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Tje Mysterious Rolling Pop Can

What You Need

1 polystyrene plastic bookshelf, 1 rabbit fur or wool sweater, 1 empty pop can

What To Do

1. Rub the surface of the plastic bookshelf thoroughly with a rabbit fur or an old wool sweater. (Figure 2) You will hear a crackling sound as the shelf is being charged. This procedure places a negative charge on the surface of the shelf.

2. Place an empty pop can, on its side, at either end of the bookshelf.

What Happens?

The empty pop can rolls the length of the shelf. See Figure 3. It may keep going and roll off the other end. Alternatively, it may stop and roll back. (Tilt the shelf slightly and the can may even roll uphill!)

Question for Readers

Why does the can roll along the charged shelf? Why does it not simply stick to the shelf and stay in one place? Is it attracted, repelled, or both?

The interesting phenomenon of the pop can propelled by static charge was observed while the author was doing a presentation for visiting young students at his BIG Little Science Centre in Kamloops, BC. It is now a regular part of the
presentation!

The negatively charged surface of the plastic shelf will repel free electrons on the aluminum can (an excellent conductor) to the far side of the can. (Like charges repel.) The side of the metal can that is nearer the shelf will have an induced
positive charge. There will be attraction between the shelf (–) and the oppositely charged near side of the can (+), and repulsion between the shelf (–) and the like-charged far side of the can (–). Which way the can rolls depends on which
forces are larger.

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Tje Mysterious Rolling Pop Can

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