As a physics professor, I have two jobs. The first, obviously, is to help students understand physics. That makes me something of a coach. But I want to talk about my second job: evaluating what students understand about physics. You might call this grading them.

Evaluating a student’s understanding of a topic is like taking a measurement. However, it requires measuring something that is difficult to see. It’s not like I can stick a ruler into a student’s brain to determine the size of their physics stuff. Now, most teachers use indirect means, usually a multiple-choice test or an exam in which students work through a problem. These are poor measures of student understanding. Someone could simply guess, or flub the answer through a silly mistake.

So how can I accurately assess a student’s understanding of physics? Until someone invents a way of reading a student’s mind, I must do something else. I use a combination of written tests and video assessments.

What is a video assessment? Glad you asked. Let me explain. Suppose I want my student to demonstrate their understanding of the momentum principle. A written test might do that, but I give my students the chance to create a short video—less than five minutes—in which they solve a problem. They post it, send me a link, and I assign a grade. Students who don’t like their grade can keep sending me videos.

You will be surprised how quickly a short video conveys just what students know—or don’t know—about the subject. I can tell how well they understand a concept simply by looking at the problem they choose to solve. When students approach a new idea, they tend to pick trivial problems that require little more than plugging data into an equation. As their understanding of the material deepens, they tackle more complicated questions, or even make up problems on their own.

I’ll give you an example. During the second semester of introductory (calculus-based) physics, one idea I expect students to understand is the electric field due to a dipole (an electric dipole has a positive and negative charge very close to each other). In class, I discuss the derivation of the electric field due to a dipole along the axis of the dipole and perpendicular to the axis. The textbook covers this as well. Since there are a limited number of problems a student can solve to demonstrate an understanding of this topic, I suggest they submit a video in which they derive the electric field due to a dipole. But here is the cool part: Even though I cover this in class and the text book goes over it in detail, a video tells me within minutes if a student truly grasps the concept.

But wait! It gets even better. Making videos actually helps students. I know this, because a former student told me. “At first, I really didn’t like making videos for physics,” she said. “But after a while I realized that by making videos, I started learning physics. At first I was tempted to wait until I understood the idea to make a video, but this was a mistake. The videos aren’t just for assessment, they are for learning.”

Boom! I win. Every physics professor knows that the person who learns the most in class is the instructor. When students make a video, they are giving me a physics lesson. Each student is an an instructor of one—or a million, depending upon how many people watch the video.

Another benefit of having students make videos is it exposes them to more information. One common (and not-so-great) strategy students use when solving a problem is searching online for videos. Apparently they think finding just the perfect video will give them a thorough understanding of the topic, or at least an answer to the problem. But they often realize that many of these online solutions are bogus. Yes, it’s true—some of the stuff you find online is wrong. I know you are surprised by this.

By now you are surely thinking that you ought to use video assessments too. Great! Where do you start? Let me point you toward Any Rundquist—SuperFly Physics. He didn’t create video reassessments, but he’s inspired just about everything I’ve done. And check out Josh Gates—Newton’s Minons, because he has great stuff too. If you’d like to see how I do it in my class, this course page includes loads of info.

And to all you students, if you use a phone to shoot your video, please don’t use portrait mode.

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