 1019 ×e ). left i=4×1.6×10-19×1019e=6.4A, left Answer: (b)
Q.18
An electron charge (e) is revolving in a circular orbit of radius r round a nucleus of charge Ze with speed v. The equivalent current is
0%
a)zero
0%
b) ev/2πr
0%
c)Zev/2πr
0%
d)e2πr/ v
Explanation
i=q/T=ev/2πrAnswer: (b)
Q.19
In absence of electric field, the mean velocity of free electrons in a conductor at absolute zero temperature T is
0%
a) zero
0%
b) independent of temperature
0%
c)proportional to √T
0%
d)proportional to T
Explanation
Answer: (a)
Q.20
Two resistance are joined in parallel whose resultant is 6/5 Ω. One of the resistance wire is broken and the effective resistance become 2&Ω. Then the resistance in ohm of the wire that got broken was
0%
a) 3/5
0%
b) 2
0%
c)6/5
0%
d)3
Explanation
(6/5)=R1R2 / ( R1 + R2) Answer:(d)
Q.21
In a circuit containing two unequal resistors connected in parallel
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a) the current is same in both the resistors
0%
b) the current is large in larger resistor
0%
c) voltage drop is same across both the resistors
0%
d) voltage drop is larger across larger resistance
Explanation
Answer: (c)
Q.22
In an electrolyte if e denotes the charge on each type of ion and n+ and n- denote the number of positive and negative ions per unit volume and V+ and V- denotes the drift velocities of positive and negative ions, then the current density is given by
0%
a)en+V+
0%
b) en-V-
0%
c)en+V+ + en-V-
0%
d)eA[n+V+ + n-V-]
Explanation
Answer: (c)
Q.23
A certain piece of copper wire is to be shaped in to a conductor of minimum resistance. Its length and diameter should be
0%
a) L, D
0%
b) 2L, (1/2) D
0%
c)(1/2) L, 2D
0%
d)2L, 2D
Explanation
Answer: (c)
Q.24
A square aluminum rod is 1 m long and 5mm on edge. What must be the radius of another aluminum rod whose length is 1m and which has the same resistance as previous rod
0%
a) 5.0 mm
0%
b) 4.2 mm
0%
c) 2.8 mm
0%
d) 1.4 mm
Explanation
Answer:(c)
Q.25
Certain substances lose their electrical resistance at very low temperature. These substances are called
0%
a) good conductors
0%
b) semi-conductors
0%
c) super conductors
0%
d) dielectrics
Explanation
Answer: (c)
Q.26
A wire of length 5 m and radius 1 mm has a resistance of 1 ohm. What length of wire of same material at the same temperature and of radius 2mm will also have same resistance of 1 ohm
0%
a)1.25 m
0%
b) 2.5 m
0%
c)10.0 m
0%
d)20.0 m
Explanation
Answer: (d)
Q.27
The resistance of carbon filament at 0°C is 104ohms. It is connected in series to an iron wire. The temperature coefficient of resistivity of carbon and iron are -0.0003 and +0.0052 per °C respectively. What must be the resistance of the iron wire so that the combined resistance does not change with temperature?
0%
a) 1.5 ohms
0%
b) 6 ohms
0%
c)1.5 ohms
0%
d)156/104 ohms
Explanation
Decrease in resistance of carbon=Increase in resistance of IronRocαc×t=RoIαI×t 104×0.0003=RoI × 0.0052 RoI=6 ΩAnswer: (b)
Q.28
A wire has resistance 12 ohms. It is bent in the form of an equilateral triangle. The effective resistance between any two corners of triangle is
0%
a) 9/4 ohms
0%
b) 12 ohms
0%
c)6 ohms
0%
d)8/3 ohms
Explanation
Answer:(d)
Q.29
The equivalent resistance of the parallel combination is
0%
a) smaller than the largest resistance
0%
b) larger than the largest resistance
0%
c) smaller than the smallest resistance
0%
d) larger than the smallest resistance
Explanation
Answer: (c)
Q.30
Resistivity of iron is 1×10-7 ohm×metre. The resistance of the given wire of a particular thickness and length is one ohm. If the diameter and the length of wire both are doubled, the resistivity will be in in ohm ×metre
0%
a)1×10-7
0%
b) 2×10-7
0%
c)4×10-7
0%
d)8×10-7
Explanation
Answer: (a)
0 h : 0 m : 1 s
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