**CBSE Sample Papers for Class 12 Physics Solved 2016 Set 10**

** Section A**

1.What is the function of the ozone layer?

2.Name the experiment which establishes the wave nature of a particle.

3.What is the dimensional formula for induced emf?

4.Why a DC voltmeter and DC ammeter cannot read AC?O

5.What is the condition when terminal potential difference of a cell can be greater than its emf?

** Section B**

6.Why is a choke coil needed in the use of fluorescent tubes with AC mains? Why can we not use an ordinary resistor instead of the choke coil?

7.A cylindrical metallic wire is stretched to increase its length by 5%. Calculate the percentage change in its resistance.

8.The magnetic field in an electromagnetic wave, oscillates with an amplitude of 2 x 10^{-7 T }What would be the amplitude of the electric field, if the wave is travelling in water of refractive index 4/3?

9.A TV tower has a height of 145 m. How much population the TV broadcast covers, if the average population density around the tower is 1000 km^{-2}. Radius of the Earth is 6.4 xlO^{6} m.

Or

Calculate the length of a half-wave dipole antenna at

(i)40 MHz (ii) 400 MHz

What inference do you draw from the result?

10.A semiconductor has equal electron and hole concentration of 5xl0^{8}m –^{3}. On doping with certain impurity, electron concentration increases to 8 x 10^{12} m^{3}.

(i)Identify the new semiconductor obtained after doping.

(ii)Calculate the new hole concentration.

** Section C**

12.Define relaxation time of the free electrons, drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.

14.A small bulb is placed at the bottom of a tank, containing water at a depth of 80 cm. What is the area of the surface of water, through which light from the bulb can emerge out?Refractive index of water is 4/3.

15.Deduce de-Broglie wavelength of electrons accelerated by a potential of V volt. Draw a schematic diagram of a localised wave describing the wave nature of a moving electron.

16.State Bohr’s postulate for the permitted orbits for the electron in a hydrogen atom. Use this postulate to prove that the circumference of the nth permitted orbit for the electron can contain exact n wavelengths of the de-Broglie wavelength associated with the electron in that orbit.

17.Draw a ray diagram to show the image formation in a refraci ng type astronomical telescope in the near point adjustment. Write down the expression for its magnifying power. Why should the diameter of the objective of telescope be larger?

Or

A small air bubble in a sphere of glass of radius 2 cm appears to be 1 cm from the surface when looked at along a diameter. If the refractive index of glass is 1.5, then find the actual position of the air bubble.

18.The half-life of a radioactive substance is 30s. Calculate

(i)the decay constant.

(ii)time taken for the sample to decay to 3/4th of its initial value.

20.State Gauss’s law in electrostatics. Using this law, derive an expression for the electric field due to a uniformly charged infinite plane sheet.

21.A TV transmission tower antenna is at a height of 20 m. How much service area can it cover, if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) related to case (i).

22.27 drops of same size are charged at 220 V each. They coalesce to form a bigger drop. Calculate the potential of bigger drop

** Section D**

** Section E**

24.Explain principle and working of a cyclotron. Also, give its limitations. Draw its labelled diagram.

Or

(i)Explain the elements of Earth’s magnetic field and their meaning. Show these elements in a labelled diagram and deduce various relations between them.

(ii)Horizontal and vertical components of Earth’s magnetic field at a place are 0.15 G and 0.30 G, respectively. Calculate the angle of dip and resultant intensity of Earth’s magnetic field.

25.(i) What do you mean by the polarisation of light? Define law of Malus and then show that the intensity of light become half when ordinary light is incident on a polariser.

(ii) Two polarising sheets have their polarising direction parallel, so that the intensity of the transmitted light is maximum. Through what angle must the either sheet be turned, if the intensity is to drop by one half?

Or

(i)Define Brewster’s law. Show that the sum of angle of polarisation and angle of refraction is 90 °.

(ii)Discuss the intensity of transmitted light when polaroid sheet is rotated between two crossed polaroids.

26.(i) Distinguish between metals, insulators and semiconductors on the basis of energy band diagram.

(ii) Draw a circuit diagram showing the biasing of an LED. State the factor which controls

(a)wavelength of light,

(b)intensity of light emitted by the diode.

Or

(i)What is the difference between an intrinsic semiconductor and p-type semiconductor. Explain, why a p-type semiconductor is electrically neutral, although n_{h} >> n_{e} ?

(ii)In the figure shown alongside, analyse the action of the gates (1) and (2) and identify the logic gate of the complete circuit so obtained. Give its symbol and the truth table.

**Answers**

** Section A**

**1.What is the function of the ozone layer?**

**Sol.**The ozone layer blocks the passage of the harmful ultraviolet light from the sun to the Earth. Practically, all radiations less than 3 x 10-^{7}m are absorbed by the ozone layer.

**2.Name the experiment which establishes the wave nature of a particle.**

**Sol. **Davisson-Germer experiment.

**3.What is the dimensional formula for induced emf?**

**Sol.**

**4.Why a DC voltmeter and DC ammeter cannot read AC?O**

**Sol.**Because a average value of AC is zero, over a complete cycle

**5.What is the condition when terminal potential difference of a cell can be greater than its emf?**

**Sol.**When current is being supplied to the cell by some external source e.g. at the time of charging a secondary cell

** Section B**

**6.Why is a choke coil needed in the use of fluorescent tubes with AC mains? Why can we not use an ordinary resistor instead of the choke coil?**

**Sol.**In fluroescent tubes, choke coil drops AC voltage to a safe value without any powerloss (theoretically) in itself. If we use an ordinary resistor, it can also drop the voltage. So, there is a powerloss and the resistor is heated up

**7.A cylindrical metallic wire is stretched to increase its length by 5%. Calculate the percentage change in its resistance.**

**Sol.**

**8.The magnetic field in an electromagnetic wave, oscillates with an amplitude of 2 x 10 ^{-7 T }What would be the amplitude of the electric field, if the wave is travelling in water of refractive index 4/3?**

**Sol.**

**9.A TV tower has a height of 145 m. How much population the TV broadcast covers, if the average population density around the tower is 1000 km ^{-2}. Radius of the Earth is 6.4 xlO^{6} m.**

**Or**

**Calculate the length of a half-wave dipole antenna at**

**(i)40 MHz (ii) 400 MHz**

**What inference do you draw from the result?**

**Sol.**

**10.A semiconductor has equal electron and hole concentration of 5xl0 ^{8}m –^{3}. On doping with certain impurity, electron concentration increases to 8 x 10^{12} m^{3}.**

**(i)Identify the new semiconductor obtained after doping.**

**(ii)Calculate the new hole concentration.**

**Sol.**

** Section C**

**Sol.**

**12.Define relaxation time of the free electrons, drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.**

**Sol.**

**Sol.**

**14.A small bulb is placed at the bottom of a tank, containing water at a depth of 80 cm. What is the area of the surface of water, through which light from the bulb can emerge out?Refractive index of water is 4/3.**

**Sol.**

**15.Deduce de-Broglie wavelength of electrons accelerated by a potential of V volt. Draw a schematic diagram of a localised wave describing the wave nature of a moving electron.**

**Sol.**

**16.State Bohr’s postulate for the permitted orbits for the electron in a hydrogen atom. Use this postulate to prove that the circumference of the nth permitted orbit for the electron can contain exact n wavelengths of the de-Broglie wavelength associated with the electron in that orbit.**

**Sol.**

**17.Draw a ray diagram to show the image formation in a refraci ng type astronomical telescope in the near point adjustment. Write down the expression for its magnifying power. Why should the diameter of the objective of telescope be larger?**

**Or**

**A small air bubble in a sphere of glass of radius 2 cm appears to be 1 cm from the surface when looked at along a diameter. If the refractive index of glass is 1.5, then find the actual position of the air bubble.**

**Sol.**

**18.The half-life of a radioactive substance is 30s. Calculate**

**(i)the decay constant.**

**(ii)time taken for the sample to decay to 3/4th of its initial value.**

**Sol.**

**Sol.**

**20.State Gauss’s law in electrostatics. Using this law, derive an expression for the electric field due to a uniformly charged infinite plane sheet.**

**Sol.**

**21.A TV transmission tower antenna is at a height of 20 m. How much service area can it cover, if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) related to case (i).**

**Sol.**

**22.27 drops of same size are charged at 220 V each. They coalesce to form a bigger drop. Calculate the potential of bigger drop**

**Sol.**

** Section D**

**Sol.**

** Section E**

**24.Explain principle and working of a cyclotron. Also, give its limitations. Draw its labelled diagram.**

**Or**

**(i)Explain the elements of Earth’s magnetic field and their meaning. Show these elements in a labelled diagram and deduce various relations between them.**

**(ii)Horizontal and vertical components of Earth’s magnetic field at a place are 0.15 G and 0.30 G, respectively. Calculate the angle of dip and resultant intensity of Earth’s magnetic field.**

**Sol.**

**25.(i) What do you mean by the polarisation of light? Define law of Malus and then show that the intensity of light become half when ordinary light is incident on a polariser.**

**(ii) Two polarising sheets have their polarising direction parallel, so that the intensity of the transmitted light is maximum. Through what angle must the either sheet be turned, if the intensity is to drop by one half?**

**Or**

**(i)Define Brewster’s law. Show that the sum of angle of polarisation and angle of refraction is 90 °.**

**(ii)Discuss the intensity of transmitted light when polaroid sheet is rotated between two crossed polaroids.**

**Sol.**

**26.(i) Distinguish between metals, insulators and semiconductors on the basis of energy band diagram.**

**(ii) Draw a circuit diagram showing the biasing of an LED. State the factor which controls**

**(a)wavelength of light,**

**(b)intensity of light emitted by the diode.**

**Or**

**(i)What is the difference between an intrinsic semiconductor and p-type semiconductor. Explain, why a p-type semiconductor is electrically neutral, although n _{h} >> n_{e} ?**

**(ii)In the figure shown alongside, analyse the action of the gates (1) and (2) and identify the logic gate of the complete circuit so obtained. Give its symbol and the truth table.**

**Sol.**

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