CBSE Class 12 Physics Practical Exam 2027: Experiments, Viva Questions, Marking Scheme

In short: CBSE Class 12 Physics practical exam carries 30 marks — a significant portion of your total score. This guide covers all prescribed experiments, circuit diagrams, observation tables, calculations, viva questions with answers, and expert tips to score full marks.

CBSE Class 12 Physics Practical Marking Scheme

Total Marks: 30

Experiment-wise Marking (8 marks each)

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List of CBSE Class 12 Physics Experiments

Section A: Experiments

Section B: Experiments

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Section A Experiments — Detailed Procedures

Experiment 1: Metre Bridge — Resistance Measurement

Aim: To find the resistance of a given wire using a metre bridge and hence determine the resistivity of its material.

Apparatus: Metre bridge, galvanometer, resistance box, jockey, one-way key, cell, unknown resistance wire, connecting wires

Circuit Diagram: Metre bridge with resistance box R in left gap, unknown resistance S in right gap, galvanometer connected to jockey.

Theory: At balance point: R/S = l/(100-l) Therefore: S = R(100-l)/l

Procedure:

1. Connect the circuit as per diagram
2. Take out suitable resistance R from resistance box
3. Touch jockey at different points on wire to find null point
4. At null point, galvanometer shows zero deflection
5. Note the balance length l
6. Repeat for different values of R
7. Calculate S for each reading

Observation Table:

Mean S = 3.98 Ω

Resistivity Calculation: ρ = RS × πd²/4L

Where: d = diameter of wire, L = length of wire

Precautions:

• Clean the wire ends before connecting
• Jockey should be pressed gently
• Null point should be between 30-70 cm
• Avoid heating of wire by passing current for short duration

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Experiment 2: Galvanometer Resistance by Half-Deflection Method

Aim: To determine the resistance of a galvanometer by half-deflection method.

Apparatus: Galvanometer, resistance box (2), cell, key, connecting wires

Theory:

• When full deflection θ is obtained with resistance R in circuit
• Half deflection θ/2 is obtained when shunt S is connected
• Galvanometer resistance G = RS/(R-S)

Procedure:

1. Connect galvanometer with high resistance R and cell
2. Adjust R to get full-scale deflection (θ)
3. Now connect shunt resistance box S in parallel with galvanometer
4. Adjust S to get half deflection (θ/2)
5. Note values of R and S
6. Calculate G = RS/(R-S)

Observation Table:

Mean G = 86.7 Ω

Precautions:

• Use high resistance R to protect galvanometer
• Take readings quickly to avoid heating
• Ensure tight connections

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Experiment 3: Resistors in Series and Parallel

Aim: To verify the laws of combination of resistances using a metre bridge.

Apparatus: Metre bridge, two resistance coils, resistance box, galvanometer, jockey, cell, key

Theory:

• Series: Rs = R₁ + R₂
• Parallel: 1/Rp = 1/R₁ + 1/R₂ or Rp = R₁R₂/(R₁+R₂)

Procedure:

1. Find individual resistances R₁ and R₂ using metre bridge
2. Connect R₁ and R₂ in series, find combined resistance Rs
3. Connect R₁ and R₂ in parallel, find combined resistance Rp
4. Verify with theoretical values

Observation Table:

Conclusion: Laws of combination of resistances verified within experimental error.

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Experiment 4: EMF Comparison Using Potentiometer

Aim: To compare the EMF of two cells using a potentiometer.

Apparatus: Potentiometer, two cells (E₁ and E₂), galvanometer, resistance box, jockey, key

Theory: E₁/E₂ = l₁/l₂

Where l₁ and l₂ are balancing lengths for cells E₁ and E₂.

Procedure:

1. Connect the potentiometer circuit with driver cell
2. Connect cell E₁ in secondary circuit
3. Find balance point, note length l₁
4. Replace E₁ with E₂, find balance point l₂
5. Calculate E₁/E₂ = l₁/l₂

Observation Table:

Mean E₁/E₂ = 1.475

Precautions:

• EMF of driver cell should be greater than cells being compared
• Positive terminals should be connected to same end
• Jockey should not be dragged on wire

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Experiment 5: Internal Resistance of Cell Using Potentiometer

Aim: To determine the internal resistance of a cell using a potentiometer.

Theory: r = R(l₁ - l₂)/l₂

Where:

• l₁ = balance length with cell in open circuit
• l₂ = balance length with resistance R across cell
• r = internal resistance

Procedure:

1. Find balance length l₁ with key K₂ open (open circuit)
2. Close key K₂ (resistance R in circuit), find balance length l₂
3. Calculate r = R(l₁ - l₂)/l₂
4. Repeat for different values of R

Observation Table:

Mean internal resistance r = 0.82 Ω

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Experiment 6: Galvanometer to Ammeter Conversion

Aim: To convert a galvanometer into an ammeter of given range and verify the same.

Theory: Shunt resistance S = Ig × G / (I - Ig)

Where:

• G = galvanometer resistance
• Ig = current for full-scale deflection
• I = desired range of ammeter

Procedure:

1. Find G and Ig of galvanometer
2. Calculate shunt S for desired range
3. Connect S in parallel with galvanometer
4. Verify using standard ammeter

Calculation Example:

• G = 60 Ω, Ig = 15 mA, I = 3 A
• S = (0.015 × 60)/(3 - 0.015) = 0.9/2.985 = 0.30 Ω

Verification Table:

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Experiment 7: Galvanometer to Voltmeter Conversion

Aim: To convert a galvanometer into a voltmeter of given range and verify the same.

Theory: High resistance R = (V/Ig) - G

Where:

• V = desired voltage range
• Ig = current for full-scale deflection
• G = galvanometer resistance

Procedure:

1. Find G and Ig of galvanometer
2. Calculate high resistance R for desired range
3. Connect R in series with galvanometer
4. Verify using standard voltmeter

Calculation Example:

• G = 60 Ω, Ig = 15 mA, V = 3 V
• R = (3/0.015) - 60 = 200 - 60 = 140 Ω

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Section B Experiments — Optics and Semiconductors

Experiment 9: Focal Length of Convex Mirror

Aim: To find the focal length of a convex mirror using a convex lens.

Apparatus: Convex mirror, convex lens, optical bench, object needle, image needle, screen

Theory:

• Convex mirror alone cannot form real image
• Use convex lens to form real image
• Place convex mirror to reflect rays back
• When image coincides with object, rays are normal to mirror
• Distance from mirror to image = R (radius of curvature)
• f = R/2

Procedure:

1. Mount convex lens on optical bench
2. Place object needle, adjust to get real image on other side
3. Place convex mirror between lens and image
4. Adjust mirror until image coincides with object (no parallax)
5. Distance from mirror to image position = R
6. f = R/2

Observation Table:

Mean f = 10.1 cm

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Experiment 11: Angle of Minimum Deviation

Aim: To determine the angle of minimum deviation for a given prism and calculate its refractive index.

Theory: n = sin[(A + Dm)/2] / sin(A/2)

Where:

• A = angle of prism
• Dm = angle of minimum deviation
• n = refractive index

Procedure:

1. Place prism on drawing board
2. Fix pins P₁, P₂ for incident ray at angle i
3. Look through prism, fix pins P₃, P₄ in line with refracted image
4. Measure angle of deviation (d)
5. Repeat for different angles of incidence
6. Plot i vs d graph, find minimum deviation Dm

Observation Table:

Minimum deviation Dm = 36°

Calculation:

• A = 60° (equilateral prism)
• n = sin[(60 + 36)/2] / sin(60/2)
• n = sin(48°) / sin(30°)
• n = 0.743 / 0.5 = 1.49

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Experiment 13: Characteristics of p-n Junction Diode

Aim: To draw the I-V characteristic curves of a p-n junction diode in forward and reverse bias.

Apparatus: p-n junction diode, voltmeter, milliammeter, microammeter, variable DC power supply, resistance, connecting wires

Forward Bias Procedure:

1. Connect diode in forward bias (p to +ve, n to -ve)
2. Increase voltage in steps of 0.1 V
3. Note current for each voltage
4. Plot V vs I graph

Forward Bias Observations:

Reverse Bias Observations:

Conclusions:

• Knee voltage (cut-in voltage) ≈ 0.6-0.7 V for silicon diode
• Forward current increases exponentially after knee voltage
• Reverse current is very small (microamperes)
• Diode acts as closed switch in forward bias, open switch in reverse bias

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Experiment 14: Characteristics of Zener Diode

Aim: To draw the characteristic curve of a Zener diode and determine its breakdown voltage.

Apparatus: Zener diode, voltmeter, milliammeter, variable DC power supply, resistance

Procedure:

1. Connect Zener diode in reverse bias
2. Increase voltage gradually
3. Note current at each voltage
4. Observe sharp increase in current at breakdown voltage

Observations:

Breakdown voltage Vz ≈ 5.5 V

Application: Zener diode is used as voltage regulator because voltage remains constant at Vz even when current changes.

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Experiment 15: Characteristics of Transistor

Aim: To study the input and output characteristics of a transistor in common emitter configuration.

Apparatus: NPN transistor, two variable DC power supplies, voltmeters, milliammeters, microammeter

Input Characteristics (VBE vs IB at constant VCE):

Output Characteristics (VCE vs IC at constant IB):

Current Gain (β): β = IC/IB = 3.0 mA / 40 μA = 75

Conclusions:

• Input characteristics similar to forward-biased diode
• Output characteristics show saturation region, active region, and cut-off region
• Current gain β typically 50-200 for common transistors

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Physics Practical Viva Questions and Answers

Metre Bridge and Potentiometer

1. What is the principle of metre bridge? Wheatstone bridge principle. At balance: P/Q = R/S

2. Why is the balance point kept between 30-70 cm? To minimize percentage error in measurement. End corrections are significant at extremes.

3. What are end corrections? Corrections for resistance of thick copper strips at ends of metre bridge wire.

4. Why is potentiometer preferred over voltmeter for EMF measurement? Potentiometer draws no current at balance point, so it measures true EMF. Voltmeter draws current, so it measures terminal voltage.

5. What is the sensitivity of a potentiometer? Fall of potential per unit length. Lower the potential gradient, higher the sensitivity.

6. Why should EMF of driver cell be greater than cells being compared? Otherwise balance point won't be obtained on the wire.

Galvanometer Conversions

7. Why is shunt resistance very low? To bypass most of the current, protecting the galvanometer.

8. Why is high resistance connected in series for voltmeter conversion? To limit current through galvanometer and extend voltage range.

9. What is figure of merit of galvanometer? Current required for unit deflection. Lower figure of merit means higher sensitivity.

10. Can we convert ammeter to voltmeter? Yes, by connecting high resistance in series.

Optics

11. Why can't we find focal length of convex mirror directly? Convex mirror always forms virtual image, which cannot be obtained on screen.

12. What is angle of minimum deviation? The angle of deviation when angle of incidence equals angle of emergence.

13. At minimum deviation, what is the path of ray inside prism? Parallel to the base of the prism.

14. Why does refractive index depend on wavelength? Different wavelengths travel at different speeds in the medium (dispersion).

Semiconductors

15. What is knee voltage? The forward voltage at which diode starts conducting significantly (~0.7V for Si, ~0.3V for Ge).

16. Why is Zener diode used in reverse bias? In reverse bias, it maintains constant voltage (breakdown voltage) for varying current.

17. What is the difference between avalanche and Zener breakdown? Zener: due to high electric field breaking covalent bonds (low voltage). Avalanche: due to collision ionization (high voltage).

18. What are the three regions in transistor output characteristics? Cut-off region (both junctions reverse biased), Active region (one forward, one reverse), Saturation region (both forward biased).

19. Why is common emitter configuration most used? It provides both current gain and voltage gain, giving high power gain.

20. What is the relation between α and β? β = α/(1-α) or α = β/(1+β)

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Tips to Score 30/30 in Physics Practical

Before Exam

1. Complete practical record with all experiments, diagrams, and calculations
2. Practice circuit diagrams — draw them neatly with proper symbols
3. Memorize formulas for all experiments
4. Understand the theory behind each experiment
5. Practice viva questions — they carry 5 marks

During Exam

1. Read the experiment carefully before starting
2. Draw circuit diagram first — it shows you understand the setup
3. Take multiple readings — minimum 5-6 for accuracy
4. Calculate carefully — show all steps
5. Write precautions — at least 4-5 points
6. Mention sources of error — shows deeper understanding

Common Mistakes to Avoid

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Frequently Asked Questions

How many experiments are asked in Class 12 Physics practical?

Two experiments — one from Section A (electricity) and one from Section B (optics/semiconductors). Each carries 8 marks.

Is the investigatory project evaluated in practical exam?

Yes, it carries 3 marks. You should have a complete project report ready with proper documentation.

What if my readings don't match expected values?

Small deviations are acceptable. Write your actual readings honestly. Examiners check your procedure and understanding, not just final values.

Can I use a calculator in practical exam?

Yes, calculators are allowed. Bring a scientific calculator for trigonometric calculations.

How important is the practical record?

Very important — it carries 6 marks. Ensure all experiments are complete with diagrams, observations, and teacher's signature.

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