This question differs from the more usual TC calculation questions so I am not quite sure what the examiners expected. How much this was due to the desire to "ring the changes" and how much it is the examiners not risking putting dodgy numbers into the question, we may never know!
There has been a pretty broad hint that any TC calc question in 2012 can be expected to be unlike the traditional ones.
I think I'd have interpreted the question a little differently from you-
1. I'd have made the initial diagram more PHYSICALLY representative. Your feed and relay ends are arguably ok (but I'd have depicted rail leads closer to the IRJs, track cable from loc links). Certainly I'd have shown the train shunt as an axle with contact resistance and the ballast resistance as being a result of a large number of sleepers holding the two rails with some imperfect insulation from rail to rail and also each rail to earth. This would have depicted more of the physical reality and a greater contrast to the electrical representation (that was how I' d have shown) and the place to record some assumptions such as negligible resistance in the track connections or along the length of plain line rail.
2. I also think the question was asking you to propose some specific values for relay resistance, operating current, percentage drop-away, ballast resistivity, feed voltage etc. This is what I think the question means by "including the electrical parameters" and certainly reinforced by wording at beginning of the 2nd part of the question.
Hence although you show ability with Ohms Law, Kirchoff Law and manipulating equations, when you get to your answer as case 1 and case 2, I am left wondering what you have actually demonstrated to me; does it really show how the relay reliably picks and drops?
Perhaps some words here would have been beneficial in supporting your claim, but I'd have made things a bit simpler and more obvious by using some typical numbers that I think was the examiners' intention.
In contrast the 3 lines at the end of page 4 were very good start for the last part of the question (although I feel that some reference should have been made to why the wet condition of the ballast affects resistance presented). You have produced an equation for the current through the relay as a function of the ballast resistance and identified the other terms as constants, but to me without giving an indication of the relative magnitudes then your answer fell short of actually "showing the effect".
Overall I just felt that you gave too much in the way of mathematical expressions (it was quite a long answer to produce in the time), but not enough signal engineering in terms of making it "real" by putting in some typical numbers.
Despite that I thought that overall it was a reasonable interpretation of the question, you certainly showed ability to do the calculations required so I think a definite Pass or even perhaps a Credit. However I guess that the examiners were actually looking for something a little different and they may think you made a meal of certain elements but skimped on the explanation and showing an intuitive understanding.
I may be biased as I have never really liked maths; it'd be interesting to hear Peter's view who may differ.
There has been a pretty broad hint that any TC calc question in 2012 can be expected to be unlike the traditional ones.
I think I'd have interpreted the question a little differently from you-
1. I'd have made the initial diagram more PHYSICALLY representative. Your feed and relay ends are arguably ok (but I'd have depicted rail leads closer to the IRJs, track cable from loc links). Certainly I'd have shown the train shunt as an axle with contact resistance and the ballast resistance as being a result of a large number of sleepers holding the two rails with some imperfect insulation from rail to rail and also each rail to earth. This would have depicted more of the physical reality and a greater contrast to the electrical representation (that was how I' d have shown) and the place to record some assumptions such as negligible resistance in the track connections or along the length of plain line rail.
2. I also think the question was asking you to propose some specific values for relay resistance, operating current, percentage drop-away, ballast resistivity, feed voltage etc. This is what I think the question means by "including the electrical parameters" and certainly reinforced by wording at beginning of the 2nd part of the question.
Hence although you show ability with Ohms Law, Kirchoff Law and manipulating equations, when you get to your answer as case 1 and case 2, I am left wondering what you have actually demonstrated to me; does it really show how the relay reliably picks and drops?
Perhaps some words here would have been beneficial in supporting your claim, but I'd have made things a bit simpler and more obvious by using some typical numbers that I think was the examiners' intention.
In contrast the 3 lines at the end of page 4 were very good start for the last part of the question (although I feel that some reference should have been made to why the wet condition of the ballast affects resistance presented). You have produced an equation for the current through the relay as a function of the ballast resistance and identified the other terms as constants, but to me without giving an indication of the relative magnitudes then your answer fell short of actually "showing the effect".
Overall I just felt that you gave too much in the way of mathematical expressions (it was quite a long answer to produce in the time), but not enough signal engineering in terms of making it "real" by putting in some typical numbers.
Despite that I thought that overall it was a reasonable interpretation of the question, you certainly showed ability to do the calculations required so I think a definite Pass or even perhaps a Credit. However I guess that the examiners were actually looking for something a little different and they may think you made a meal of certain elements but skimped on the explanation and showing an intuitive understanding.
I may be biased as I have never really liked maths; it'd be interesting to hear Peter's view who may differ.
(29-05-2012, 01:48 PM)hopkin Wrote: Here attach my attempt to 2011 Qn5. Any comment or suggestion.
PJW