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12th Grade > Physics

WORK POWER AND ENERGY MCQs

Work And Energy, Sources Of Energy, Electric Potential Energy Potential And Dipoles

Total Questions : 109 | Page 1 of 11 pages
Question 1. Two metal spheres of radii in the ratio 3:4 are connected and a charge of 14μ C is given to the system and then they are separated so that there is no mutual force between them. The potential due to the larger sphere at a distance of 3m from the centre of the sphere is
  1.    72KV
  2.    36KV
  3.    24KV
  4.    12KV
 Discuss Question
Answer: Option C. -> 24KV
:
C
q1+q2=14μC...(1)V=q1c1=q2c2;q1q2=4πϵ0r14πϵ0r2;q1q2=r1r2=34q1=6μC;q2=8μCV2=14πϵ0q2r=9×109×8×1063V2=24×103V
Question 2. Potential at the centroid of an equilateral triangle of side ‘a’,  when equal charges each of magnitude ‘Q’ are placed at the vertices of the triangle is?
 
  1.    Zero
  2.    14πϵ03Qa
  3.    14πϵ03√3Qa
  4.    14πϵ0Q√3a
 Discuss Question
Answer: Option C. -> 14πϵ03√3Qa
:
C
V1=V1+V2+V3;V1=V2=V3
=3V1
=314πϵ0Qa3
=14πϵ033Qa
Question 3. Two point charges 4μC and 9μCare separated by a distance of 50 cm.  The potential at the point between them where the field has zero strength is
  1.    4.5×105V
  2.    9×105V
  3.    9×104V
  4.    Zero
 Discuss Question
Answer: Option A. -> 4.5×105V
:
A
Assume the point at which field is zero is at a distance X from 4μC
X=dq2q1+1=5094+1=5032+1=1005=20cmV=V1+V2=14πϵ0[q1r1+q2r2]=9×109[4×1060.2+9×1060.3]=9×103[20+30]=9×103×50=4.5×105V
Question 4. A body of mass m is moving in a circle of radius r with a constant speed v. The force on the body is mv2r and is directed towards the centre. What is the work done by this force in moving the body over half the circumference of the circle
  1.    mv2πr2
  2.    Zero
  3.    mv2r2
  4.    πr2mv2
 Discuss Question
Answer: Option B. -> Zero
:
B
Work done by centripetal force is always zero, because force and instantaneous displacement are always perpendicular.
W=F.s=Fscosθ=Fscos90=0
Question 5. A body of mass 4 gm carrying a charge 1μ C starts from rest at the positive plate and reaches the negative plate of a parallel plate condenser with final velocity of 0.3 m/s. The potential difference between the plates is?
 
  1.    180 V
  2.    360 V
  3.    90 V    
  4.    18 V
 Discuss Question
Answer: Option A. -> 180 V
:
A
12mv2=qV
12×4×103×9×102=106V
V=180V
Question 6. Four identical charges +50 μC each are placed, one at each corner of a square of side 2m. How much external energy is required to bring another charge of +50 μC from infinity to the centre of the square (Given 14πϵ0=9×109 Nm2C2)
  1.    64 J
  2.    41 J
  3.    16 J
  4.    10 J
 Discuss Question
Answer: Option A. -> 64 J
:
A
Potential at the centre of square
V=4×(9×109×50×1062)=902×104V
Work done in bringing a charge (q = 50μC) from to centre (O) of the square is
W=q(V0V)=qV0
W=50×106×902×104=64J
Question 7. Two insulated charged conducting spheres of radii 20 cm and 15 cm respectively and having an equal charge of 10 C are connected by a copper wire and then they are separated. Then
  1.    Both the spheres will have the same charge of 10 C
  2.    Surface charge density on the 20 cm sphere will be greater than that on the 15 cm sphere
  3.    Surface charge density on the 15 cm sphere will be greater than that on the 20 cm sphere
  4.    Surface charge density on the two spheres will be equal
 Discuss Question
Answer: Option C. -> Surface charge density on the 15 cm sphere will be greater than that on the 20 cm sphere
:
C
After redistribution, charges on them will be different, but they will acquire common potential
i.e. kQ1r1=kQ2r2Q1Q2=r1r2
As σ=Q4πr2σ1σ2=Q1Q2×r22r21σ1r
i.e. surface charge density on smaller sphere will be more.
Question 8. In the following diagram the work done in moving a point charge from point P to point A, P to B and P to C is respectively WA, WB and WC , then
In The Following Diagram The Work Done In Moving A Point Cha...
 
  1.    WA=WB=WC  
  2.    WA=WB=WC=0  
  3.    WA>WB>WC  
  4.    WA
 Discuss Question
Answer: Option B. -> WA=WB=WC=0  
:
B
According to the figure, there is no other charge. A single charge when moved in a space of no field, does not experience any force hence no work is done. WA=WB=WC=0
Question 9. A particle has a mass 400 times that of the electron and the charge is double that of an electron. It is accelerated by 5V of potential difference. Initially the particle was at rest, then its final kinetic energy will be
  1.    5 eV
  2.    10 eV
  3.    100 eV
  4.    2000 eV
 Discuss Question
Answer: Option B. -> 10 eV
:
B
ΔE=2e×5V=10eV Final kinetic energy =10eV
Question 10. If a unit positive charge is taken from one point to another over an equipotential surface, then
  1.    Work is done on the charge
  2.    Work is done by the charge
  3.    Work done is constant
  4.    No work is done
 Discuss Question
Answer: Option D. -> No work is done
:
D
On the equipotential surface, electric field is normal to the charged surface (where potential exists) so no work will be done.

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