# Generating buy-in is an ongoing process

I had a great moment in my calculus-based introductory E&M course today. I had spent the last few minutes explaining with great enthusiasm and great clarity (according to all the nodding I was seeing) how the electric field due to an infinite sheet of charge does not depend on the distance from the sheet of charge. I pointed out the progression of the 1/r2 dependence for the point charge, the 1/r1 dependence for an infinite line of charge, and the 1/r0 = constant dependence for an infinite sheet of charge. I argued that you could easily see that it was constant because the electric field lines have a constant density (neither converge nor diverge) no matter how far you get away from the sheet of charge.

Then I asked them the following clicker question…

Clicker question from Knight – Physics for Scientists and Engineers

…and 2/3rds of them incorrectly chose option D which says that the electric field due to this sheet of charge changes with distance from the sheet.

And I was delighted! This created a great moment to generate some more buy-in for the methods I use in the course.

I will roughly paraphrase what I said to them. I’m pretty certain it was more enthusiastic and less coherent when I was saying it in class:

Before this clicker question, you all sat there nodding in agreement with me as I explained this idea to you. And then when I turned around and asked you to apply the concept on a fairly straight-forward question, 2/3rds of you did not answer the question correctly. This right here highlights exactly why I run this course in such an interactive way. Until you have had to wrestle with the concept and make the understanding your own, you can easily fool yourself into thinking that you have learned the idea.

Today was a great day.

1. bwfrank says:

Totally Cool. One thing I’ve seen David Hammer do in between two questions like this is explicitly ask a clicker question that asks students to rate how well they think they understand that idea.

• Joss Ives says:

I think it was Andy that really likes using cards instead of clickers for the reason that he can ask them to use the height to which they raise their answers to indicate their confidence in their answer.

2. bretbenesh says:

This is fantastic. On the first day of class, I often use the line: “How many of you have ever told a professor, ‘I understand everything well in class, but then I struggle when I try to do the homework on my own.'” This normally gets vigorous head-nodding.

• Joss Ives says:

That’s a really good example. I need to file that away for later

3. Ido says:

So interesting! I had a different misconception in mind, one that is not reflected in the answers – but perhaps I am too far from being a student.
I thought that they would assume that b and d have a smaller field, since they are not on the lines. Kinda like negative interference. I wonder whether they would have chosen a > c = e > b = d.
Ah, students work in mysterious ways…

• Joss Ives says:

Hi Ido. Good catch on there being no distractors for that common misconception. It’s a big one that is often quite challenging to unstick. I think this is a clicker question that I blindly copied from somewhere that blindly copied it from the Knight text and somewhere along the way that distractor got dropped. I will have to fix that up in the future.