Circuits/currents Labs for AP and Honors

This is relevant to both AP and honors classes. I have reviewed several labs from my AP classes and I found nearly everyone still fails to understand what causes currents. We began this subject with a discussion of how a current through a conducting element or object RESULTS from a field through it. The field through a conducting element is directly related to the voltage across it. Thus the current through an element depends on the voltage across the element and the property of the element known as its conductance, or
I = V x conductance or more commonly

I= V/resistance where resistance ,R , = 1/ conductance.

Thus if you hook up a battery to three bulbs, R1, R2, and R3 in parallel and then disconnect R1, the total current through the battery drops but the current through R2, which is due to the voltage across R2, does not drop because the voltage across it did not drop and its resistance did not change. [ If they were in parallel, the current through R1 never went through R2 or R3.] Yes the total current did drop but not the currents in each of the two remaining bulbs. The battery produces less current but about the same voltage. If R2 and R3 are in parallel with each other and then the pair is in series with R1, if R3 is disconnected, less current will flow through R1 so V1 across R1 must have decreased and V2 across R2 will increase because V1 + V2 still must equal Vtotal from the battery as before. Thus R2 will have more current through it, more voltage across it and produce more power. The reverse will be true for R1.

In fact when one in three parallel bulbs goes out, slightly more voltage is available to the remaining bulbs because the current flowing through the battery and thus through the small internal resistance in the battery is lower, reducing the internal voltage drop in the battery. This means the battery output voltage will go up slightly. AP students should have noticed that the voltage in part A of the lab with its small total current was higher tan in Part B, with its larger total current. Honors students should also become aware of this small effect when you find the drop in R internal for the series (low current lower V drop across Rinternal) and parallel cases (high current higher V drop across Rinternal).

Another point about those labs. No one bothered to fill out the ehadings on the first sheet of the lab report. While I admire your attempt to conserve the precious resource, ink, you will have to fill these out on each first page to receive full credit on this lab and all future ones. This goes for Honors as well. Name , Date of Lab performance, Period of your class, Lab group members who participated, must appear on the first sheet of all labs.

If R2 and R3 are in parallel with each other and then the pair is in series with R1, if R3 is disconnected, less current will flow through R1 so V1 across R1 must have decreased and V2 across R2 will increase because V1 + V2 still must equal Vtotal from the battery as before. Thus when R3 is disconnected, R2 will have more current through it, more voltage across it and produce more power. The reverse will be true for R1 when R3 is disconnected.