Viva questions for engineering: how vivas are scored, the 20 every examiner asks, and branch-wise lists

What is the aim of this experiment, in your own words?

Sir, the aim was to find the Young's modulus of the given steel wire by Searle's method: we loaded the wire in steps, measured the extension with the spherometer, and used the slope of the load-extension graph with the wire's length and diameter to get the modulus.

Say the aim as a sentence, not as the record's heading. The examiner is checking you understood the purpose.

What is the principle behind it?

Hooke's law: within the elastic limit, stress is proportional to strain, and Young's modulus is the ratio of the two. Stress is load over cross-sectional area, strain is extension over original length, so the modulus comes from load, extension, length and area.

Explain the formula and the meaning of each term.

Y equals M g L over pi r squared times l. M is the load in kilograms, g the acceleration due to gravity, L the original length of the wire, r its radius, and l the extension. The unit is newtons per square metre, pascal, and for steel it is of the order of two into ten to the eleven.

Why did you use this apparatus or this method and not another?

Searle's apparatus uses a reference wire alongside the experimental wire, so any change in temperature or sag of the support affects both and cancels out. A single-wire method would include those errors in the extension reading.

What is the least count of the instrument you used, and how did you find it?

The spherometer's least count was 0.01 millimetre: the pitch, one millimetre for one full rotation, divided by the hundred divisions on the circular scale. The micrometer screw gauge for the diameter had the same least count. I always write the least count before the readings.

What are the sources of error in this experiment?

Backlash error in the spherometer, which I avoided by turning it in one direction only; the wire not being perfectly uniform, so I took the diameter at three places; and kinks in the wire, which is why we loaded it first to straighten it before the zero reading.

Which reading in your table looks suspicious, and why?

The fourth loading reading, sir. The extension jumped by more than the step before and after. I think the wire slipped slightly in the chuck or the load was still swinging when I read it. That is why I used the mean of loading and unloading and the graph's slope rather than a single pair.

Why does the graph pass or not pass through the origin?

In theory it should, because zero load gives zero extension. Mine has a small positive intercept because the initial straightening of the wire adds a little extension at the first load. The slope is unaffected, which is why we take the modulus from the slope and not from one point.

If I change one condition, what happens?

If the wire's diameter were doubled, the area becomes four times, so for the same load the extension would be one-fourth; the modulus stays the same because it is a property of the material. If the load went beyond the elastic limit the graph would curve and Hooke's law would no longer apply.

What is the difference between accuracy and precision, in your readings?

Precision is how close repeated readings are to each other; my three diameter readings were within 0.02 millimetre, so they were precise. Accuracy is how close the result is to the true value; my modulus was about eight percent above the standard value, so it was precise but not fully accurate, probably from a zero error.

What precaution matters most in this experiment, and why?

Loading and unloading in equal steps and waiting for the wire to settle before each reading, because a reading taken while the load swings gives a random extension. On a machines or high-voltage bench the equivalent is checking the supply is off before touching connections.

Where is this used in practice?

Young's modulus is what a structural engineer uses to predict how much a beam or a cable will stretch under load: the steel cables of a suspension bridge, the rods in reinforced concrete, the shaft of a motor. Choosing a material starts with this number.

What is the standard or code that governs this test?

For tensile testing of metals the Indian Standard is IS 1608, which is aligned with ISO 6892. In the lab we did not test to the full standard, but the specimen shape, the loading rate and the reporting of the result follow the same idea.

What result did you get, and does it match the expected value?

I got 2.16 into ten to the eleven pascal; the standard value for mild steel is about two into ten to the eleven. The eight percent difference is within what I expect from the least count of the diameter reading, because radius is squared in the formula and that doubles any error in it.

What did you actually do in this experiment, as opposed to your partner?

I took all the spherometer readings and plotted the graph; my partner measured the diameter and the length. We swapped for the next experiment. I can explain both parts, and the calculation in the record is mine.

What went wrong, and how did you fix it?

The first set of readings did not come back to zero on unloading, which meant the wire had slipped. We re-clamped it, loaded once to straighten it, reset the zero and repeated the set. That is why there are two tables in my record.

What is the limitation of this method?

It works only within the elastic limit and for a thin wire that can be loaded safely; it cannot give the yield or ultimate strength. For those you need a universal testing machine, which is the next experiment in the strength of materials lab.

How would you improve this experiment?

Use a digital dial gauge or an extensometer in place of the spherometer to remove backlash and reading error, and take the diameter with a digital micrometer at five points. A longer wire would also give larger extensions for the same load and reduce the relative error.

Explain this term or this line to me.

Sir, this is the pi r squared term, the cross-sectional area of the wire. I used the radius, half the measured diameter, and squared it, which is why I was careful with the diameter: a two percent error in diameter becomes a four percent error in area.

What did you learn from this experiment?

That the slope of a graph is a more reliable result than any single reading, and that the measurement with the smallest least count, the diameter, dominates the error because it is squared. I now check which term in a formula is most sensitive before I start taking readings.

Digital electronics viva: what is the difference between combinational and sequential circuits?

A combinational circuit's output depends only on the present inputs, like an adder or a multiplexer. A sequential circuit's output depends on present inputs and the previous state, so it has memory, like flip-flops, counters and registers. A clock usually drives sequential circuits.

Digital electronics viva: why are NAND and NOR called universal gates?

Because any logic function, AND, OR, NOT and therefore every circuit, can be built using only NAND gates or only NOR gates. In the lab I built a NOT gate by tying both NAND inputs together and an AND by following a NAND with that NOT.

Digital electronics viva: what is the difference between a latch and a flip-flop?

A latch is level-triggered: it passes the input while the enable is high. A flip-flop is edge-triggered: it changes state only at the rising or falling clock edge, which makes timing predictable. That is why counters and registers use flip-flops.

Digital electronics viva: what is a race-around condition and how does a master-slave JK flip-flop remove it?

In a JK flip-flop with J and K both high, the output toggles repeatedly while the clock is high, which is the race-around condition. The master-slave version lets the master change while the clock is high and the slave copy it when the clock goes low, so the output toggles exactly once per clock.

Digital electronics viva: what is the difference between a multiplexer and a decoder?

A multiplexer selects one of many inputs and sends it to a single output using select lines, like an 8-to-1 mux with three select lines. A decoder takes an n-bit code and activates one of two-to-the-n outputs, like a 3-to-8 decoder used for address selection in memory.

Digital electronics viva: what is the difference between a synchronous and an asynchronous counter?

In a synchronous counter all flip-flops get the same clock, so they change together and it is fast. In an asynchronous or ripple counter each flip-flop is clocked by the previous one's output, so the change ripples through with a delay that limits the maximum frequency.