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

  • Context: High School 
  • Thread starter Thread starter Elbobo
  • Start date Start date
  • Tags Tags
    Charge Gain
Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of charge transfer when two objects are rubbed together, specifically addressing why one object gains a negative charge while the other gains a positive charge. It explores concepts related to electron movement, charge conservation, and the factors influencing which object gains or loses electrons.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that when two objects are rubbed together, one gains electrons and the other loses them, resulting in one gaining a negative charge and the other a positive charge due to charge conservation.
  • Others express confusion about how to determine which object is gaining or losing electrons, highlighting that it often depends on the specific materials involved in the interaction.
  • A participant mentions that empirical examples, such as a plastic rod rubbed against wool, are often given without clear reasoning for which object gains or loses electrons.
  • Some participants refer to the Triboelectric Series as a helpful tool for predicting charge transfer based on the materials' electron affinities.
  • There is a discussion about the concept of electron affinity and how it influences whether an object gains or loses electrons when rubbed against another object.

Areas of Agreement / Disagreement

Participants generally agree on the principle of charge conservation and the movement of electrons, but there is no consensus on how to determine which object gains or loses electrons without prior knowledge of the materials involved.

Contextual Notes

Participants acknowledge that the understanding of charge transfer is often based on empirical observations and material properties, which can vary widely. The discussion highlights the limitations of generalizing these principles without specific context.

Who May Find This Useful

This discussion may be useful for students and educators interested in electrostatics, materials science, and the principles of charge transfer in various contexts.

Elbobo
Messages
145
Reaction score
0
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?
 
Physics news on Phys.org
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.
 
If one object gains electrons, another object must have lost them. The object that loses electrons "gains" a positive charge.
 
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?
 
Elbobo said:
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.
 
Ah, so in other words, I had no way of knowing at the time.

Thank you.
 
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.
 
Elbobo said:
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.
 

Similar threads

High School Electric charge on rubbed objects
  • · Replies 1 ·
Replies
1
Views
1K
High School Why Inflating Balloons Causes them to Gain a Positive Charge
  • · Replies 5 ·
Replies
5
Views
4K
High School Does the Electric Field Change When One Charge Moves Relative to Another?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Meaning of "Static Electricity" and Physical Interpretation
  • · Replies 20 ·
Replies
20
Views
2K
Undergrad Understanding dissipation of energy in a resistor through the Drude model
  • · Replies 1 ·
Replies
1
Views
3K
High School Problems I Have With Electrostatics and Pith Ball Experiments
  • · Replies 10 ·
Replies
10
Views
5K
High School How is surface charge accumulated?
  • · Replies 36 ·
2
Replies
36
Views
7K
High School Definition of ground potential in infinite sheet charge distributions
  • · Replies 11 ·
Replies
11
Views
2K
High School Conductors in the triboelectric series
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Electrostatics and electric shocks
  • · Replies 5 ·
Replies
5
Views
2K