I’ve started using Socrative for assessing students’ short response answers. It serves a couple of purposes. One, I gain valuable insight into what each student does and doesn’t understand. Two, students see each other’s responses, critique them, and see examples of strong responses. Today, one of the questions I posed was from Knight’s College Physics: “The n = 3 state of hydrogen has E3 = -1.51 eV. Why is the energy negative? What is the physical significance of the specific number 1.51 eV?”
The responses clearly demonstrated that there was a substantial number of students who attributed the 1.51 eV as the difference in energy between the ground state and n=3 instead of the difference in energy of the electron between n=3 and infinity.
If I had asked someone to volunteer and answer the question, I never would have realized how few students understood!