Textbook picture: Is direction of this eddy current wrong?

[greypilgrim]

Hi.

This is from a high school textbook, in a chapter about eddy currents and Lenz's law. In a), a magnetic field is turned on, in b) it's turned off, which both induces a current in a suspended aluminium ring:

I think the direction of the current is wrong in both pictures. Any opinions?

 

[Philip Wood]

Agree with you. Flux (broken lines) due to induced current (if circuit were complete) is in right direction in both cases, but induced current is in wrong direction in both cases.

Other minor nitpicks: (1) Since the conductor is a single loop, not a solenoid, the flux due to the induced current won't be parallel to axis throughout the cross-section, (2) I'd have thought also that the flux lines tightly wrapped around the conductor ought to be broken rather than full, but maybe I'm being stupid here.

Pity, because the artwork is so beautiful.

 

[256bits]

Does the author state the I ( cuurent ) protocol. Perhaps it refers to electron flow.

 

[greypilgrim]

Does the author state the I ( cuurent ) protocol. Perhaps it refers to electron flow.

I've been using this book for almost two years now and only realized now that they indeed indicate the flow of negative charges, i.e. electrons. They don't write the $I$ arrow often, in the DC chapter they write only + and - poles which make it clear (obviously this is not possible for induced currents), which is why it probably took me so long.

I think I see this convention in an increasing number of "modern" school textbooks, probably to make it easier for students to use their mental picture of moving electrons. But are there major university level textbooks or publications that do this as well? None that I know of and I highly doubt there are, since this would mean redefining current density as well and therefore introducing minuses in the fourth Maxwell equation and the continuity equation, which would just look weird.

So the confusion will start at the latest when those students enter university and probably much earlier when they try and find explanations or exercises on the internet where the other convention is used. I don't think those "modern" textbooks are doing the students much favor...

 

[Philip Wood]

I don't think those "modern" textbooks are doing the students much favor...

Agree entirely. In my experience students learning electricity for the first time quickly get used to electrons moving in the opposite direction to the conventional current in metals. It's only a minor and temporary nuisance.