CBSE Physics XII
Assertion–Reason
Mastery Guide
A complete framework for solving every A–R question with confidence. Learn the 4 cases, avoid common traps, and master the 10-second method.
The 4 Answer Types
A
Both True + Correct Explanation
Reason directly explains the Assertion
B
Both True + Wrong Explanation
True statements but logically independent
C
A True, R False
Assertion is correct, Reason is factually wrong
D
A False, R True
Assertion is wrong, Reason is factually correct
Detailed Case Studies
Assertion (A)
Kinetic energy of photoelectrons depends on frequency of incident light.
Reason (R)
According to Einstein's equation, $KE = h\nu - W_0$
1
Is Assertion true?
Yes. KE depends on frequency, not intensity.
2
Is Reason true?
Yes. That equation directly shows KE depends on $\nu$.
3
Does Reason explain Assertion?
Yes — the equation proves the statement directly.
✓
Correct Answer: A
💡
Teaching tip: If Reason contains a formula that directly proves Assertion, it's usually option A.
Assertion (A)
Net electric flux through a closed surface can be zero.
Reason (R)
Electric field inside the surface may be zero.
1
Is Assertion true?
Yes — if no net charge is enclosed inside.
2
Is Reason true?
Yes — the field can be zero in some cases.
3
Does field being zero explain zero flux?
No. Flux depends on enclosed charge, not the field value at every point. These are independent ideas.
✓
Correct Answer: B
💡
Teaching tip: If both statements are correct but talk about different underlying ideas, it's usually option B.
Assertion (A)
Drift velocity of electrons in a conductor is very small.
Reason (R)
Because electrons move randomly at high speed.
1
Is Assertion true?
Yes — drift velocity is extremely small (~mm/s).
2
Is Reason true?
No. Random thermal speed is high, but drift velocity is slow because of the electric field effect, not random motion. The reason mixes two concepts.
✓
Correct Answer: C
💡
Teaching tip: If the Reason mixes two different concepts or uses one to incorrectly explain the other, it's often false — option C.
Assertion (A)
Magnetic force on a charge is maximum when velocity is parallel to the magnetic field.
Reason (R)
$F = qvB\sin\theta$
1
Is Assertion true?
No. If velocity is parallel to $\vec{B}$, then $\theta = 0$, so $\sin 0 = 0$ and $F = 0$. Force is actually zero, not maximum.
2
Is Reason true?
Yes — the formula $F = qvB\sin\theta$ is correct.
✓
Correct Answer: D
💡
Teaching tip: Whenever the assertion uses words like "maximum" or "always", check the edge cases very carefully.
Common CBSE Trap Questions
Electric Field Inside Conductor
A: Electric field inside a conductor in electrostatic equilibrium is zero.
R: Charges redistribute themselves on the surface.
Redistribution directly causes the internal field to become zero.
✓ Answer: A
Intensity Trap
A: Increasing intensity increases kinetic energy of photoelectrons.
R: Intensity increases number of photons.
KE depends on frequency, not intensity. Assertion is false.
✓ Answer: D
AC Phase — Pure Inductor
A: In a pure inductor, current lags voltage by 90°.
R: Inductive reactance opposes change in current.
The opposition to current change is exactly what causes the 90° lag.
✓ Answer: A
Resonance
A: Current is maximum at resonance in an LCR circuit.
R: Impedance is minimum at resonance.
$I = V/Z$ . Minimum $Z$ directly gives maximum $I$.
✓ Answer: A
The 10-Second Method
Ask yourself these 3 questions in order:
1
Is the Assertion scientifically correct?
2
Is the Reason scientifically correct?
3
If both correct → Does the Reason directly explain the Assertion?
Both true
+ formula/cause
explains directly
A
Both true
but statements
are independent
B
Assertion true
Reason is
factually wrong
C
Assertion false
Reason is
factually correct
D
High-Frequency Topics in CBSE
Master these 7 topics and your A–R accuracy goes up drastically.
Photoelectric Effect
Magnetic Force Direction
Gauss's Law
AC Phase Difference
Drift Velocity
Logic Gates
p-n Junction