Charge Transfer
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claimed by Andrew White Fall 17
If a charged conductor comes in contact, or is in close enough proximity, with another conductor, it is possible to transfer this charge to the second conductor. This process is called charge transfer. Charges cannot be created or destroyed; this is known as the Law of Conservation of Charge. Therefore, in the transfer of charge between two objects, the amount of charge gained by one object is equal to the amount of charge loss by the other. There are multiple ways that charge can be transferred such as through direct contact and through inductance.
Insulators vs Conductors
In an insulator, electrons are bounded tightly to atoms, which prevents charged particles from moving through the material. If charge is transferred to an insulator at a given location, the charge will remain at the location that the transfer occurred.
On the other hand, electrons are able to flow freely from particle to particle within conductors. When charge is transferred to a conductor, the charge is distributed evenly across the surface of the object via electron movement. The electrons will be distributed until the repelling force between the excess electrons is minimized. This is the main difference between insulators and conductors: insulators do not have mobile charged particles whereas conductors have mobile charged particles that allow for charge transfer through the free movement of electrons. Examples of insulators include rubber and air and examples of conductors include metals and salt water.
Transfer Charges by Conduction
Electrons move from one object to another (especially with metals) through points of contact. An example of this is rubbing a glass rod with silk. The glass rod will become positively charged and the silk will become negatively charged; this means that electrons were transferred from the glass rod to the silk, since protons are not removed from the nuclei. Rubbing two objects together is not necessary for charge transfer, but because rubbing creates more points of contact between two objects, it facilitates charge transfer.
Transfer Charges by Induction
Unlike the transfer of charges by conduction, objects that becomes charged to each other do not require points of contact. When an object is charged, it has an electric field. This electric field will repel or attract electrons in another object. This electron movement is called transfer of charges by induction. A neutral object can be charged by another charged object through a process called polarization. This is when electrons in the object is repelled or attracted to one side of the object by the charged second object. For example, if a negatively charged sphere is placed near a neutral sphere, the electrons in the neutral sphere will be repelled by the charged sphere. The neutral sphere is now polarized, with one side of it being negatively charged and the other side being positively charged. The negatively charged side of the sphere can be removed through grounding or with a conductor. Once removed, the originally neutral sphere will now be positively charged. Another example of induction is the balloon and black pepper experiment. A balloon can be given a negative charge by rubbing it on hair. When the balloon is placed near grounded black pepper, the black pepper particles will be polarized so that it becomes positively charged on top and will be attracted to the negatively charged balloon.
See also
http://www.physicsbook.gatech.edu/Charge_Motion_in_Metals
http://www.physicsbook.gatech.edu/Polarization
References
Internet resources on this topic:
http://www.physicsclassroom.com/class/estatics/Lesson-1/Conductors-and-Insulators
http://www.physicsclassroom.com/class/estatics/Lesson-1/Charge-Interactions
Chabay, Ruth W., Bruce Sherwood. Matter and Interactions, Volume II: Electric and Magnetic Interactions, 4th Edition. Wiley, 19/2015. VitalBook file.