I strongly recommend this lecture series. Walter Lewin is a fine teacher, goes to great lengths to try to explain the principles, and does some fascinating experiments. However as my understanding improves, I am finding errors in some of the explanations (though obviously not in the actual equations).

I sought to inform MIT of a particular error, but as expected they did not want to know, and did not respond to my email. So I am posting the evidence here. You might want to watch the first 4 minutes of the following lecture before reading on:

I also I sent this email to Walter Lewin:

One of the great things about your lectures, is that you succeed in making the viewer see beyond the equations, to the physical principles. But that does mean there is the potential for error, and I think your explanation at the start of 8.02 lecture 7, is incorrect. I would suggest the following addition to the lecture notes:

“At the start of the lecture, Professor Lewin discusses the work done in moving apart the plates of a capacitor where the charges are fixed. He correctly states that the work done is equal to half the electric field between the plates, times the charge on each plate, times the distance moved. However his explanation for the factor of a half, is incorrect and confusing. The correct explanation is as follows:

The electric field in a capacitor, is by definition the force on a test charge placed between the two plates. If an electron was placed between the two plates, it would feel a force towards the positive plate, due to: a). The attraction from the positive plate, and b). The repulsion by the negative plate. Forces a). and b). of course being equal, as the charge on each plate is equal in magnitude.

However when the Professor winds the negative plate away from the positive plate, the force he is opposing is only due to a). the attraction of the negative plate towards the positive plate. Clearly the negative plate is not repelling itself towards the positive plate. Hence the factor of a half.”

Whilst this is only a small part of a generally excellent set of lectures; the first few times I watched the lecture, the fact that your explanation went completely over my head, left me thinking that my understanding was severely lacking, which made the rest of the lecture harder to follow. I suspect other viewers may have had similar problems.

He did reply, thanking me for the suggestion. However that does not tell me whether he agrees that his lecture contained an error, or whether he was not interested and was just replying out of politeness.

It certainly seems the errors are few and far between; but if you have found any others, please leave a comment. The only other substantial misunderstanding I have found, is in Walter Lewin’s description of how a battery cell functions, during the first 7 minutes of the lecture below. I have been unable to find a proper explanation for a cell online, therefore I am working on it, and will add my own when I have fully figured it out, so bookmark this page if you are interested. At least Walter Lewin had the courage to try to produce a proper explanation, but he actually ends up getting much of the logic round his neck.