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Let's say you place a metal tray of foam pieces on top of a Van de Graaff generator. You turn it on and nothing happens right away but in a few seconds the foam pieces begin flying out of the pan. Why does this happen?

AThe Van de Graaff generator produces an anti-gravity current.
BThe foam pieces have a lower density than air once they're charged.
CThe foam pieces acquire the same charge as the round shell of the Van de Graaff generator.
DThe foam pieces build up an opposing electric field to the Van de Graaff generator and cancel its effect.

Verified Answer :

AL2006

'A' is absurd.
'B' is false.
'D' is fanciful.

'C' is true.  When all the foam pieces have the same charge,
and enough of it, they repel each other with enough force to
fly apart from each other.

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