When two identical socks are removed from a clothes dryer, they will usually

1) repel each other 2) cling to each other 3) no way to predictl

Moving charge by friction

Some atoms/moleculesattract electrons morestrongly than others.

Affinity: a natural liking or sympathy

far:weaker

attraction

close: strongerattraction

electronjumpsElectron Affinity

rabbit’s furglasswoolsilk

rubberhard plastic

F 1r2

Different materials have different electrons affinities

fur

plastic rod

When fur is rubbed on a plastic rod, both acquire an equal but opposite charge.

If the fur merely rests on top of the plastic, each will acquire equal and opposite charges.

1) True.2) False. Friction is necessary to provide enough heat (energy) for electrons to jump from the fur to the plastic.

True. It is not necessary to rub the fur on the rod. If they touch with no motion between them, electrons will still jump from the plastic to the fur. Why is the fur rubbed? Because “contact” is made by a bunch of random, jagged edges. (Remember friction!)

In the figure above, there are only about three contact points where the atoms are close enough for the electrons to make the jump. If instead the fur and plastic are rubbed, many, many more atoms come into contact with each other and more electrons can make the big move. The plastic and fur thus acquire more charge, which makes it more noticeable.

plastic

fur

Far apart: electrons stay with their own atoms

both atoms neutral

Close: difference in electron affinity. Electron jumps.

Move apart: electron stays with the atom it jumped to

both atoms charged

electron

Note: only contact between atoms is necessary. The heat of friction plays no role.

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An electron and a proton are set free, near each other, deep in outer space. The electron moves towards the proton with

1) constant velocity ( v constant )2) increasing velocity but constant acceleration ( a constant )3) increasing velocity and increasing acceleration

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An electron and a proton are set free, near each other, deep in outer space. The electron moves towards the proton with?

There is a net force on the electron due to the proton’s charge, so the electron accelerates (1 is out). As the electron moves closer to the proton, the force it experiences grows stronger (Coulomb’s Law holds F 1/r2). If the force becomes stronger, and the mass does not change, then Newton’s Second Law (F = ma or a = F/m) says that the acceleration increases. The answer must be (3).

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An electron and a proton are set free, near each other, deep in outer space.

What about the proton?