# Why aren't all conductors always charged?

If you place a conductor beside an insulator, the insulator will become negatively charged and the conductor will become positively charged. Air is an insulator. So why don't all conductors placed in air automatically become positively charged and the air around it become negatively charged?

See, for example, the "Cause" section on the Triboelectric effect Wikipedia page.

• Where did you take that from?? If you place a conductor beside an insulator, nothing much happens (you can argue a bit with chemical potentials, but if one is an insulator then there obviously can't be much transfer of charge). Commented Jan 31, 2014 at 21:17
• @leftaroundabout This is from the Wikipedia page on the Triboelectric effects: "Although the word comes from the Greek for "rubbing", τρίβω (τριβή: friction), the two materials only need to come into contact and then separate for electrons to be exchanged".
– dfg
Commented Jan 31, 2014 at 21:22
• Where the "two materials" are an insulator and a conductor.
– dfg
Commented Jan 31, 2014 at 21:22
• You should be careful about reading too much into the choice of words in the Wikipedia. Both the degree of adhesion and the effectiveness of charge transfer by these meas vary enormously depending on the choice of materials, the pressure of contact, and the finish of the surfaces. Commented Jan 31, 2014 at 22:26
• The rubbing just facilitates contact; energy only goes into heat. The energy input comes from separating the attracting oppositely charged surfaces. In practice, you will notice the electrostatic effects when "rubbing" since surface separation at the edges occurs. Commented Feb 1, 2014 at 1:48

The triboelectric effect isn't a result of placing two materials next to each other, it comes as a result of rubbing them together. This is important because the two materials do not naturally want to become charged, you have to add energy, typically in the form of the friction that comes from rubbing. This overcomes the activation energy, so to speak, that is required for the electrons to jump from one material to the other. Everyday interactions between objects and air molecules are not energetic enough to cause this effect.

• I'm sorry I don't understand. The insulator wants the electrons, the conductor doesn't. Why would there need to be any more energy? The article even says that the charges would exchange without rubbing.
– dfg
Commented Jan 31, 2014 at 22:07
• It's not just rubbing, but it does require an input of energy. The usual non-rubbing demo is plastic backed adhesive tape (Scotch brand works really well). Peal two strips off the roll and bring them near each other without letting them touch other things and the generally repel. Ground them, stick one onto the back of the other and peal them apart again and they generally attract. Why the different behaviors is a nice little puzzle for first year students. Commented Jan 31, 2014 at 22:21

To charge something you need at least some driving force: friction, electric field, potentials, temperature gradient, etc. The fact that you just gather two different materials together does not mean they will be charged.

• But there is a driving force - the insulator "wants" the electrons but the conductor doesn't so the insulator has a stronger "pull" on the electrons that the conductor. So the electrons should move shouldn't they?
– dfg
Commented Jan 31, 2014 at 21:20
• Insulator does not mean I am hungry for electron. Insoluator means I am not so confident in transfering a physical quantity. Do you think insulator wants to get cold or hot, or charged? Insulator are refringent to transfert heat or charge. Commented Jan 31, 2014 at 21:25
• But the entire reason electrons don't move freely in insulators is that the insulator atoms have a really strong pull on the electrons. Why else would there be no free electrons?
– dfg
Commented Jan 31, 2014 at 21:30
• Does it mean that it has to be charged ? Commented Jan 31, 2014 at 21:31
• @dfg: the entire reason electrons don't move freely in insulators is that there's a band gap too high for thermal fluctuations to kick electrons in the conduction band. You can't just argue with "pulling strongly on the electrons". Commented Jan 31, 2014 at 21:33