Class 12 Physics Most Important Questions + Answer | Trimasik Pariksha 2026 | MP Board Wallah

Main Ideas / Lessons Conveyed (Physics: Class 12 Important Questions)

1) Course context + exam-focused plan

The teacher begins a live class for the quarterly examination and explains that:

• The class timing was shifted slightly, and students weren’t informed earlier.
• Students should quickly inform/prepare for the quarterly exams.
• Approximately five chapters will be covered, listed as:
  1. Electric Charge (electric charge, electric field, dipole moment, electric flux, Gauss’s law, and applications)
  2. Electric Potential & Capacitance
  3. Magnetic Effects of Current
  4. Magnetism

He emphasizes that important exam topics repeat and these will be asked.

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Detailed Methodology / Instruction-Style Points

A) Chapter 1: Electric Charge — key definitions and formulas

Electric charge distribution

• Surface charge distribution: charge spread over a surface.
• Volume charge distribution: charge spread throughout a volume.

Volume charge density

• Concept: volume charge distribution represented as a density.

• Formula: [ \rho = \frac{q}{V} ]

• Exam note: the unit is highlighted as coulomb per meter (as stated).

Quantization of charge

• Charge comes in discrete units.
• Charge on any object/conductor is an integer multiple of electron charge.

• Electron charge magnitude: [ e = 1.6 \times 10^{-19}\ \text{C} ]

• Principle: [ Q = ne ] where ( n ) is an integer.

• Common exam numeric highlighted:

  • Number of electrons in 1 coulomb: [ 6.25 \times 10^{18} ]

• Unit emphasized: coulomb (C).

Coulomb’s law

For two point charges ( q_1 ) and ( q_2 ) separated by distance ( r ):

• Proportional form: [ F \propto \frac{q_1 q_2}{r^2} ]

• Full form: [ F = \frac{1}{4\pi\varepsilon_0}\frac{q_1 q_2}{r^2} ]

• Unit emphasized: newton

• Constant emphasized: [ \varepsilon_0 = 8.854 \times 10^{-12} ]

Axial position of an electric dipole (derivation-focused)

• Setup:
  • Dipole charges: ( +q ) and ( -q )
  • Center is ( O )
  • Separation from center to charges is ( l )
  • Point ( P ) is on the extended axis at distance ( d ) from the center
• Distances from point ( P ):
  • From ( +q ): ( d-l )
  • From ( -q ): ( d+l )

• Electric field due to each charge uses: [ E = \frac{1}{4\pi\varepsilon_0}\frac{q}{(\text{distance})^2} ]

• Direction handling:

  • Field due to ( +q ) is outward
  • Field due to ( -q ) is inward
• Final combined expression emphasized: [ E \approx \frac{1}{4\pi\varepsilon_0}\frac{4qdl}{(d^2-l^2)^2} ]

Electric flux

• Formula: [ \Phi = E\, dS \cos\theta ]

• Unit emphasized (as stated): coulomb per newton·meter² (and/or meter² form).

Gauss’s theorem

• Core statement:

  • Total flux over any closed surface: [ \Phi = \frac{q_{\text{inside}}}{\varepsilon_0} ]

• Typical derivation setup:

  • Point charge ( +q ) at center ( O )
  • Spherical Gaussian surface of radius ( r )
  • For spherical symmetry: ( \cos\theta = 1 )

• Calculation highlights:

  • Flux: [ \Phi = \int \vec{E}\cdot d\vec{S} ]

  • For spherical symmetry:

    • [ E = \frac{1}{4\pi\varepsilon_0}\frac{q}{r^2} ]

    • [ \int dS = 4\pi r^2 ]

  • Result: [ \Phi = \frac{q}{\varepsilon_0} ]

Deriving Coulomb’s law from Gauss’s theorem

• Start with point charge at center and spherical Gaussian surface.

• Use:

  • Flux form (aligned direction): [ \Phi = E\, dS ]

  • Gauss theorem: [ \Phi = \frac{q}{\varepsilon_0} ]

• Substitute sphere area ( 4\pi r^2 ) to solve for ( E ).

• Force on test charge: [ F = q_{\text{test}}E ]

• Final result emphasized: [ F = \frac{1}{4\pi\varepsilon_0}\frac{q_{\text{test}}\, q}{r^2} ]

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B) Chapter 2 / Related Topics: Current, Resistance, Cell

Ohmic vs non-ohmic resistance

• Ohmic:
  • Obeys Ohm’s law
  • ( V\text{–}I ) graph is a straight line
• Non-ohmic:
  • Does not obey Ohm’s law
  • ( V\text{–}I ) graph is not a straight line

Ohm’s law (requirements)

• Relationship: [ V \propto I ]

• Conditions: length, cross-sectional area, and temperature must remain constant.

Internal resistance of a cell

• Meaning:
  • Internal resistance is the obstruction to current inside the cell.
• Factors affecting it:
  • distance between electrodes
  • concentration of electrolyte
  • temperature
  • immersed area of electrodes

• Using series-resistance method:

  • Treat cell emf as ( \mathcal{E} ), internal resistance ( r ), external resistance ( R )

  • Apply:

    • externally: [ I = \frac{V}{R} ]

    • cell voltage: [ V = \mathcal{E} - Ir ]

• Internal resistance result emphasized: [ r = \frac{\mathcal{E}R}{V-R} ]

Trailing drift velocity (drift velocity derivation)

• Setup:
  • Conductor of length ( l ) and area parameter (named ( a ) in subtitles)
  • Electrons drift due to applied electric field
• Direction note:
  • Electric field is opposite to electron motion.

• Key steps:

  • Electric force on electron: [ F = qE ] (electron charge is negative)

  • Newton’s second law: [ F = ma ] giving acceleration: [ a = \frac{-eE}{m} ]

  • Equation of motion: [ v_d = u + a\tau ]

  • Interpret:

    • ( u ) = thermal velocity (taken as 0 in the model)
    • ( \tau ) = time between collisions
  • Emphasized result: [ v_d \propto \frac{E\tau}{m} ] with sign handled using electron nature.

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C) Electric field / magnetic field concepts and vector rules

Electric field line direction

• Electric field lines go from positive to negative (2-mark point).

Two properties of electric field lines

• They do not intersect.
• They may be straight or curved (and can extend infinitely).

Equipotential surfaces

• Potential is constant at every point on an equipotential surface.
• Electric field lines are perpendicular to equipotential surfaces.
• Angle between electric field lines and equipotential surface:
  • 90°

Kirchhoff’s laws

• KCL (Kirchhoff’s Current Law):
  • At a junction, algebraic sum of currents = 0
  • Connects to conservation of charge
  • In-going current = out-going current
• KVL (Kirchhoff’s Voltage Law):
  • In a closed loop, sum of potential drops across resistors = total emf
  • Example phrasing: corresponding element relation like ( IR = E )
  • Connects to conservation of energy

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D) Capacitance and Capacitors

Capacitance definition

• Potential increases with added charge: ( V \propto q )

• Definition: [ C = \frac{q}{V} ]

• Unit: farad

Spherical capacitor

• Setup:
  • Two concentric spheres: inner radius ( a ), outer radius ( b )
• Charges:
  • inner sphere: ( +q )
  • outer sphere’s inner surface: ( -q )

• Capacitance result emphasized: [ C = \frac{4\pi\varepsilon_0 ab}{b-a} ]

• Unit emphasized again (subtitle text appears garbled).

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E) Magnetism Chapter: Magnetic Effects of Current

Biot–Savart law

• Magnetic field due to a current element: [ dB \propto \frac{I\, d\ell \sin\theta}{r^2} ]

• Full form conceptually includes ( \mu_0/4\pi ): [ dB = \frac{\mu_0}{4\pi}\frac{I\, d\ell \sin\theta}{r^2} ]

• He emphasizes it as the most important topic of the chapter.

Ampere’s law

• Core statement:
  • Line integral of magnetic field around a closed loop equals ( \mu_0 ) times enclosed current.
• Exam tip:
  • Typically solved with both LHS and RHS shown.

Moving charges produce fields

• He states that a moving charge produces both magnetic and electric fields.

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F) Wheatstone Bridge (Important Question)

Equilibrium condition

• Four resistors arranged in a diamond/quadrilateral.
• Galvanometer placed in one diagonal.
• At equilibrium:
  • Galvanometer current ( I_G = 0 )
• Ratio condition: [ \frac{P}{Q} = \frac{R}{S} ]

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G) Ammeter / Voltmeter / Shunt (Exam-prone instrumentation)

• Ammeter
  • Measures current
  • Placed in series
  • Resistance: zero (as stated)
• Voltmeter
  • Measures potential difference
  • Connected in parallel
  • Resistance: infinite (as stated)
• Shunt
  • Low resistance wire across galvanometer to protect it from burning/damage.

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H) Paramagnetic / Diamagnetic / Ferromagnetic Topics Checklist

He lists likely exam points:

• Difference between paramagnetic, diamagnetic, ferromagnetic
• HV/angles-type concept (as written in subtitles: “H V I by Theta …”; likely angle/susceptibility relation)
• Soft iron and steel
• Magnetic field
• Neutral/indifferent position (as stated: “negative situation / indifferent position”)

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Sources / Speakers Featured

• Single main speaker (teacher): an unnamed “Sir” (subtitles also mention names like Pushpa Latha Pandey, Anjali Patel, Roop Narayan, Khushi, Bunty, Raunak, etc., but these appear to be students/participants rather than separate presenters).
• Students / participants mentioned: Khushi, Bunty, Raunak, Nakul, Pushpa, Raunak Sahu, Shalini Prajapati, Nitin, Harish, Neetu, Visha, Arjun, Lalita, Vishal, Mahesh, Aashiyana, Pihu, Harshit, Suresh, Karan, Santosh, Om, and others (as called out in subtitles).