#### 12th Standard English Medium Physics Reduced syllabus Public Exam Model Question Paper with Answer key - 2021

12th Standard

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Physics

Time : 02:45:00 Hrs
Total Marks : 70

Part I

Answer all the questions.

Choose the most suitable answer from the given four alternatives and write the option code with the corresponding answer.

15 x 1 = 15
1. Two identical conducting balls having positive charges q1 and q2 are separated by a center to center distance r. If they are made to touch each other and then separated to the same distance, the force between them will be

(a)

less than before

(b)

same as before

(c)

more than before

(d)

zero

2. Rank the electrostatic potential energies for the given system of charges in increasing order.

(a)

1 = 4 < 2 < 3

(b)

2 = 4 < 3 < 1

(c)

2 = 3 < 1 < 4

(d)

3 < 1 < 2 < 4

3. Two metallic spheres of radii 1 cm and 3 cm are given charges of -1 x 10-2 C and 5 x 10-2 C respectively. If these are connected by a conducting wire, the final charge on the bigger sphere is

(a)

3 × 10-2 C

(b)

4 × 10-2 C

(c)

1 × 10-2 C

(d)

2 × 10-2 C

4. There is a current of 1.0 A in the circuit shown below. What is the resistance of P ?

(a)

1.5 Ω

(b)

2.5 Ω

(c)

3.5 Ω

(d)

4.5 Ω

5. A piece of copper and another of germanium are cooled from room temperature to 80 K. The resistance of

(a)

each of them increases

(b)

each of them decreases

(c)

copper increases and germanium decreases

(d)

copper decreases and germanium increases

6. A non-conducting charged ring of charge q, mass m and radius r is rotated with constant angular speed ω. Find the ratio of its magnetic moment with angular momentum is

(a)

$\\ \frac { q }{ m }$

(b)

$\\ \frac { 2q }{ m }$

(c)

$\\ \frac { q }{ 2m }$

(d)

$\\ \frac { q }{ 4m }$

7. The BH curve for a ferromagnetic material is shown in the figure. The material is placed inside a long solenoid which contains 1000 turns/cm. The current that should be passed in the solenonid to demagnetize the ferromagnet completely is

(a)

1.00 m A (milli ampere)

(b)

1.25 mA

(c)

1.50 mA

(d)

1.75 mA

8. Th e fl ux linked with a coil at any instant t is given by $\Phi$B =10t2−50t+ 250 . The induced emf at t = 3s is

(a)

−190 V

(b)

−10 V

(c)

10 V

(d)

190 V

9. $\frac{20}{\pi^2}H$inductor is connected to a capacitor of capacitance C. The value of C in order to impart maximum power at 50 Hz is

(a)

50 μF

(b)

0.5 μF

(c)

500 μF

(d)

5 μF

10. Let E = Eo sin[106 x -ωt] be the electric field of plane electromagnetic wave, the value of ω is

(a)

0.3 × 10−14 rad s−1

(b)

3 x 10−14 rad s−1​​​​​​​

(c)

0.3 x 1014 rad s−1

(d)

3 x 1014 rad s-1

11. In an electron microscope, the electrons are accelerated by a voltage of 14 kV. If the voltage is changed to 224 kV, then the de Broglie wavelength associated with the electrons would

(a)

increase by 2 times

(b)

decrease by 2 times

(c)

decrease by 4 times

(d)

increase by 4 times

12. The work functions for metals A, B and C are 1.92 eV, 2.0 eV and 5.0 eV respectively. The metals which will emit photoelectrons for a radiation of wavelength 4100$\\ \\ \mathring { A } \\$ is/are

(a)

A only

(b)

both A and B

(c)

all these metals

(d)

none

13. Mp denotes the mass of the proton and Mn denotes mass of a neutron. A given nucleus of binding energy B, contains Z protons and N neutrons. The mass M(N,Z) of the nucleus is given by(where c is the speed of light)

(a)

M(N,Z)=NMn+ZMp-Bc2

(b)

M(N,Z)=NMn=ZMp+Bc2

(c)

M(N,Z)=NMn=ZMp-B/c2

(d)

M(N,Z)=NMn=ZMp+B/c2

14. In a Young’s double-slit experiment, the slit separation is doubled. To maintain the same fringe spacing on the screen, the screen-to-slit distance D must be changed to,

(a)

2D

(b)

$\frac{D}{2}$

(c)

$\sqrt{2}$D

(d)

$\frac{D}{\sqrt2}$

15. If a half –wave rectifi ed voltage is fed to a load resistor, which part of a cycle the load current will flow?

(a)

00–900

(b)

900–1800

(c)

00–1800

(d)

00–3600

1. Part II

Answer any 6 questions. Question no. 16 is compulsory.

6 x 2 = 12
16. The image of an object formed by a lens on the screen is not in sharp focus. Suggest a method to get a dear focussing of the image on the screen without disturbing the positions of the object, the lens or the screen.

17. If the intensity of radiation in a photocell is increased how does the stopping potential
vary?

18. What is nuclear chain reaction?

19. What is the role of nanostructure in the morpho butterfly wings?

20. Write the expression for angular resolution.

21. How does the power of a convex lens vary, if the incident red light.is replaced by violet light?

1. Part III

Answer any 6 questions. Question no.27 is compulsory.

6 x 3 = 18
22. Calculate the equivalent resistance for the circuit which is connected to 24 V battery and also find the potential difference across 4 Ω and 6 Ω resistors in the circuit.

23. If the resistance of coil is 3 Ω at 20oC and α = 0.004/oC then determine its resistance at 100oC.

24. Show the time period of oscillation when a bar magnet is kept in a uniform magnetic field is
$T=2\pi \sqrt { \frac { 1 }{ { p }_{ m }B } }$ in second, where I represents moment of inertia of the bar magnet, pm is the magnetic moment and is the magnetic field

25. The magnetic field shown in the figure is due to the current carrying wire. In which
direction does the current flow in the wire?.

26. Explain the principle and working of a moving coil galvanometer.

27. A circular antenna of area 3 m2 is installed at a place in Madurai. The plane of the area of antenna is inclined at 47o with the direction of Earth’s magnetic field. If the magnitude of Earth’s field at that place is 40773.9 nT find the magnetic flux linked with the antenna.

1. Part IV

Answer all the questions.

5 x 5 = 25
1. You are given two converging lenses of focal lengths 1.25 cm and 5 cm to design a compound microscope. If it is desired to have a magnification of 30, find out the seperation between the objective and the eyepiece

2. In Young's experiment, the upper slit is covered by a thin glass plate of refractive index 1.4 while the lower slit is covered by another glass plate having the same thickness as the first one but having refractive index 1.7. Interference pattern is observed using light of wavelength 5400A . It is observed that the point P on the screen where the central maximum (n = 0) fell before the glass were inserted now has $\cfrac { 3 }{ 4 }$ th original intensity. It is further observed that what used to be the fifth maximum earlier, lies below the point P while the sixth minimum lies above P. Calculate the thickness of the glass plate.

1. The eyepiece 'and objective of a microscope having focal lengths of 0.03 m and 0.04 m respectively are separated by a distance 0.2 m. Now the eyepiece and the objective are to be interchanged such that the angular magnification of the instrument remains the same. What is the separation between the lenses?

2. Explain current transfer characteristics.

1. Show that for a straight conductor, the magnetic field
$\overset { \rightarrow }{ B } =\frac { { \mu }_{ ° }I }{ 4\pi a } (cos\varphi _{ 1 }-cos\varphi _{ 2 })\hat { n }$
$=\frac { { \mu }_{ ° }I }{ 4\pi a } (sin{ \theta }_{ 1 }+sin{ \theta }_{ 2 })\hat { n }$

2. A particle of charge q moves with velocity $\vec { v }$ along positive y -direction in a magnetic field $\vec { B }$ .Compute the Lorentz force experienced by the particle
(a) when magnetic field is along positive y-direction
(b) when magnetic field points in positive z - direction
(c) when magnetic field is in zy - plane and making an angle θ with velocity of the particle. Mark the direction of magnetic force in each case.

1. Compute the work done and power delivered by the Lorentz force on the particle of charge q moving with velocity $\vec { v }$ Calculate the angle between Lorentz force and velocity of the charged particle and also interpret the result.

2. An electron moving perpendicular to a uniform magnetic field 0.500 T undergoes circular motion of radius 2.80 mm. What is the speed of electron?

1. For a BJT, the common - base current gain α = 0.98 and the collector base junction reverse bias saturation ICU = 0.6μA. This BJT is connected in the common emitter mode and operated in the active region with a base drive current ID= 20 μA. The collector current IC for this mode of operating is

2. Let E be the electric field of magnitude 6.0 × 106 N C-1 and B be the magnetic field
magnitude 0.83 T. Suppose an electron is accelerated with a potential of 200 V, will
it show zero deflection?. If not, at what potential will it show zero deflection.