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27 January 2012

Common misconceptions -- parallel resistors

Parallel resistors each take the same voltage, which is equal to the total.

Now ask a student: "Two 100 ohm resistors are connected in parallel to a 12 V battery.  Determine the voltage across one of the two resistors."  What does the student say?

Generally, that student reasons, "Parallel resistors take the same voltage.  The battery provides 12 V to two resistors equally, so that's 6 V across each."  D'oh.

How do I attempt to remedy this misconception?  Give everyone the chance to predict and then MEASURE the voltage across several resistors, as in this laboratory exercise.  When a student comes to my desk for me to sign off on his correct measurement, I throw the common misconception in his face.  I say, "Hey, Will, that doesn't make sense.  Seems to me, you've got two parallel resistors here, you should only get half the battery's voltage across each."  Will generally has two points to his rebuttal:  (1) "That's not the correct rule, Mr. Jacobs, the voltage across parallel resistors is equal to the total." And, most importantly, (2) "That's not what I measured.  I get the same voltage across everything."

1 comment:

  1. I find they are great with the parallel = same volts, until I introduce the combination circuit, then they still want the parallel voltage to equal the total voltage.

    A great demo to see the parallel = same volts is to use a car battery and attach extentions poles to the terminals. You can attach two separate head lights across and they have the same bright. Also demonstrates that 2 in series will share the volts.