Why can't an object gain a (+) charge when rubbed against another?

[Elbobo]

Not a homework question, but I was curious because problems in class assumed the object gained electrons versus losing them.

Why can't it be the other way around?

 

[HallsofIvy]

The question in the title is NOT the same as the question in the body.

If two objects are rubbed against each other and one gains a negative charge the other must gain a positive charge: charge is conserved.

But that's not really what you are asking about. Electrons are in the outer shells of atoms and, in metallic substances "free", moving from one atom to another easily. Protons are bound firmly in the nucleus of atoms- they are not going to move.

One object gains a negative charge because some of the "loose" electrons have moved from the other body. The other object gains a positive charge because it has lost electrons. Neither body loses or gains protons because they are too firmly bound to the nucleus of atoms.

 

[Doc Al]

If one object gains electrons, another object must have lost them. The object that loses electrons "gains" a positive charge.

 

[Elbobo]

Yes, I understand that, but questions I have encountered have stated things like this:

"A plastic rod is rubbed against a wool shirt,
thereby acquiring a charge of −4.9 μC."

How am I supposed to know that the rod is the object gaining electrons versus losing them besides it being given?

My teacher has given other questions that have you assume the object (say obj. A) being rubbed on is becoming negative. Why can't the electrons on object A be stripped of its electrons instead?

 

[Doc Al]

Yes, I understand that, but questions I have encountered have stated things like this:

"A plastic rod is rubbed against a wool shirt,
thereby acquiring a charge of −4.9 μC."

How am I supposed to know that the rod is the object gaining electrons versus losing them besides it being given?

Those are just empirical facts. There's no (simple) way of knowing which object loses electrons and which gains them--it depends on the objects being rubbed.

Rub a glass rod with silk, and the rod loses electrons; rub a plastic rod with wool and the rod gains electrons.

 

[Elbobo]

Ah, so in other words, I had no way of knowing at the time.

Thank you.

 

[zoobyshoe]

A handy list, called The Triboelectric Series has been prepared for easy reference. See this link:

http://www.siliconfareast.com/tribo_series.htm

If you're dealing with some material you're not sure of you can derive a test from that list. It looks like the majority of plastics tend to become negative and that hair and skin become positive. I cut a small piece off a garbage bag and rub it on my hair. Indeed, it now sticks tightly to my skin, so I believe it has a negative charge. It also sticks very well to my ceramic coffee mug, so I believe the mug is positive. I rub a plastic bottle on my hair and now the piece of garbage bag won't stick to it, so I assume it is also negative.

 

[Archosaur]

How am I supposed to know that the rod is the object gaining electrons versus losing them besides it being given?

A good term to know is "electron affinity". A simple way of thinking about it is that it is a quality of a material that determines how much it "likes" electrons.

If you rub object A against another object B and object B has a lower electron affinity than object A, then object A gains electrons.

I saw that someone posted a link to a "triboelectric series". All that is is a list of materials in order from the least to the greatest electron affinity, such that if you rub any material with something lower on the list, it loses electrons, and if you rub any material with something higher on the list, it gains electrons.