Electroscope Properties (need explaination)

[lnvoker]

So I came upon this problem on a quiz (didnt count for marks) about my Static Electricity Unit.
It was like... An electroscope is negatively charged, with the leaf at an angle of about 45 degrees. A metal ball on an insulating handle is slowly brought to (but not touching) the plate of the electroscope and the leaf falls. This suggest that the metal ball is...

• uncharged or positively charged
• uncharged or negatively charged
• positively charged
• negatively charged

I put #1 as my answer, but got it wrong since my teacher told the class the it would USUALLY be positive charged and not uncharged.

What I'm confused about is how leaf falls when the metal ball is brought upon the electroscope. The only possibility for it to fall would be if the leaves where both neutral or both + or -. (which i have no idea how this would come)

The question before this was similar, but it stated that: A negatively charged rod is slowly brought near the plate of the electroscope (negatively charged), the leaf... (obviously rises since - repels the electrons to the leaves and - and - repel so the leaves repel.

I just don't get the question after. And I'm somewhat confused how the electrons...charges move in the electroscope.

Help would be appreciated.

Thanks

 

[Delphi51]

The positive rod will attract the negative charge on the electroscope, and that negative charge will move from the leaves to the bulb, leaving less excess charge on the leaves.

You should try a + charged rod close to a neutral electroscope. The rod will induce a charge separation on the electroscope - negative on the bulb and positive on the leaves, causing them to repel.