How the viva works
Most questions fall into four groups. Prepare a one-line answer for each group, for every experiment in your syllabus.
Before the experiment
- Aim and principle: what are you finding, and which law does it use?
- Apparatus: why this wire, this cell, this range?
During the experiment
- Procedure: why this step, and what happens if you skip it?
- Observations: least count, zero error, units, number of readings.
After the experiment
- Calculations: graph slope, mean value, significant figures.
- Result: percentage error and its causes.
Beyond the experiment
- Applications: where is this used in real life?
- Variations: what changes if you use a different material or setting?
Measuring instruments
What is the least count of an instrument?
It is the smallest value the instrument can measure accurately. For a vernier callipers it equals one main scale division minus one vernier scale division. For a screw gauge it is the pitch divided by the number of divisions on the circular scale.
What is zero error, and how is it corrected?
It is the reading shown when the jaws are closed and the true reading is zero. It can be positive or negative. The corrected reading is the observed reading minus the zero error, taken with its sign.
What is the pitch of a screw gauge?
It is the distance the screw moves along its axis in one complete rotation of the circular scale, usually 1 mm or 0.5 mm.
What is backlash error in a screw gauge?
It is the error caused by wear between the screw and nut threads, which gives a different reading when the screw is turned back instead of forward. It is avoided by always rotating the screw in one direction.
Why take readings at several places along a wire?
The wire may not have a uniform thickness or may be slightly oval. Measuring at different places and in two perpendicular directions, then taking the mean, gives a more reliable diameter.
Why is a ratchet used in a screw gauge?
It slips once the screw presses with a fixed, small force, so the object is not compressed and the reading is not changed by how hard the observer tightens it.
Electricity experiments
Meter bridge
Why is the null point best found near the middle of the wire?
The percentage error in the unknown resistance is least when the balance length is close to 50 cm, so adjust the known resistance until the null point lies between 40 cm and 60 cm.
Why is the wire made of manganin or constantan?
These alloys have a high resistivity and a very small temperature coefficient, so the wire’s resistance stays steady as it heats slightly during the experiment.
Why are the two resistances interchanged?
Interchanging them and finding the null point again helps cancel end-corrections due to the copper strips and connections. The mean of the two results is used.
Potentiometer
Why must the driver cell’s emf be greater than the emf being measured?
The potential drop across the whole wire must exceed the emf under test, or no balance point can be found on the wire.
Why is a potentiometer better than a voltmeter for measuring emf?
At balance no current flows through the test cell, so the reading equals its emf. A voltmeter draws current and reads a lower terminal voltage.
How can the sensitivity of a potentiometer be increased?
Lower the potential gradient by using a longer wire or reducing the driver current with the rheostat, so each centimetre of wire stands for a smaller potential difference.
Ohm’s law, resistivity and galvanometer
What does the V–I graph of a conductor show, and what is its slope?
For an ohmic conductor it is a straight line through the origin. The slope of the V against I graph gives the resistance. The graph curves if the wire heats up.
Why should the circuit be closed only while taking readings?
Continuous current heats the wire, changing its resistance and spoiling the readings.
How is a galvanometer converted into an ammeter or a voltmeter?
For an ammeter, a small shunt resistance S = IgG/(I − Ig) is connected in parallel. For a voltmeter, a large series resistance R = V/Ig − G is added.
Why is an ammeter connected in series and a voltmeter in parallel?
An ammeter must carry the circuit current and has very low resistance. A voltmeter measures the potential difference across a component and has very high resistance so it draws almost no current.
Optics and semiconductors
Lens and prism
How is the focal length of a convex lens found by the u–v method?
Measure the image distance v for several object distances u. Use 1/f = 1/v − 1/u for each pair, or plot 1/v against 1/u, whose intercepts on both axes give 1/f.
What is parallax, and why is it removed?
Parallax is the apparent shift between the image and a pin when the eye moves. When there is no relative movement the two are at the same position, which locates the image accurately.
What happens to the angle of deviation as the angle of incidence increases?
It first decreases to a minimum value, called the angle of minimum deviation, and then increases. At minimum deviation the refractive index is μ = sin[(A + δm)/2] / sin(A/2).
Diode and transistor
What is the knee or cut-in voltage of a diode?
It is the forward voltage below which the current is negligible. It is about 0.7 V for silicon and about 0.3 V for germanium.
Why is a resistor connected in series in forward bias?
Beyond the knee voltage the current rises steeply, and the resistor limits it to a safe value so the diode is not damaged.
Why is a milliammeter used for forward bias and a microammeter for reverse bias?
The forward current is in milliamperes, while the reverse saturation current is only a few microamperes.
What is a Zener diode used for?
It is operated in reverse breakdown, where its voltage stays almost constant over a wide range of current, so it is used as a voltage regulator.
Mechanics and properties of matter
Why is the amplitude of a simple pendulum kept small?
The formula T = 2π√(L/g) holds only for small angles, below about 15°. At larger amplitudes the motion is not simple harmonic.
Why is time noted for 20 to 30 oscillations?
It reduces the effect of the human reaction time error in starting and stopping the stopwatch, because that error is spread over many oscillations.
How is the length of the pendulum measured?
From the point of suspension to the centre of the bob, which means the length of the string plus the radius of the bob.
Why is the bob’s thread made of a thin, light, inextensible string?
A thick or stretchy thread changes the effective length and adds its own mass, so the pendulum no longer behaves like an ideal simple pendulum.
Common error sources and how to reduce them
| Experiment | Main error sources | How to reduce them |
|---|---|---|
| Vernier callipers | Zero error, parallax while reading, tight jaws | Note zero error and correct it, read the scale straight on, close the jaws gently |
| Screw gauge | Zero error, backlash, over-tightening | Turn in one direction, use the ratchet, measure at several points |
| Meter bridge | End-corrections, heating of wire, poor contacts, null point far from centre | Interchange R and S, close the key only for readings, clean plugs, keep l between 40 and 60 cm |
| Potentiometer | Weak or unsteady driver cell, uneven wire, jockey pressed hard | Use a fresh driver cell, tap the jockey, check for opposite deflections at the ends |
| Galvanometer / ammeter conversion | Inaccurate resistance box values, battery run-down | Check connections, use a steady source, take a mean of readings |
| Convex lens (u–v) | Parallax, index (end) error, reading distances from the wrong point | Remove parallax carefully, measure index error and correct it, keep the optical bench aligned |
| Prism (i–δ) | Pins not in line, blunt pins, locating the minimum deviation | Use sharp pins placed far apart, take readings at small intervals near the minimum |
| Diode characteristics | Temperature changes, wrong ammeter range, reading before the circuit settles | Use correct meter ranges, wait for steady readings, limit current with a series resistor |
| Simple pendulum | Large amplitude, reaction time, wrong length, air currents | Keep angle small, time many oscillations, measure length to the bob’s centre, avoid draughts |
Combining errors
For a product or quotient, the fractional errors add:
For a power Z = An, the fractional error is multiplied by n:
This is why the diameter of a wire, which appears squared in its cross-sectional area, must be measured with extra care.
Exam-day tips
- Know your own data. Examiners often ask about a reading in your observation table. Be ready to explain every value and unit.
- Answer in one or two sentences. Give the reason first, then stop.
- Don’t bluff. If you are unsure, say what you do know about the principle and reason from there.
- Learn the “why” of each precaution. “Why do we tap the jockey?” is far more common than “What is the formula?”.
- Mention the least count and the number of readings whenever you describe a measurement.
- Keep the practical file and graph paper neat. Presentation is part of the marks.
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