Status check

Just wanted to check in with everyone using the webTA site. I have a couple of questions for you!

1. Am I keeping pace with the topics in class? (I have access to the notes and the CAPA's but without feedback from you I can't tell exactly where you are with the material).

2. Are people finding the material both here on the blog and on the main website useful? Any suggestions regarding what is good/bad, could be improved/added/discarded?

Any feedback you have for me would be very much appreciated! Hope your courses are going well, I know this can be a busy time of year!

Question: Current and a switch

I'm just wondering. If there is an open switch AFTER a resistor, the current still goes through the resistor but does not continue in the circuit, right?

If there is an open circuit, there is no current flow. For current to flow there has to be a continuous path from a high potential to a low one, otherwise charge would have to build up somewhere... which cannot happen. If the charge built up in this way, the charges flowing in behind them would feel more and more repulsive force until the potential required to pile in more equaled the potential of the battery and there was no driving force left. This is basically what happens in a capacitor, and a real circuit will have some of this occuring, but the end result in both cases is that the current stops flowing.

Ever been on a really crowded bus? There is somewhat of a driving force from the front of the bus to the back of the bus as people get on and the bus driver yells to everyone to go to the back. As the bus fills up, and fills up, people at the front push people in the middle who push people at the back closer and closer together, but eventually there is nowhere for them to go... at some point, the flow of people towards the back has to stop until people get off and make room again.

A wire is like an already crowded bus. The charges close to the battery (the front of the bus) feel a driving force from the battery, those charges move slightly, and because they are displaced in the wire, there is a net charge which repels like charges near them and pushes them ahead a bit. This continues all around the wire. But, if there is a break in the loop, there is nowhere for the charges to go (they can't just jump out of the wire), so they just sit there and wait until there is a way for them to move.

The short answer to your question is: if there is an open switch anywhere in a loop, there will be no current flowing through any part of that loop. If there are alternate closed paths in the circuit, then current can flow through those parts of the circuit, but they must constitute a closed loop, with some source of potential a.k.a. electromotive force (EMF) (a battery, DC or AC power supply, a charged capacitor...).

Hope this helps. Please post a comment to this if you need clarification