Sense And Nonsense In Elementary Electricity And Magnetism

Today my students wrote the final exam in the first-year university course in electricity and magnetism (+ a two-week introduction to quantum physics at the end of the course) that I taught this past semester.

I reproduce the first question on the exam below. Worth 20% of the marks on the exam (each of the ten parts is worth 2%), each statement is either sensible or has some degree of nonsense, and each student must first say if the statement is true or false, and then back it up with an explanation of some sort (which might include diagrams or formulas), including a correction if the statement is false. (Each question is worth 1 mark, and the total number of marks on the exam is 50. Eight problems, worth 5 marks each, complete the exam.)

We practiced dealing with such statements in the course, so these types of situations have been practiced, and their existence on the final exam was advertised in advance. (Of course, the statements themselves were not made available in advance.)

I believe the best way to train critical thinking is to present students with a certain amount of nonsense, and ask them to sort it out for themselves. Enough of this and they will form the habit of carefully questioning what they read, see, and hear.

It’s not so much the T or F response that gets them a mark; what I’m after is the quality of their explanations. A correct decision about T or F followed by a non sequitur, even if it is correct, is worth no marks. (That is, if you decide that a statement is true, and it really is true, but then your explanation (while correct), has nothing to do with the given statement, or doesn’t grasp the error in a false statement, then you get zero, or at least not full marks.)

Some of the questions have clear errors in them, whereas others are deliberately muddled.

So here they are, for your pleasure. I’ll include answers in a few days (once marking is done!) in case they will be of interest to some readers.


a. If you cut a bar magnet in half, one half will be an isolated North pole, and the other half will be an isolated South pole.

b. In a normal household circuit, light bulbs are wired in parallel because less wire is needed, and therefore the cost is less.

c. Equipotential surfaces are regions of space where the electric field is constant.

d. The functioning of an electrical transformer can be explained using Faraday’s law of induction.

e. Two long, straight, parallel wires, each carrying a current I, do not exert a force on each other, because there is no force between parallel magnetic fields.

f. Electric currents create magnetic fields, and, similarly, magnetic currents create electric fields.

g. Electricity is produced at Niagara Falls by rotating loops of wire within a magnetic field.

h. Electrical power is sometimes transmitted across very long distances; the voltages used in these cases are very high so that it can reach customers faster.

i. In the photoelectric effect, when the intensity of the incident light is doubled, the kinetic energy of the ejected electrons is also doubled.

j. Passing electrons through a double-slit experiment proves that light is a swarm of particles.

Update: “Answers”/discussion is here.

(This post first appeared at my other (now deleted) blog, and was transferred to this blog on 22 January 2021.)