Your Favourite Electrical Engineering College Course Question

Question

Hello, this is my first post. I&rsquo;m interested in hearing about electrical engineering assignment questions you encountered during college or sixth form that were particularly engaging, really made you think and therefore memorable. Or if you could design one such question now, what would it be?

mapj1 · Accepted Answer

built 1920/30's 
 Well if it is as-built, you might start by refusing the cup of tea, as it probably has lead pipes, and that may also save you needing to use the outside loo... It's not the age of the property, it's what has been done to it since, and of course, why you have been asked to attend in the first place. But we all know I hope that "real world" questions confuse both candidates and examiners and are to be avoided. ;-) 
 I do remember a nasty exam one where two capacitors at different starting voltages are connected by a switch, and to calculate the final voltage. The trick is to realize that charge (C.V)on either side of the insulators is redistributed but conserved, but energy (1/2 CV^2) is not - in fact it never is, the only question is where it goes, and there is always a loss somewhere but not one actually shown in the question.. There is a related variation question about switching inductors - where the momentum like quantity L.I is conserved but 1/2 LI^2 energy is not. 
 Actually anyone who has stared sadly at the burnt points on a car with conventional ignition can testify where that energy goes to, or indeed the larger version when the older London underground trains change winding configuration as they pick up speed and there is a loud "pock" and blue flash and in that case it feels like actual momentum is not always conserved either.... Mike.

Andy Millar · Answer

The one which always seemed pointless at the time, but then I actually probably found I used the most in my career, were the ones along the lines of "a box with four terminals, with resistive components inside, has the following voltage and current readings when a voltage of X is applied to these terminals with short circuit / open circuit across these terminals, draw the equivalent circuit". At the time I remember thinking "or just open the box and look inside?" but then spent years using Thevenin, Kirchhoff and Norton more than any other theorems other than Ohm's law, and realising why we were given that question over and over again... 
 I even nearly used it last week at home, I'm making some tweaks to a passive pre-amp on my stereo and needed to know what value pots I'd used on the volume and balance controls, and what the equivalent resistances were at the volume we normally listen at, and was remembering that I could work all that out without opening the box. But then cheated and opened the box anyway! (To solve the somewhat more basic problem of the mains switch, which switches all the AV equipment, which had finally burnt out after 30-odd years for the reasons Mike and Philip mention! It's a switch and a chunky relay now.)

AJJewsbury · Answer

oops - so it was. 
 Might have to look into the circumstances a bit more, but with 7/029 having a c.s.a. of neatly 3mm&sup2; - directly embedded or clipped direct (no thermal insulation in original 1920s houses) - it must be good for at least 30A - so an extra 2/3rds of 10A-ish shouldn't be an issue in practice. If you're relying on the BS 3036 for overload protection and have to take the 0.725 factor into account (or the "course" ratings as they were) it does start to look a bit slimmer (but then is the extra load available to make an overload feasible?) 
 On the other hand if the three sets had different orientations (e.g. one East, one South and one West) so they'd never be generating full whack at the same time, the whole question might be moot. 
 Or just say it's not a BS 7671 matter? 
 What would you say if you found two 3kW appliances plugged into the same double socket? 
 - Andy.

Philip Oakley · Answer

mapj1 said: I do remember a nasty exam one where two capacitors at different starting voltages are connected by a switch, and to calculate the final voltage. The trick is to realize that charge (C.V)on either side of the insulators is redistributed but conserved, but energy (1/2 CV^2) is not - in fact it never is, the only question is where it goes, and there is always a loss somewhere but not one actually shown in the question.. 
 I love that one (now). 
 The hidden question is to name all 5 components... Such as the inductor, and the antennae, and perhaps the arc lamp (in the switch), and perhaps its oscillation frequency.. (wire resistance being only half a mark) 
 Reminds me of an old Scottish Power story from Jim Toal about a switch (South Gyle IIRC) that was always burning out from arcing, and when he attended, asked the surprised lineman to connect a spare drum of cable to the switch. 
 Turned out that the switch was lightly loaded, and when interrupted the resonant frequency was high enough that the arc never had time to quench. Adding the extra capacitance of the unterminated drum of cable drop the frequency enough to stop the arcing. Proper fixes were planned later...

Alan Currie · Answer

I recall the demonstration of the Wheatstone bridge, or Kirchhoff's theory. I liked how electromagnetic force could be explained with physics and vectors, forces similar to other theory like when considering a sine wave for AC. That or the VIR triangle which is so invaluable if you have to undertake a practical maintenance role. I still remember the current as the push, and therefore more dangerous with voltage being seen as the pressure exerted on the internal of the cable. It reminds me of parallel and series circuits now, haha. Not sure if all that reminiscing helps you much?

Philip Oakley · Answer

A subtly different question was when another student asked about forbidden bands in semi conductors and I pursued the point because of a fob-off answer. 
 How can you have a probability of an electron being in a place where it cant be? Lots of fun. Sill have my university notes. I even took a set into work once because it came up in reality (about staring and sustaining voltages for sparks and arcs in arc lamps.

Your Favourite Electrical Engineering College Course Question

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