01,02 ce pp (urgent)

2012-01-02 11:37 pm

圖片參考：http://imgcld.yimg.com/8/n/HA01097219/o/701201020050813873418380.jpg

37: Can some1 explain why when want to convert a galvanometer to a voltmeter, we usually add a resistor with high resistance to it in series? What will it be/what will it measure when we connect it in other ways shown in (b) to (d)?

34: Why the diagram is a correct ans? I know the voltage across each bulb is 6V, but we it's very clear that the two wire cross each other (in the red circle). Why is it ok? Wouldn't the circuit be shorted?
If the two wires are not connected together but simply overlap with each other, it should be shown on the right with a 'semi-open circle', shouldn't it?

thanks

回答 (1)

天同

2012-01-03 1:06 am

✔ 最佳答案

37. A voltmeter measures the potnetial difference (p.d.) across two points in a circuit. In so doing, it is envisaged that it should have a high internal resistance such that it will not draw any current (or as small current as possible) from the circuit. Otherwise the original condition of the circuit would be altered and the voltage measurement would not be accurate.

A typical galvanometer has a very low resistance. Thus, it is essential to connect a high resistance in series with the galvanometer to achieve the purpose as explained above.

In option B: the "voltmeter" would divert a substantial amount of current flowing through it from the circuit, thus altering much the original circuit condition.
In options C abd D: it is clear that most of the current from the circuit would flow through the galvanometer direct (because of its low resistance), instead of flowing through the component to which its p.d. is to be measured.

34. It is just a mis-print in the circuit connected to the secondary coil.

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