Electrical potential

The potential of the ruler should be lowered than before the -ve charged object is brought nearby.

The -ve charged object will lower the potential of space surrounding it. As such, it also affects the potential of the ruler that is near to it, unless the ruler is earthed.

Be aware that the potential on the metal ruler itself is constant throughout despite there are +ve and -ve charges on it.

2011-02-22 13:34:55 補充：
Unless an object is completely isolated, objects carry -ve charges do not necessarily have -ve potential and objects carry +ve charge do not necessarily have +ve potential.
If a small +ve charge object is brought near a large -vely charged object, the potential of the +ve charge object would be -ve.

2011-02-22 13:39:01 補充：
In your example, the induced charges on the ruler must be less than the charge on the object that is brought nearby, the potential of th ruler is mainly determined by the potential set up by the charged object. Assume that the charged object is -ve, the potential on the ruler should also be -ve.

2011-02-22 13:42:24 補充：
Q: the potential of both ends are the same even they are in opposite charge?
Yes
Q: Will the value of potential depends on the +ve & -ve charges?
Yes,but the potential of the ruler is mainly determined by the nearby charged object.The induced charges only modify slightly the potential on the ruler.

2011-02-22 13:43:39 補充：
Q: Can I say one end having +ve potential & another end having -ve potential but the overall potential of the ruler be the same?
No. The potential on the two ends are the same, and is -ve

2011-02-22 13:44:25 補充：
You could pm me if you still do not understand, as there is no enough space here to write more.