Class 12 Physics Practical Exam Guide 2027 - Viva Questions and Tips

In short: The Class 12 Physics practical exam is worth 30 marks — that's nearly half of what some students score in theory. The marks split is: 8 for experiments, 5 for the investigatory project, 5 for the practical record, 5 for viva, and 7 for two activities. Most students lose marks in viva and sources of error. This guide covers all prescribed experiments, the exact marks breakdown, top 30 viva questions with concise answers, and tips to maximize your practical score.

Marks Distribution — CBSE Class 12 Physics Practical (30 Marks)

Note: Actual marks split may vary slightly year to year. Check latest CBSE circular for exact breakup.

Complete List of Experiments

Section A: Experiments

Section B: Experiments

Activities (for Record and Demonstration)

Section A Activities:

1. Resistance of a wire changes with length
2. Resistance of a wire changes with area of cross-section
3. Resistance of a wire changes with material
4. Study of series and parallel combinations — using appropriate instruments
5. Identify polarity of LED and check if current flows in one direction

Section B Activities:

1. Observe refraction and lateral shift through a glass slab
2. Observe diffraction pattern through a single slit
3. Study nature and size of image from convex lens using a candle
4. Identify semiconductor diode, LED, transistor, resistor, capacitor from a mixed collection
5. Use multimeter to identify components and check continuity

Top 30 Viva Questions with Answers

Metre Bridge and Potentiometer:

Q1: What principle does the metre bridge work on? Wheatstone bridge principle. When the bridge is balanced: P/Q = R/S, no current flows through the galvanometer.

Q2: Why do we use a jockey instead of pressing the wire directly? A jockey makes a point contact with the wire, ensuring precise measurement of the balancing length. Pressing directly could damage the wire or give inaccurate readings.

Q3: Why should the null point be near the middle of the bridge wire? When the null point is near the middle (between 30-70 cm), the sensitivity of the bridge is maximum and percentage error in measurement is minimum.

Q4: What is the advantage of a potentiometer over a voltmeter? A potentiometer measures EMF accurately because it draws no current from the cell at the null point (null method). A voltmeter always draws some current, so it measures terminal voltage, not true EMF.

Q5: What is internal resistance of a cell? The resistance offered by the electrolyte and electrodes of the cell to the flow of current within the cell. It causes the terminal voltage to be less than EMF when current flows.

Q6: Why do we use a standard cell (Daniel cell) in potentiometer experiments? It has a known, stable EMF that doesn't change with time or temperature. This allows calibration of the potentiometer wire.

Q7: What is the sensitivity of a potentiometer? How can it be increased? Sensitivity is the smallest potential difference that can be measured. It is increased by using a longer wire or by decreasing the current through the wire (using a rheostat).

Optics:

Q8: What is the angle of minimum deviation? The smallest angle of deviation produced by a prism. At this angle, the refracted ray inside the prism is parallel to the base, and the angle of incidence equals the angle of emergence.

Q9: Why does white light split into colours through a prism? Because different wavelengths of light have different refractive indices in glass. Violet (shortest wavelength) deviates most, and red (longest wavelength) deviates least.

Q10: What is the difference between a real and virtual focus? Real focus: rays actually converge at the point (concave mirror/convex lens). Virtual focus: rays appear to diverge from the point (convex mirror/concave lens).

Q11: How do you find the focal length of a convex mirror? By using a convex lens. Place the convex mirror behind the lens, adjust until the image coincides with the object. The distance between the mirror and the image gives R, and f = R/2.

Q12: What is parallax? How is it removed? Parallax is the apparent shift in the position of an image when viewed from different positions. It is removed by adjusting the position of the image pin until there is no relative shift between the image and the pin when the eye is moved sideways.

Q13: What precaution should you take with the spectrometer? Level the spectrometer using the leveling screws. Focus the telescope on a distant object for parallel light. The prism should be placed centrally on the prism table.

Semiconductor Devices:

Q14: What is a p-n junction diode? A semiconductor device formed by joining p-type and n-type semiconductors. It allows current to flow easily in forward bias (low resistance) but blocks it in reverse bias (high resistance).

Q15: What is the knee voltage of a silicon diode? Approximately 0.7V. Below this voltage, negligible current flows. Above this, current increases rapidly.

Q16: What happens in the breakdown region of a Zener diode? In reverse bias, when voltage reaches the breakdown voltage, the current increases sharply but the voltage across the diode remains almost constant. This property is used for voltage regulation.

Q17: What is the function of a transistor in CE configuration? It acts as a current amplifier. A small change in base current (IB) produces a large change in collector current (IC). Current gain β = ΔIC/ΔIB (typically 50-300).

Q18: What is the difference between forward and reverse bias? Forward bias: positive terminal connected to p-side, depletion layer narrows, current flows. Reverse bias: positive terminal connected to n-side, depletion layer widens, no significant current (only leakage).

General Physics Viva:

Q19: What are systematic and random errors? Systematic errors are consistent, repeatable errors in one direction (faulty instrument, zero error). Random errors are unpredictable fluctuations in measurements (parallax, environmental changes).

Q20: What is the least count of your ammeter/voltmeter? Least count = smallest division on the scale. For ammeter: typically 0.01A or 0.05A. For voltmeter: typically 0.1V or 0.05V. Always state it when recording observations.

Q21: Why do we take multiple readings? To minimize random errors. The mean of several readings is more reliable than a single reading. Ideally, take at least 5 readings.

Q22: What is zero error? How do you correct it? When an instrument shows a non-zero reading even when it should read zero. Positive zero error: subtract the error from readings. Negative zero error: add the error to readings.

Q23: Name the colour code of resistors. BBROY: Black(0), Brown(1), Red(2), Orange(3), Yellow(4), Green(5), Blue(6), Violet(7), Grey(8), White(9). Tolerance: Gold(±5%), Silver(±10%).

Q24: What is Kirchhoff's junction rule? At any junction in a circuit, the sum of currents entering equals the sum of currents leaving. Based on conservation of charge.

Q25: Why is copper wire used in most electrical experiments? Copper has low resistivity (1.7 × 10⁻⁸ Ωm), is ductile, and a good conductor. It allows current to flow with minimal heat loss.

Additional Scoring Questions:

Q26: What is the principle of a galvanometer? A current-carrying coil in a magnetic field experiences a torque. The deflection is proportional to the current: I = (C/nBA)θ.

Q27: How do you convert a galvanometer to a voltmeter? Connect a high resistance in series with the galvanometer. V = Ig(G + R), where R is the series resistance.

Q28: What is total internal reflection? When light travels from a denser to rarer medium at an angle greater than the critical angle, it is completely reflected back into the denser medium. Basis of optical fiber.

Q29: What is the difference between EMF and terminal voltage? EMF is the potential difference across the cell when no current flows (open circuit). Terminal voltage is when current flows: V = E - Ir (less than EMF due to internal resistance).

Q30: What is the significance of the V-I graph being a straight line? It verifies Ohm's law. The slope of the V-I graph gives the resistance of the conductor. A straight line means the conductor is ohmic (resistance is constant).

Sources of Error — Mention These for Full Marks

Tip: Always mention 2-3 sources of error in your experiment answer. Examiners specifically look for this and it carries 0.5-1 mark.

Investigatory Project Ideas

Project Report Format:

1. Title page (name, class, roll number, school, year)
2. Certificate (teacher-signed)
3. Acknowledgement
4. Index
5. Introduction / Theory
6. Materials used
7. Procedure
8. Observations and Data
9. Calculations and Results
10. Conclusion
11. Bibliography

Frequently Asked Questions

Can I choose which experiment to perform?

No. The examiner assigns experiments through chits. You may get any combination — one from Section A (electrical) and one from Section B (optics/semiconductors). Prepare all experiments.

What if my readings don't match the expected values?

Record your actual readings honestly. Calculate the percentage error and mention possible sources of error. Examiners give marks for correct procedure, proper recording, and honest analysis — not just perfect results.

How much time do I get for the practical exam?

Typically 3 hours. This includes performing experiments, recording observations, completing calculations, and the viva. Plan your time: 45 min per experiment, 20 min for activities, 15 min for viva preparation.

Is the investigatory project presented on exam day?

Your project file is submitted and evaluated. The examiner may ask 1-2 questions about your project during viva. Be prepared to explain your project's objective, method, and conclusion.

How can I score 30/30 in practicals?

Complete lab record (all experiments + activities), neat and well-maintained. Perform experiments carefully with multiple readings. Know your sources of error. Prepare viva questions (top 30 above). Submit a good investigatory project. Attend all practical sessions throughout the year.

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