Competency #8
Answer and Explanation
A Van de Graaff generator uses a moving belt to remove electrons from the metal sphere at the top. If a stack of aluminum pie pans are placed on top of this device, they will ‘fly' off, one right after another. Which statement best describes this phenomenon?
The correct answer is B.
Like all metals, aluminum is electrically (and thermally) conductive.
The pie pans are each electrically neutral, meaning they contain equal numbers of protons and electrons.
- The pans become negatively charged and are thus repelled from each other.It is true that if all the pans were negatively charged, they would repel each other. However, as we will see below, the pans become positively charged.
- The pans become positively charged and are thus repelled from each other.The belt is positively charged and thus removes electrons from the metal sphere. The metal sphere then becomes positively charged. Opposite charges attract. Since the pie pans are in electrical contact, some electrons (negative charges) will flow from the pie pans into the metal sphere (positively charged). As electrons leave the pie pans, they, in turn, become positively charged. Like charges repel each other, and since the pie pans are all now positively charged, they ‘fly' off the Van de Graaff generator.
- Some pans become positively charged and others become negatively charged.Since the pans are electrically conductive, all of the pans will be negatively charged or all of the pans will be positively charged.
- The north poles of the pans are repelled by the north pole of the Van de Graaff generator.North and south poles refer to magnetic fields. The phenomenon described above is based on separation of electrical charge.
- Other__________________________
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