ISC Class 12 Physics Important Questions 2027 (Repeated in Board Exams)

Obtain an expression of intensity of electric field at a point lying on the axis of an electric dipole.

An electric dipole is kept in a uniform electric field E with its axis making an angle θ with the field. Show forces acting on each charge and hence prove that torque acting is given by τ = pE sinθ

Arrangement of an oxygen ion and two hydrogen ions in a water molecule is shown in Figure 4. Calculate electric dipole moment of water molecule. Express your answer in terms of e (charge on hydrogen ion), l and θ.

A tiny bird carrying a charge of q = 8 × 10^-17 C dives vertically down towards a large positively charged metallic plate kept on a table in a horizontal position. What should be the surface charge density (σ) of the plate so that the bird stops in its flight, close to the plate? (Mass of the bird = 50g)

In case of a short electric dipole, what is the locus of a point having zero potential?

Four-point charges Q₁ = +17·7μC, Q₂ = −8·85μC, Q₃ = −17·7μC and Q₄ = 35·4μC are kept as shown in Figure 1 below. Calculate electric flux emanating from the closed surface S.

Obtain an expression for equivalent capacitance C when three capacitors C₁, C₂ and C₃ are connected in series.

Obtain an expression for intensity of electric field at a point in broadside position of an electric dipole.

A hollow sphere of radius R has a point charge q at its centre. Electric flux emanating from the sphere is X. How will the electric flux change, if at all, when (a) radius of the sphere is doubled? (b) charge q is replaced by an electric dipole?

Using Kirchhoff's laws of electrical networks, calculate the current I₃.

An electric current (I) flowing through a metallic wire is gradually increased. The graph of heating power (P) developed in it versus the current (I) is: (options (a)-(d) are the graphs shown)

The drift velocity of electrons in a conductor is v_d = -eEτ/m. How does the drift velocity change with a change in the potential difference across the conductor?

The current (I)-voltage (V) graphs for a conductor are given at two different temperatures T₁ and T₂. (a) At which temperature T₁ or T₂ is the resistance higher? (b) Which temperature (T₁ or T₂) is higher?

Draw a labelled diagram of an arrangement used in Physics laboratory to compare emfs of two given cells X and Y.

In the circuit shown in Figure 5, the central zero galvanometer G shows no deflection when the key K is closed. Calculate the value of resistor R₁.

Figure 2 shows a part of Wheatstone bridge circuit. With the help of Kirchhoff's loop rule, calculate the current flowing through the 4Ω resistor.

In a potentiometer, a cell is balanced against 110 cm when the circuit is open. A cell is balanced at 100 cm when short-circuited through a resistance of 10 Ω. Find the internal resistance of the cell.

Study the two circuits shown in Figure 6. The cells in the two circuits are identical. The resistance of R is the same in both. If the same current flows through R in both circuits, calculate the internal resistance of each cell in terms of the resistance R.

A copper wire of length 'l' is connected to a source. If the copper wire is replaced by another copper wire of the same area of cross-section but length '4l', how will the drift velocity change? Explain your answer.

(a) Find the value of current 'I' in Figure 2. (b) State the conservation principle on which your calculation is based.

When a battery is connected between the terminals P and Q, as shown in Figure 1, it is found that no current flows through 5Ω resistor. Then, the value of resistor X is: (a) 10Ω (b) 20Ω (c) 25Ω (d) 45Ω

State the condition for a balanced Wheatstone bridge.

Figure 5 shows a battery consisting of three cells. Calculate: (a) emf of the battery. (b) internal resistance of the battery.

Calculate the radius of the circular path described by the electron (v = 8×10⁷ ms^-1, B = 1·3×10^-3T).

Using Ampere circuital law, obtain an expression for magnetic field 'B' at a point at a perpendicular distance 'r' from a long current carrying conductor.

A galvanometer of resistance 100 Ω gives a full-scale deflection for a potential difference of 200 mV. What must be the resistance connected to convert the galvanometer into an ammeter of the range 0-200 mA?

The variation of magnetic susceptibility (x) with absolute temperature (T) for a diamagnetic substance is: (options (a)-(d) are the graphs shown)

A student records data for the magnitude (B) of the magnetic field at axial points at different distances x from the centre O of a circular coil of radius a carrying current I: B = B₀ at x=0, B₀/(2√2) at x=a, B₀/(5√5) at x=2a, B₀/(10√10) at x=3a. Verify (for any two) that these observations agree with the expected theoretical values of B.

An electron moving along positive X axis with a velocity of 8×10⁷ ms^-1 enters a region having uniform magnetic field B = 1·3 × 10^-3 T along positive Y axis. Explain why the electron describes a circular path.

Obtain an expression for magnetic field (B) at the centre of a circular coil having 'n' turns of radius 'R' when it is carrying a current 'I'.

Figure 6 shows a long straight conductor X carrying a current I₁. A point P is at a perpendicular distance 'r₁' from it. How much force does conductor X exert on a short conductor Y of length 'l' which is carrying a current 'I' and is kept at point P parallel to the conductor X?

In Figure 2, CD is an infinitely long and straight metallic wire carrying a current I. Point P is at a perpendicular distance r from it. Which combination of I and r gives maximum magnetic field at point P, pointing into the plane of the paper? (a) I = 4A towards C, with r = 0·500 m (b) I = 4A towards D, with r = 0·250 m (c) I = 8A towards C, with r = 0·125 m (d) I = 8A towards C, with r = 0·050 m

A circular coil having N turns of radius R carrying a current I produces a magnetic field B at its centre O. If this coil is opened and rewound such that the radius of the newly formed coil is 2R, carrying the same current I, what will be the magnetic field at the centre O? (a) 2B (b) B (c) B/2 (d) B/4

An alternating emf E = 5·0 Sin(314t)V is applied to a circuit containing a resistor connected in series with an unknown component X. The current in the circuit is I = 3·0 Sin(314t − π/3) A. (a) Identify the component X. (b) Calculate rms value (I_rms) of the current. (c) Find the frequency of the source. (d) Calculate power factor. (e) Find the impedance (Z) of the circuit.

Figure 9 shows a metallic rod MN of length l = 3m moving in an external uniform magnetic field B = (1/π) T directed into the plane of paper. The position of the rod changes with time as x = πt (x in metre, t in second). (1) Calculate the motional emf developed in the rod. (2) Name the law used to find the direction of induced current.

Three students X, Y and Z studied variation of A.C. with frequency in a series LCR circuit using an AC source of same emf and inductance of the same value, obtaining resonance curves. (a) Who used minimum resistance? (b) In which case will the quality Q factor be maximum? (c) What did the students conclude about the nature of impedance at resonant frequency (f₀)?

State any one difference between a transformer used at the power generating station and the transformer in a city which is used to supply power to households.

An ideal capacitor is connected across 220V, 50Hz, and 220V, 100Hz supplies. Find the ratio of current flowing through it in the two cases.

An ac generator generating an emf e = 300 sin (100πt)V is connected to a 100Ω resistor. Calculate the average power consumed by the resistor.

What is motional emf?

Explain why a blue coloured spark appears in a switch contact when a circuit containing a load like a heater, geyser or an electric iron is switched off.

When a coil is connected to a 200V dc supply, the current flowing through it is 1A. However, when it is connected to the 200V, 50Hz ac supply, the current is 0·5A. (1) Explain why current flowing is less when the coil is connected to an ac supply. (2) Calculate coefficient of self-inductance (L) of the coil.

A 20cm length of a long iron cored solenoid has 25 turns. If the area of cross section of the solenoid is 1 × 10^-4 m², calculate its coefficient of self-inductance (L). (Relative permeability of iron = 3000.)

State why soft iron is preferred to steel as a material for the core of a transformer.

When current flowing through a solenoid decreases from 15A to 0 in 0·2s, an emf of 30V is induced in it. Calculate the coefficient of self-inductance of the solenoid.

Name the electromagnetic radiation that has been used in obtaining the image below.

Name the electromagnetic wave travelling from the satellite to the dish antenna shown in the image above.

Which physical quantity is the same for UV rays, red light and radio waves when they are travelling in vacuum?

A student studies details of a microorganism with the help of an instrument. Name the instrument used by him.

With reference to the lens maker's formula: Apply the formula (expression) of refraction at a single spherical surface to (a) refraction at first spherical surface. (b) refraction at second spherical surface.

Draw a labelled ray diagram of an image formed by this instrument (compound microscope), assuming (a) a small upright object. (b) image lies at least distance of distinct vision.

A bi-convex lens of focal length f₁ is placed in air (Figure 3a). The radii of curvature of its first and second surfaces are R and 2R respectively. The lens is cut along the plane CD. Compare the focal length f₂ of the resulting lens (Figure 3b) with that of the original lens (Figure 3a).

Assertion: The focal length of a convex mirror will increase, if the mirror is placed in water. Reason: The focal length of a convex mirror is equal to half its radius of curvature. (a) Both Assertion and Reason are true, and Reason is the correct explanation for Assertion. (b) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion. (c) Assertion is true and Reason is false. (d) Assertion is false and Reason is true.

Why are giant telescopes of reflecting type? Give any one scientific reason.

A thin convex lens of power 5D is kept in contact with a thin concave lens of power 8D. Calculate the focal length of the combination.

The image shows a compound microscope. Which one of the two lenses L₁ (near the eye/view) or L₂ (near the object) has a larger focal length?

A monochromatic ray of light incident on one refracting surface of an equilateral prism suffers a deviation as shown in Figure 7. Calculate the refractive index of the material of the prism.

'Since prism B (minimum deviation 47°) has larger minimum deviation than prism A (34°), the student claims it must also have the greater dispersive power.' Is this statement always correct? Justify your answer.

[context: an optical sensor for a drone: LED light passes through a convex spherical refracting surface (R = 20 cm, air μ₁ = 1·0 to glass μ₂ = 1·5), then a thin biconvex lens of focal length 10 cm, then strikes a concave mirror of radius of curvature 30 cm focusing on a photodetector] If the drone operates in a highly humid or misty environment where a film of water completely coats the lens surfaces, how will the refractive power of this lens change?

For any prism, show that refractive index 'n' of its material is given by: n = sin((A+δ_m)/2) / sin(A/2), where the terms have their usual meaning.

The focal length of a double convex lens is equal to the radius of curvature of either surface. What is the refractive index of its material?

What is meant by a thin prism?

The two parts are arranged as shown in Figure 4B. What is the focal length of the final combination?

When a beam of white light is incident on a prism, the prism: (a) only disperses the incident light. (b) only deviates the incident light. (c) deviates as well as disperses incident light. (d) neither deviates nor disperses incident light.

For refraction at convex spherical surface of denser medium η₂ surrounded by a rarer medium of refractive index η₁, derive a formula connecting object distance u, image distance v, and radius of curvature R for the object in rarer medium and a real image formed inside the denser medium.

What is the ability of an optical instrument to form enlarged images called?

Name any two phenomena which take place in the formation of a rainbow.

Teacher: Two students performed the same Young's double slit experiment. One student obtained wider fringes. What is the possible reason for this observation? Student: The student who obtained wider fringes must have increased the distance between the two slits. State whether the answer is correct or incorrect; correct it with a brief explanation.

A physics teacher drew a ray diagram (Young's double slit, Figure 8) and asked: There is a point P on the screen where the path difference between two interfering light waves is exactly 1·2 × 10^-6 m. If the light emitted by the source has a wavelength of 600 nm, will a bright or a dark fringe form at P? Student: Dark fringe will be obtained. State whether the student's answer is correct or incorrect; correct it with a brief explanation.

Use Huygen's wave theory to prove Snell's law.

Using Huygen's wave theory and a labelled diagram, show that angle of reflection (r) is equal to angle of incidence (i).

In Young's double slit experiment, show that fringe width ω (fringe separation) is given by ω = λD/d where the terms have their usual meaning.

What will be the change in the interference pattern if the given light is replaced with monochromatic light of wavelength 500nm?

With what type of source of light are plane wavefronts associated?

In a single slit Fraunhofer diffraction experiment, how does the angular width of central maximum change when the slit width is increased?

What is the essential condition for obtaining a sustained interference?

Two identical rectangular slits 5mm apart are illuminated with a monochromatic light of wavelength 600nm. The screen is kept 1·2m away from the slits. Calculate the distance between the 5th bright fringe on one side and the 3rd bright fringe on the other side of the central bright band.

If the photon energy in image B = 2·8 eV (work function 2·3 eV), calculate the maximum kinetic energy of the emitted photo electrons.

What happens when an electron collides with a positron?

Study the images shown in Figure 9 (light incident on three surfaces A, B, C; all three surfaces have same work function = 2·3eV). Why are no photoelectrons emitted in image 'A'?

When can a gamma ray photon produce a proton and an antiproton pair?

Two charged particles having same charge but different masses are passed through a potential difference V. When V is varied, de-Broglie wavelength (λ) of the particles varies as shown in graphs. Which graph is for heavier particles and why?

How does stopping potential in photoelectric emission vary if the intensity of the incident radiation increases?

State the conclusion that was drawn from Davisson-Germer's experiment.

What is 'Pair production'?

Plot a labelled graph of stopping potential (V_s) versus frequency (f) of incident UV radiation.

State how the value of Planck's constant can be determined from this graph (stopping potential vs frequency).

If kinetic energy of moving electrons is made four times, then their de Broglie wavelength becomes: (a) eight times. (b) four times. (c) two times. (d) half.

What is meant by the following statement? 'Angular momentum of an orbiting electron is quantised.'

Study the binding energy per nucleon versus mass number graph. Indicate which region corresponds to: (1) Nuclei prone to fission. (2) Nuclei prone to fusion. (3) Most stable nuclei.

Figure 1 is the Energy level diagram for Hydrogen atom. Study the transitions shown and answer: (a) State the type of spectrum obtained. (b) Name the series of spectrum obtained.

Find the energy released during the fission reaction {}²³⁵₉₂U + {}¹₀n → {}²³⁶₉₂U → {}⁹⁰₃₆Kr + {}¹⁴³₅₆Ba + 3{}¹₀n, given masses {}²³⁵U = 235·0439u, {}⁹⁰Kr = 89·9195u, {}¹⁴³Ba = 142·9206u, {}¹n = 1·0087u.

Prove that 1u = 931·5 MeV where the terms have their usual meaning.

What are isotones?

In an atom X (energy levels E₁ = 0·00eV, E₂ = 1·78eV, E₃ = 2·95eV, ground state 0 eV for reference), electrons absorb energy exciting them to higher levels and emit photons on returning. What wavelength of radiation is needed to excite the atom to energy level E₂ from E₁?

Show that radius (r_n) of the nth Bohr orbit varies directly with square of the principal quantum number (n) of the orbit.

Write Bohr's equations for circular motion of an electron around the nucleus and conservation of angular momentum. Using them, obtain an expression for orbital velocity of an electron.

During Rutherford's gold foil experiment, most alpha-particles did not deflect, however some showed a deflection of 180°. What hypothesis was made to justify the deflection of alpha-particle by 180°?

A group of students visited Bhabha Atomic Research Centre and observed reactors (Apsara a swimming pool type reactor; water slows fast neutrons and absorbs heat). (1) What is the use of a nuclear reactor? (2) What is the name given to a material which slows down fast moving neutrons? (3) State how a nuclear reactor can be shut down in case of an emergency.

According to Bohr's theory of hydrogen atom, calculate (1) angular momentum of the electron in second Bohr orbit. (2) radius of the third Bohr orbit.

Find the binding energy per nucleon of {}²³⁵U given mass of neutral {}²³⁵₉₂U = 235·0439u, mass of a proton = 1·0073u, mass of a neutron = 1·0087u.

Radius of the first Bohr orbit of hydrogen atom is 0·05nm. Calculate the radius of the third orbit.

Identify the phenomena that occur when a P-N junction diode is forward biased. (P) The width of the depletion layer decreases. (Q) The height of the potential barrier is lowered. (R) The effective resistance of the diode increases significantly. (S) The reverse current flows mainly due to the diffusion of majority charge carriers. (a) Both (P) and (Q) (b) Both (R) and (S) (c) Both (P) and (S) (d) Both (Q) and (R)

Explain why resistance of a semiconductor material decreases when its temperature is increased.

In an energy band diagram of a certain material, forbidden band is absent. Identify the material.

What is a solar cell?

In which of the following figures is the p-n diode forward biased? (a) Only (P), (Q) and (S) (b) Only (R) (c) Only (P) and (R) (d) Only (Q) and (S)

What is meant by potential barrier?

How did Annie obtain an extrinsic semiconductor from an intrinsic semiconductor?

N-type semiconductor is that which has: (a) majority of holes as charge carriers. (b) majority of free electrons as charge carriers. (c) trivalent element added as an impurity. (d) an equal number of holes and free electrons.

With reference to a semiconductor diode, define the following term: depletion region.

Draw a labelled diagram of a full wave rectifier. Show graphically how its output voltage varies with time.