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800 most-asked Class 11 & 12 (+1 / +2) questions across Physics, Chemistry, Maths and Biology — each with a model answer and the exact marking-scheme points examiners reward. Revise smart, walk in calm.

800 board questions•Model answers•Marking-scheme points•Every chapter•CBSE · ISC · State boards

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Class 11 Physics · +1Class 11 Chemistry · +1Class 11 Maths · +1Class 11 Biology · +1Class 12 Physics · +2Class 12 Chemistry · +2Class 12 Maths · +2Class 12 Biology · +2
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PhysicsClass 123 marksmedium

Electric Charges and Fields

Two point charges of 2 microcoulomb and 3 microcoulomb are placed 30 cm apart in air. Calculate the electrostatic force between them.

Reveal model answer + marking points▾

Given q1 = 2 x 10^-6 C, q2 = 3 x 10^-6 C, r = 30 cm = 0.3 m. Using F = k q1 q2 / r^2 = (9 x 10^9)(2 x 10^-6)(3 x 10^-6)/(0.3)^2 = (9 x 10^9 x 6 x 10^-12)/0.09 = 0.054/0.09 = 0.6 N. The force is repulsive since both charges are positive.

F = k q1 q2 / r^2

Marking-scheme points

  • ✓Convert r = 30 cm = 0.3 m
  • ✓F = (9e9 x 2e-6 x 3e-6)/(0.3)^2
  • ✓F = 0.6 N (repulsive)
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PhysicsClass 123 marksmedium

Electric Charges and Fields

Calculate the electric field intensity at a point 30 cm from a point charge of 5 microcoulomb in air.

Reveal model answer + marking points▾

Given Q = 5 x 10^-6 C, r = 0.3 m. The field due to a point charge is E = kQ/r^2 = (9 x 10^9)(5 x 10^-6)/(0.3)^2 = (45000)/0.09 = 5 x 10^5 N/C. The field is directed radially outward since the charge is positive.

E = kQ/r^2

Marking-scheme points

  • ✓E = kQ/r^2
  • ✓= (9e9 x 5e-6)/(0.3)^2 = 45000/0.09
  • ✓E = 5 x 10^5 N/C, directed radially outward
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PhysicsClass 123 marksmedium

Electric Charges and Fields

Define electric dipole moment. Derive the expression for the electric field at a point on the axial line of a short dipole.

Reveal model answer + marking points▾

An electric dipole is a pair of equal and opposite charges (+q and -q) separated by a small distance 2a. The dipole moment p = q x 2a, directed from the negative to the positive charge. On the axial line at distance r from the centre, the field due to +q is k q/(r-a)^2 (away) and due to -q is k q/(r+a)^2 (towards). Net E = kq[1/(r-a)^2 - 1/(r+a)^2] = kq[(4ar)/((r^2-a^2)^2)]. For a short dipole (r much greater than a), E = k(2p)/r^3, directed along the dipole moment.

E(axial) = 2kp/r^3

Marking-scheme points

  • ✓Dipole moment p = q x 2a (from -q to +q)
  • ✓Axial field = kq[1/(r-a)^2 - 1/(r+a)^2]
  • ✓For short dipole: E(axial) = 2kp/r^3
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PhysicsClass 123 marksmedium

Electrostatic Potential and Capacitance

Three capacitors of 2 microfarad, 3 microfarad and 6 microfarad are connected first in series and then in parallel. Find the equivalent capacitance in each case.

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