Practice Problem #05: A Circuit with a Switch

Remarks:

I gave this problem on the First Hour Exam one year ago. About half the class nailed it and about half the class bombed it.

There are not many calculations to do.

Hint: It's all about three things:

  • A voltage source maintains a constant voltage. Absolutely true.
  • A conducting wire ensures tha the voltage is the same everywhere on that wire. Absolutely true.
  • Ohm's Law says that V_{drop} = IR.

    Note in particular that any ideas that you might have about how "current will go...." are very likely to be based on some non-physical idea about how a circuit works. Also remember this: a voltage source has zero impact on the current that goes through it. Voltage sources do not regulate current at all. The current through a voltage source could be zero, positive or negative. The only thing a voltage source does is that it ensures that the voltage at the front of the source is fixed relative to the voltage at the back of the source.

    Yes, this is just exactly the kind of problem I could put on an exam. It's such a great problem that I am seriously tempted to just put it on the exam to see how people do.