Board Boosters
The questions your board exam loves to ask
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.
PhysicsClass 122 markseasy
Electric Charges and Fields
State Coulomb's law of electrostatics and write its mathematical form.
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Coulomb's law states that the force of attraction or repulsion between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them, acting along the line joining them. Mathematically, F = k q1 q2 / r^2, where k = 1/(4 pi epsilon0) = 9 x 10^9 N m^2 C^-2 in free space.
F = k q1 q2 / r^2
Marking-scheme points
- ✓F is proportional to product of charges and to 1/r^2
- ✓F = k q1 q2 / r^2 along the line joining them
- ✓k = 1/(4 pi epsilon0) = 9 x 10^9 N m^2 C^-2
The concept
Coulomb's law gives the electric force between two charges: it grows with charge and falls with distance squared.
Why it matters
It's the electrostatic backbone of atoms, capacitors and all of electronics.
💡 Memory trick
Like gravity but for charge: F = k q1 q2 / r^2. Same charges repel, opposite attract.
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.
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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)
PhysicsClass 122 markseasy
Electric Charges and Fields
Define electric field intensity at a point. State its SI unit.
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The electric field intensity at a point is the force experienced by a unit positive test charge placed at that point. It is a vector quantity given by E = F/q0, where q0 is the small positive test charge. Its SI unit is newton per coulomb (N/C) or equivalently volt per metre (V/m).
E = F/q0
Marking-scheme points
- ✓E = force per unit positive test charge = F/q0
- ✓Vector quantity, directed along the force on a positive charge
- ✓SI unit: N/C or V/m
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.
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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.
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Electric Charges and Fields
Derive the expression for the torque acting on an electric dipole placed in a uniform electric field.
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Electric Charges and Fields
State Gauss's law in electrostatics and write its mathematical form.
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Electric Charges and Fields
Using Gauss's law, derive the expression for the electric field due to an infinitely long straight uniformly charged wire.
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Electrostatic Potential and Capacitance
Define electric potential at a point. State its SI unit and write the expression for the potential due to a point charge.
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Electrostatic Potential and Capacitance
Calculate the electric potential at a point 9 cm away from a point charge of 4 microcoulomb in air.
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Electrostatic Potential and Capacitance
Write the relation between electric field and electric potential. What does the negative sign indicate?
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Electrostatic Potential and Capacitance
Define capacitance of a conductor. State its SI unit.
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Electrostatic Potential and Capacitance
Derive the expression for the capacitance of a parallel plate capacitor with air between the plates.
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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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Electrostatic Potential and Capacitance
Write the expression for the energy stored in a charged capacitor in three equivalent forms.
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Electrostatic Potential and Capacitance
How does the introduction of a dielectric slab between the plates of a capacitor affect its capacitance? Explain.
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Current Electricity
State Ohm's law. Define resistance and give its SI unit.
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Current Electricity
Define resistivity of a material. How does the resistance of a wire depend on its length and area of cross-section?
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Current Electricity
What is drift velocity? Derive the relation between current and drift velocity.
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Current Electricity
Three resistors of 2 ohm, 3 ohm and 6 ohm are connected in parallel. Find their equivalent resistance. What would it be in series?
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