Summary for Chapter 22: Electrostatics

There are several important concepts in this chapter.

Electrical Charge

• The first is electrical charge, or just charge. At the atomic level, charge is associated with protons and electrons. They have the same magnitude of charge, but their charge is opposite in sign. Protons have Positive charge and Electrons have Negative charge.
  
• Charge is measured in Coulombs, abbreviated C. It takes 6.25x10¹⁸ protons to make 1C of charge, i.e. 6,250,000,000,000,000,000 protons. 6.25x10¹⁸ electrons will make - 1C of charge.
  
• Electrical charges will exert forces on each other. Two positive charges will repel each other and two negative charges will repel each other. (Like charges, or charges of the same sign, repel each other.) A positive and a negative charge will attract each other. (Unlike charges attract each other.)
  
• This is a strong force.  As a result most objects usually have about the same amount of positive and negative charge.  If they have exactly the same amount of positive and negative charge the net charge is zero and we say they are neutral.

Electric Force

• The force is proportional to the product of the charges and inversely proportional to the square of the distance between them. (If I double one charge the force doubles. If I double both charges the force increases by four. If I increase the distance between the charges the force decreases, and visa versa.)

• Electrical forces can do work and there is a potential energy associated with this force.

Electric Potential

• We often use the ideal of electric potential and it is associated with the idea of electric potential energy. Regions of excess positive charge are regions of high electric potential (and HIGH potential energy for POSITIVE charges).
  
• Regions of excess negative charge are regions of low electric potential (and LOW potential energy for POSITIVE charges).
  
• For NEGATIVE charges it is reversed and a high potential is a low potential energy and visa versa.
  
• POSITIVE charges are attracted to NEGATIVE charges or regions of LOW potential.
  
• NEGATIVE charges are attracted to POSITIVE charges or regions of HIGH potential.

Conductors and Insulators

• Materials are classified by how freely charge and move in or on the material. Materials that allow charge to move easily are called electrical conductors, or conductors for short. Materials that strongly resist the movement of charge are called electrical insulators, or just insulators.
  
• Metals are good conductors, both of electric charge and heat. Materials like glass and wood are electrical insulators and thermal insulators.

Producing Charge Imbalance

Because of the strong electrical force between charged objects, it usually requires an effort to produce a charge imbalance.  There are several ways to produce such a charge imbalance.

1. Friction.  When two objects rub against each other they rip atoms or molecules off of each other.  Some of these may be charged (ions) and as a result charge is transferred from one material to another.  For example rubbing glass with silk tends to leave the silk negatively charged (excess electrons or negative ions) and the glass positively charged (deficit of electrons).
2. Chemical reactions.  This is how batteries produce charge imbalances to produce a difference in potential.  (Note that chemical reactions are really electrical in nature.)
3. Changing Magnetic fields can exert forces on charges to produce a charge imbalance.  We will learn more about this in Chapter 24.