Exam feedback from Physics lecturersSemester 2 2009-10

Unit name and number: PH20014 / 61: Electromagnetism 1

Lecturer: RJ (1-4), PAS (5-10)

Report:

RJ: A mixed bag of answers, overall. There were some excellent scripts and some not so good, and plenty in the middle.

Q1: Many very good answers. A few just gave a sentence reading out the maths, not offering a meaning.

Q2: There were only a few perfect answers to this question, though plenty more that showed the right ideas. Virtually everyone understood that simple geometry and arithmetic are enough to solve this. Most knew that no flux crosses the face containing Q. Quite a few got in a muddle by thinking that the Greek letter must always be the total flux. Another common mistake was to assume that all 5 faces carry the same flux - they don't. Finally, some didn't realise that only half the total flux passes through the cube.

Q3: (a) Answered well by many, though too many failed to explain what they were doing - things like defining what A and V are were often missing. (b) Also done well by many. Some said nothing about why the divergence of Ampere's Law is an issue. Others worked back from the answer. (c) Answered less well overall. There were a lot of vague answers, many of which only really repeated things that were in the question.

Q4: (a) Virtually everyone got the mark for this one. (b) A lot of people messed up the starting equation for U. Other common mistakes were to miss that B0 isn't the amplitude in this case, and to forget that average and instantaneous values differ by a factor of 2. (c) Most could write down Malus's Law, but very few realised that is the angle between E and k, and this question gave you B and k.

PAS: Q5. Book work mostly done well. For (a) the intention was that one relation, involving the three relevant vector fields, would be given for (i) and (ii). Some candidates gave two relations in parts (i) and (ii). This wasnt necessary but wasnt penalised.

Q6. This application of Amperes law and integration to find the stored energy is not hard. The details were discussed in a lecture, worked through in a problems class, and in the worked solutions to problem sheets. These are all valid sources for learning course material. Course content to be revised is not limited to lecture notes.

Q7. Bookwork. Note question says provide the words or short sentences of explanation. If in the proof it is easy to observe that J has been replaced by E it is not an EXPLANATION to give an answer without any words of comment at all -J = E. An appropriate answer (remember 2 minutes work per mark): Assuming that the conductor obeys Ohms law so that J = E, where the conductivity of the metal is . I also think that it is nave for any candidate to think that writing down a repetition of a self-evident fact from the question can earn a mark in a level 2 exam!

Q8. Basically book work. Again, READ THE QUESTION, which says that Indicating the effect of and . Unlabelled vague shapes not showing the effects of and lost marks.

Q9. See Comments for Q6 there are worked examples and question sheets with answers for help with solving this question on Moodle. Common mistakes were getting the D/E relationship wrong or misunderstanding the geometry. The interface between the materials is z=0. The k-component of the field is perpendicular (normal) to this interface. If candidates miscalculated fields in (a) then they werent penalised for using these results in (b). Part (c) uses a routine result from Fresnel equations. A good number of candidates were able to do this.

Q10. Mostly done well but part (b) seemed hard for some candidates to answer. Not sure why clear sub-part of relevant lecture.