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Published on: 25/10/2025
Download CBSE Class 12th Standard CBSE Physics question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Physics
Questions + Answers key
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1.
What is not true?
It is not possible to create or destroy net charge carries by any isolated system
Charges can be created or destroyed in equal and unlike pairs only
proper signs have to be used while adding the charges in a system
Excess of electrons over protons in a body is responsible for positive charge of the body.
2.
Out of glass (rod) and silk (cloth) work function of glass is
smaller
larger
equal
none of the above
3.
Which of the following is not an insulator?
glass
rubber
ebonite
human body
4.
A charge of 10\(\mu C\) lies at the centre of a square. Work done in carrying a charge of \(2\mu C\) from one corner of square to the diagonally opposite corner is
20 J
5 J
zero
20 \(\mu\) J
5.
The correct relation between electric intensity E and electric potential V is
\(E=-{dV\over dr}\)
\(E={dV\over dr}\)
\(V=-{dE\over dr}\)
\(V={dE\over dr}\)
6.
Force \(\overrightarrow { F } \) acting on a test charge qo in a uniform electric field \(\overrightarrow { E } \) is
\(\overrightarrow { F } =q_{ o }\overrightarrow { E } \)
\(\overrightarrow { F } =\frac { \overrightarrow { E } }{ q_{ o } } \)
\(\overrightarrow { F } =\frac { \overrightarrow { q_o } }{\overrightarrow { E } } \)
\(\overrightarrow { F } =q_{ o }^{ 2 }\overrightarrow { E } \)
7.
Electric field due to an electric dipole is
spherically symmetric
cylindrically symmetric
asymmetric
none of the above
8.
Electric field intensity (E) due to an electric dipole varies with distance (r) of the point from the centre of dipole as:
\(E\alpha {1\over r}\)
\(E\alpha{1\over r^4}\)
\(E\alpha{1\over r^2}\)
\(E\alpha {1\over r^3}\)
9.
The SI unit of electric field intensity is
N
N/C
C/m2
N/m2
10.
At a particular point, electric field depends upon
Source charge Q only
test charge qo only
both Q and q0
neither Q nor qo
11.
Raghav lives in an area where birds in large groups play around producing pleasing humming sounds. One day he notices that the high power lines soon after a strong wind have come too close which may prove fatal for the birds that would sit on them and flutter their wings for some reason or other. He complained to the authorities and the lines were set at the proper distance once again.
(1) What are the values possessed by Raghav and the authorities?
(2) What is the danger that could happen to the innocent birds in Raghav's view?
12.
State Kirchhoff's laws of current distribution in an electrical network. Using these rules determine the value of the current I1 in the electric circuit given below:
.png)
13.
A wire of \(10 \ \Omega\) resistance is stretched to thrice its original length. What will be its
(i) new resistivity and
(ii) new resistance?
14.
A wire of resistance \(5.0 \ \Omega\) is used to wind a coil of radius 5 cm. The wire has a diameter 2.0 mm and the specific resistance of its material is 2.0 x 10-7 \(\Omega m\). Find the number of turns in the coil.
15.
There is a copper wire of length 2.2 m, of area of cross-section 2.0 sq. mm, carrying a current of 6.0 A. If the number density of electrons in copper is 8.5 x 1028 m-3 , find the time taken by an electron to drift from one end to another end of the wire.
16.
At room temperature copper has free electron density of 8.4 x 1028 per m3. The copper conductor has a cross-section of 10-6 m2 and carries a current of 5.4 A. What is the electron drift velocity in copper?
1.
(d)
Excess of electrons over protons in a body is responsible for positive charge of the body.
2.
(a)
smaller
3.
(d)
human body
4.
(c)
zero
5.
(a)
\(E=-{dV\over dr}\)
6.
(a)
\(\overrightarrow { F } =q_{ o }\overrightarrow { E } \)
7.
(b)
cylindrically symmetric
8.
(d)
\(E\alpha {1\over r^3}\)
9.
(b)
N/C
10.
(a)
Source charge Q only
11.
(1) Raghav: affection towards birds, taking appropriate action. Authorities: duty conscious.
(2) The bird may get electrocuted; avoid sparking as shown in the diagrams below:
12.
First law or Current law or Junction law: It states, "In any electrical network, the algebraic sum of currents meeting at a point (or junction) is zero."
Second law or Voltage law or Loop law: It states,
"In a closed circuit, the algebraic sum of the products of the current and the resistance in each of the conductors in any closed path (or mesh) in a network plus sum of emf in that path is equal to zero."
Now from the given figure,
I1+ I2 = I3 .....(i)
I2 = I3 - I1 ........(ii)
-20I1 - 40I3 = 40
-40 (I1 + 2I3) = 80
I1+ 2I3 = 2 ...(iii)
40I3 + 20I2 = 80 + 40
20 (2I3 + I2) = 120
2I3 + I2 = 6
2I3+ (I3- I1) = 6
2I3+ I3 -I1 = 6
3I3 - I1 = 6 ......(iv)
Now solving eqns. (ii) and (iv)
I1+ 2I3 = 2
-I1 + 3I3 = 6
\(5I_3=8\Rightarrow I_3=\frac{8}{5}=1.6 \ amp.\)
2I3+ I1= 2
I1 = 2-2I3
I1= 2-2 x \(\frac{8}{5}\)
Now, \(I_1=6-2I_3\)
\(\Rightarrow\ \ I_2=6-2\times \frac{8}{5}\)
\(\Rightarrow\ I_2=6-\frac{16}{5}\)
\(\Rightarrow\ \ I_2=\frac{30-16}{5}=\frac{14}{5}\)
\(\Rightarrow \ I_2=2.8\ amp\)
13.
(i) Since resistivity depends on the nature of material of wire and is independent of dimensions of wire, so the resistivity remains unchanged.
(ii) In both the cases, the volume of the wire is same.
So
V = Al = A'l'
or A' = \(\frac{lA}{l'}=\frac{lA}{3l}=\frac{A}{3}\)
Now R = \(\frac{\rho l}{A}\)
and R' = \(\frac{\rho l'}{A'}=\frac{\rho 3l}{A/3}=\frac{9 \rho l}{A}= 9 \ R\)
R' = 9 x 10 = \(90 \ \Omega\)
14.
Here,
R = \(5.0 \ \Omega\);
r1 = 5 x 10-2 m;
D = 2.0 x 10-3 m;
\(\rho= 2.0 \times 10^{-7} \ \Omega m\).
R = \(\frac{\rho l}{\pi D^2/4}\)
or l = \(\frac{R \pi D^2}{4 \rho}\)
Let n be the number of turns in the coil. Then total length of the wire used, l = \(2 \pi r_1 n\)
or n = \(\frac{l}{2 \pi r_1}=\frac{R \pi D^2}{4 \rho \times 2 \pi r_1}=\frac{RD^2}{8 \rho r_1}\)
= \(\frac{5.0 \times (2.0 \times 10^{-3})^2}{8 \times (2.0 \times 10^{-7})\times (5 \times 10^{-2})^2}\)
= 250
15.
Here,
l = 2.2 m;
A = 2.0 sq. mm = 2 x 10-6 sq.m,
I = 6 A;
n = 8.5 x 1028 m-3
Drift velocity,
vd = \(\frac{I}{nAe}\)
= \(\frac{6}{(8.5 \times 10^{28})\times(2\times 10^{-6})\times (1.6\times10^{-19})}\)
= 2.2 x 10-4 ms-1
Time of drifting the electron,
t = \(\frac{l}{v_d}=\frac{2.2}{2.2 \times 10^{-4}}\)
= 104 s
16.
Here,
n = 8.4 x 1028 m-3,
A = 10-6 m2,
I = 5.4 A
vd = \(\frac{I}{nAe}\)
= \(\frac{5.4}{(8.4 \times10^{28})\times(10^{-6})\times(1.6 \times10^{-19})}\)
= 0.4 x 10-3 m/s
= 0.4 mm/s
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