The Oral Exam Experiment Worked

Last fall I posted that I was dropping homework from the grade book and adding an oral portion to every exam in the spring. Students were running Engineering homework problems through Gemini and handing in solutions they could not explain when I asked. I was grading a chatbot, so I stopped.

Spring semester is over. It worked. The oral portion runs about ten minutes per student. They pick one problem from their written work and walk me through it. Why mesh and not nodal. What the time constant tells you about the circuit. Where the negative sign came from. I learn more about what a student actually understands in that ten minutes than I used to learn from a semester of graded homework.

A new Lumina Foundation-Gallup study says 57% of US college students use AI in their coursework at least weekly, and one in five use it every day. At the same time, 53% say their school discourages or prohibits it. Daily use is highest among men and among business, tech, and engineering students. The students avoiding AI mostly cite ethical concerns and school policy, so the ones following the rules are falling behind on a tool they will use the rest of their careers.

My position on AI in education is simple. If we are preparing students for the jobs they are about to take, AI has to be in every class. Every engineering job they walk into will expect them to use these tools well. A program that prohibits AI is training students for a job market that no longer exists. The work is not to keep AI out of the classroom. The work is to teach students how to use it, where it fails, and when to check it against first principles.

That still leaves the assessment problem. If students use AI on everything, how do you know what they understand? You change how you measure them. Oral exams catch what written work cannot. In-class paper problems catch it. Hands-on labs, where a student wires a circuit on a breadboard, takes scope measurements, and explains what they are seeing, catch it cold. Take-home essays graded on polish do not catch anything anymore.

The AI can solve the circuit. It cannot explain why this student chose the loop they chose, and it cannot wire the breadboard when the lab is due at five. That is what we should be assessing, and that is the work employers are hiring for.

DC, AC and GROUND Coupling Settings on the Oscilloscope

A student stopped me in lab last week. She was staring at a trace that kept drifting off screen. She had the coupling set to DC and a large offset was pushing her signal out of range. The question she asked was a good one: what is the difference between AC, DC, and ground coupling, and when do you use each?

The short answer is that coupling controls what the oscilloscope passes to its vertical amplifier before it draws anything on screen. With newer scopes from different manufacturers it is sometimes difficult to find where to set coupling so you can either hunt around (usually the way I do it!) or consult the user manuals.The choice changes what you see, and getting it wrong means you either miss something or start chasing a problem that is not there. Here's a pic with short definitions for each following.

DC Coupling

DC coupling passes everything through: the full signal including any DC offset. If you have a 10 mV sine wave sitting on top of a 12V supply, you see both. This is the correct default for most measurements. Power supply work, logic signals, anything where the offset carries information requires DC coupling. You want the complete picture. That student had the scope set to DC Coupling (often the default.)

AC Coupling

AC coupling inserts a series capacitor that blocks DC and attenuates frequencies below roughly 10 to 20 Hz, depending on the scope. That same sine wave now appears centered on zero, and you can use full vertical scale to examine it. This is useful for noise on a supply output, audio signals, or serial data eye diagrams where the DC level is irrelevant. The tradeoff is real: the capacitor charges and discharges, so the trace drifts for a few seconds after you switch modes. Square waves also suffer at low frequencies because the flat tops droop as the capacitor charges through the input impedance.

Ground Coupling

Ground coupling disconnects the input entirely and connects the vertical amplifier to ground. The signal disappears. You get a flat line at 0V. This is a calibration step, not a measurement mode. Use it to find your ground reference position before you connect a live signal, especially when you need to compare multiple channels on the same scale.

The student switched to AC coupling, centered the trace, and saw exactly what she wanted to see: about 80 mV of sine wave.