ballon

European Sens

1. The problem statement, all variables and given/known data
A rubber balloon is rubbed against a wall and then sticks to the wall. This is because



2. multiple choice
a. the balloon and the wall exchange charges through contact
b. electrons hop back and forth between the balloon and the wall
c. the balloon induces an opposite charge on the wall surface
d. the balloon causes the formation of ions on the wall by using polarization



3. The attempt at a solution
I believe that the answer is A. please give me a second oppinion. thanks

dynamicsolo

The "static electricity" force holding the balloon to the wall is not caused by exchanging charges between objects. The balloon is brought to the wall bearing a net charge. What will that charge do to charges in the surface of the wall? Why would the balloon and wall attract one another? (Think about this: what would happen to that charge on the balloon if the wall were a good conductor and grounded? Would the balloon stick then?)

daniel69

No it wouldn't.

Okay, so the balloon would have to induse its opposite charge to the wall?

dynamicsolo

But what does this statement mean? You can do this with a conductor because there are mobile charges that are free to migrate elsewhere, such as in the demonstration with a charged rod touching a cork or pith ball covered in metal foil. But what would happen as soon as the balloon touched the wall, if the wall were conductive?

Do charged-up balloons stick to everything? What sort of surfaces will work?

daniel69

no it doesnt stick to everything. hmm, im not sure of the terminology, but i know it makes your hair stick up and you can stick it to like clothes.

dynamicsolo

Yes, this only works with materials which are called insulators. Charges placed on the surfaces of such materials tend to migrate rather little, so a charged patch of surface will stay charged (for a while, anyway).

Among the choices you are given to select from, I incline toward (d). The problem with choice (c), if I'm understanding the intent of the wording correctly, is that the induced charging doesn't last. The example I gave of the charged rod and the foil-covered ball is what I think that is referring to. The ball is electrically neutral and is usually on an insulating tether (so the charge about to be applied doesn't just escape up the support) and the rod is brought close to it. The charge of the rod repels like charges in the foil to the far side of the ball and attracts opposite charges (actually, it's just the mobility electrons that move, but the effect is the same). This "induced charging" causes the ball to be attracted to the rod. But as soon as the ball touches the rod, it picks up some of the rod's charge and now has the same sign of charge as the rod, so it immediately jumps away from the rod.

The balloon obviously isn't doing that, so something else must be at work. That's why I think they are talking about the electric field of the charged balloon creating a polarization in the insulating material of the wall (in the same sense that a field polarizes the dielectric material in a capacitor), which produces a locally attracting region of the wall.

drpizza

with all due respect, I disagree with dynamicsolo. A frequently taught physics concept is why does a charged balloon stick to a wall. And, the correct answer, had the question been worded in such a way is "C".

However, there's no indication that the balloon had a net charge to begin with, only: "A rubber balloon is rubbed against a wall and then sticks to the wall." In which case, the OP was correct: Choice A.

Dynamicsolo, in your first response, you indicated: "The balloon is brought to the wall bearing a net charge" you must have misread it. "...and then sticks to the wall" indicates that it didn't stick to the wall until after it was rubbed against the wall. Had it initially had a net charge, the rubbing would not have been necessary. Also, unlike the example you provided of induction with a charged rod and foil covered ball, a latex balloon is not a conductor; i.e. there isn't going to be any significant transfer of charge from a simple contact to the wall.

edit: Here's a nice animation for the more classic explanation of a balloon sticking to the wall. Unfortunately, it's not programmed to "do" anything if you rub the balloon on the wall. http://phet.colorado.edu/new/simulat...ic_Electricity

dynamicsolo

Yes, you are correct: I misread the question. I saw "balloon rubbed against" and thought a third object was being used. That being correct, the initially neutral balloon and the other object would become charged by one separating electrons from the surface of the other (and "A" would be the closest choice).

So that was my error and I agree that the neutral balloon is picking up charge (along with the patch of wall) by one removing electrons from the other. It is important that both objects be insulators, in order for the displaced charges to stay (more or less) in the area that was in contact.

The point of my choosing that example was in regard to what I interpreted "inducing opposite charge" to mean. I read that choice to mean that the charge already present on the balloon would supposedly attract opposing charges to the closest portion of the wall (and repel like charges). This is moot, since this was not the situation the question describes...

In the case of the charged balloon, I have to say that I'm unclear on just was is meant in choice (c). I considered (d) because there would be some such effect, given that the electrons won't move very freely in the insulating material of the wall.

I believe it is correct to say that the balloon, initially neutral or charged, won't stick to a conducting surface...