Physics electric circuit help!

2013-04-02 5:33 am

圖片參考：http://imgcld.yimg.com/8/n/HA01132001/o/20130401212839.jpg

Please draw the circuit diagrams for the above four pictures!!!

圖片參考：http://imgcld.yimg.com/8/n/HA01132001/o/20130401213149.jpg

And please answer this question, thanks a million!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

更新1:

Thanks for you detailed answer! Would you please list out the conditions for the second question? Under what given condition should we consider the change of voltage or current? Thanks!!

更新2:

And I want to ask why the left-upper bulb is short-circuited?

回答 (1)

天同

2013-04-02 7:25 am

✔ 最佳答案

1. If the resistance of each light bulb be R, then we would have the following equivalent resistance in each circuit:

A. 2R/3 B 3R/2 C R/3 D R/2
Clearly, the circuit in option C has the lowest resistance.

Notice that in the circuit in option D, the bulb in the left-upper position has been by-passed, leaving only two bulbs connected in parallel.

The circuit diagrams for the other options are as follows:
A. 2 bulbs in series connected with the 3rd bulb in parallel
B 2 bulbs in parallel connected with the 3rd bulb in series.
C 3 bulbs in parallel.

2. There is no general rule. All depends on the given conditions.

2013-04-05 16:18:44 補充：
Your suppl question:
Q:Under what given condition should we consider the change of voltage or current?
A: As said, there is no rule. It depends which parameter can be varied, and which one cannot.

2013-04-05 16:21:35 補充：
Q: you asked "WHY the left-upper bulb is short-circuited?"
A: It is simply given on the diagram that it is shorted.

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