12th Physics Pre Half-Yearly Question with answer key 2025

 12th Physics Pre Half-Yearly Original Question with Official Answer Key 2025 | PDF Download

12th Physics Pre Half Yearly Original Question 2025

Answer in all question. $10\times 1=20$

1. An electric dipole is placed at an alignment angle of ${30}^{\circ }$ with an electric field of $2\times {10}^5{\text{NC}}^{+}$ it experience a torque qual to 8 Nm. the charge on the dipole if the dipole of length is 1 cm is
   

     a) 4mC.          b) 8mC              c) 5mC               d) 7mC

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

$$\begin{array}{ccccccc}Q& -Q& -Q& -Q& -Q& -Q& -Q\\ r& r& r& r& r& r& r\end{array}$$

                (a) $1<4<2<3$           (b) $2<4<3<1$      (c) $2<3<1<4$

               (d) $3<1<2<4$

3. In India electricity is supplied for domestic use at 220V it is applied at 110V in USA . If the resistance of a 60W bulb for use in India is R resistance of a 60 W bulb for use in US a will be

                 a) R.          b) 2R.            c) R/4.              D) R/2

4. The carbon register of $(47+4.7)$ KΩ to be marked with ranges of different colours for its identification. the colour code sequence will be

               a) Yellow green violet gold                     b). yellow violet Orange silver                                         c) violet yellow orange silver                  d). green Orange violet gold

5. Which of the following electromagnetic radiation is used for weaving object through fog.
             a). Microwave,            b) y rays         c). X-rays           d). infrared

6. Which of the following is false for electromagnetic waves

          a) Transverse            b) non mechanical waves         c) longitudinal                                                d) produced by accelerating charges

7. An electron microscope the electrons are accelerated by voltage of 14V the voltage is charged to 224V 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 by4 times

8. A light of wavelength 500 nm is incident on a sensitive metal plate of photoelectric work function 1.235 eV the kinetic energy of the photo electron emitted is(h 6.6 × 10${}^{-34}$ Js)

          a) 0.58 eV            b). 2.48 eV               c). 1.24 eV           d) 1.16 eV

9. If the nuclear radius of ${}^{27}Al$ is 3.6 fermi , the approximate nuclear radius of ${}^{64}Cu$ in fermi is

          a). 2.4                    b). 1.2                       c). 4.8                d). 3.6

10. The nucleus is approximately spherical in shape then the surface area of nucleus having mass number. A varies as

           a). ${\text{A}}^{1\mathrm{/}3}$                  b) ${\text{A}}^{4\mathrm{/}3}$                     c) ${\text{A}}^{1\mathrm{/}3}$              d) $\text{A}$

Answer in 5 questions 5+2=10

11. State Gauss's law.

12. Define mobility

13. What are Fraunhofer lines and how do they identify elements in the sun.

14. calculate the number of electrons in a positive charge of 1 coulomb.

15. Define radioactivity ( OR ) decays rate.

16. Write the properties of nuclear forces.

Answer in 5 questions 5+3 = 15

22. Derive an expression for electrostatic potential due to a point Charge.

23. Distinguish between drift velocity and mobility.

24. Properties of electromagnetic waves

25. Derive the Duane Hunt formula for continuous X-rays.

26. Discuss the Alpha decay process with example

27. Consider a point charge +q placed at the origin and another points charge -2q placed at a distance 9m from the charge +q determine the point between the two charges at which electric potential is zero.

Answer any 3 questions. 3+5=15

23. Derive the expression for resultant capacitance when capacitors are connected in series and in parallel.

24. Obtain the condition for bridge balance in wheatstone's bridge.

25. Explain the types of emission spectrum.

26. Describe freely Davisson - Germier experiment which demonstrated the wave nature of electrons.

Download the 12th Physics Pre Half Yearly Original Question paper - click here.

12th Physics Pre Half Yearly Official Answer Key 2025

1. b) 8 mc

2. (a) 1 = 4 < 2 < 3

3. R/4

4. Yellow Violet Orange Silver

5. Infrared

6. c) Longitudinal

7. b) Decrease by 2 times

8. c) 1.24 eV

9. c) 4.8

10. b) A${}^{2\mathrm{/}3}$

Short Answers for Questions 11–26

11. State Gauss's law.
    The net electric flux through any closed surface is equal to $\frac{1}{{ϵ}_0}$ times the net electric charge enclosed within that surface.

12. Define mobility.
    Mobility is the magnitude of the drift velocity per unit electric field. Its SI unit is $m^2\mathrm{/}\sqrt{s}$.

13. What are Fraunhofer lines and how do they identify elements in the sun?
    Fraunhofer lines are dark lines in the Sun's spectrum. They are formed because the Sun's cooler outer atmosphere absorbs specific wavelengths of light. Each element absorbs unique wavelengths, so these lines act as fingerprints to identify elements present in the Sun.

14. Calculate the number of electrons in a positive charge of 1 coulomb.
    A positive charge means a deficit of electrons. Number of electrons

$$n=Q\mathrm{/}e=1C\mathrm{/}(1.6\times {10}^{-19}C)=6.25\times {10}^{18}\text{electrons}\mathrm{.}$$

15. Define radioactivity (OR) decays rate.
    Radioactivity is the spontaneous disintegration of an unstable atomic nucleus, accompanied by the emission of radiation. Decay rate (or activity) is the number of nuclei decaying per unit time.

16. Write the properties of nuclear forces.

• Very strong (strongest known force)

• Short-range (effective only within a few fermi)

• Charge independent (same for n-n, p-p, n-p)

• Act between nucleons (protons and neutrons)

17. Derive an expression for electrostatic potential due to a point Charge.
    The electrostatic potential $V$ at a distance $r$ from a point charge $q$ is defined as the work done per unit charge in bringing a test charge from infinity to that point. It is given by

$$V=(1\mathrm{/}(4\pi {ϵ}_0))\ast (q\mathrm{/}r)\mathrm{.}$$

18. Distinguish between drift velocity and mobility.

Drift Velocity	Mobility
The average velocity acquired by a charge carrier in a conductor due to an electric field.	The drift velocity per unit electric field ($v_d\mathrm{/}E$).
It depends on the electric field and the nature of the material.	It depends only on the material and temperature.

19. Properties of electromagnetic waves.

• They are transverse waves.

• They can travel through a vacuum.

• They travel at the speed of light $c=3\times {10}^8\text{m/s}$ in vacuum.

• They carry energy and momentum.

• They obey the superposition principle.

20. Derive the Duane Hunt formula for continuous X-rays.
    The Duane-Hunt law gives the maximum frequency (or minimum wavelength) of continuous X-rays. It comes from the law of conservation of energy. The maximum kinetic energy of an electron accelerated by a voltage V is eV. When this energy is entirely converted into an X-ray photon, eV = hv_max. This gives v_max = eV/h and the corresponding minimum wavelength is λ_min = hc/(eV).

21. Discuss the Alpha decay process with example.
    Alpha decay is when an unstable nucleus emits an alpha particle (helium nucleus, $zH{e}^4$). The mass number decreases by 4 and the atomic number decreases by 2.
    Example: $g_2{U}^{238}\to g_{0}T{h}^{234}+zH{e}^4$

22. Consider a point charge +q placed at the origin and another point charge -2q placed at a distance 9m from the charge +q. Determine the point between the two charges at which electric potential is zero.
    Let the zero potential point be at a distance x from +q. The potential is $V=kq\mathrm{/}x+k(-2q)\mathrm{/}(9-x)=0$.
    This simplifies to $1\mathrm{/}x=2\mathrm{/}(9-x)\to 9-x=2x\to 9=3x\to x=3\text{m}$.
    So, the point is 3 m from the +q charge.

23. Derive the expression for resultant capacitance when capacitors are connected in series and in parallel.
    Parallel: The potential difference (V) is the same across all capacitors. The total charge is $Q=Q_1+Q_2+\mathrm{.}\mathrm{.}\mathrm{.}$.
    Since $Q=CV$, we get $C_{p}V=CV+C_{2}V+\mathrm{.}\mathrm{.}\mathrm{.},so{C}_{p}arallel=C_1+C_2+\mathrm{.}\mathrm{.}\mathrm{.}$. Series: The charge (Q) on each capacitor is the same. The total voltage is $V=V_1+V_2+\mathrm{.}\mathrm{.}\mathrm{.}$. Since $V=Q\mathrm{/}C$, we get $Q\mathrm{/}{C}_s=Q\mathrm{/}{C}_1+Q\mathrm{/}{C}_2+\mathrm{.}\mathrm{.}\mathrm{.},so1\mathrm{/}{C}_{s}eries=1\mathrm{/}{C}_1+1\mathrm{/}{C}_2+\mathrm{.}\mathrm{.}\mathrm{.}$.

24. Obtain the condition for bridge balance in Wheatstone's bridge.
    The Wheatstone bridge is balanced when no current flows through the galvanometer. This happens when the potential difference between the two mid-points is zero. The balance condition is given by the ratio: $P\mathrm{/}Q=R\mathrm{/}S$, where $P,Q,R,$ and S are the four resistances in the bridge.

25. Explain the types of emission spectrum.
    Continuous Spectrum: A wide, unbroken band of colors from violet to red, produced by incandescent solids, liquids, or dense gases. Line Spectrum: A series of discrete, bright lines of specific wavelengths against a dark background, produced by excited, low-pressure gases of elements. Band Spectrum: A series of closely spaced groups of lines (bands), produced by molecules.

26. Describe the Davisson-Germer experiment which demonstrated the wave nature of electrons.
    Davisson and Germer directed a beam of electrons at a nickel crystal. They observed that the scattered electrons showed a pattern of maxima and minima (diffraction), which is a wave property. The measured wavelength matched de Broglie's predicted wavelength for electrons ($\lambda =h\mathrm{/}p$), confirming their wave nature.

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