Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Describe the difference between a conductor and an insulator.
  • Describe how like charges repel one another and unlike charges attract.
  • Define voltage in terms of a difference in electrical potential.
  • Define current in terms of a flow of charged particles.
  • Define the term dielectric.
  • Define capacitance in terms of two conductors and a dielectric.
  • Cite some common dielectric materials.
  • Describe how an electrical charge is stored in a dielectric material.
  • Define the terms electrostatic field.
  • Describe how electrostatic induction affects the charge on a capacitor.

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Capacitor 

 

A capacitor is most simply defined as two conductors separated by a dielectric. It is easier to grasp the significance of this definition by looking at a commonly used model for a capacitor that is shown here. 

A dielectric is a material that is a good insulator (incapable of passing electrical current), but is capable of passing electrical fields of force. 
 
 

 A capacitor is also called a condenser.  

Some examples dielectric materials: 

    vacuum 
    air 
    aluminum oxide 
    various ceramics 
    Barium titanate  
    glass 
    water 
    mica 
    oil 
    paper 
    polyetheylene 
    tantalum oxide 
    calcium titanate 
    waxed paper

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 Charged Capacitor 

 

A capacitor is said to be charged when there are more electrons on one conductor plate than on the other.

 The plate with the larger number of electrons has the negative polarity. The opposite plate then has the positive polarity. 

When a capacitor is charged, energy is stored in the dielectric material in the form of an electrostatic field.

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 Electrostatic Induction 
 
When an electron is added to one plate of a capacitor, one electron is driven away from the opposite plate.  

Or you can say that when an electron is pulled away from one plate of a capacitor, another electron is drawn to the opposite plate. 

No matter how you look at it, this is the principle of electrostatic induction at work in a capacitor.

 

 When this electrostatic effect increases the imbalance of electrons between the two plates: 
  • The electrostatic field grows stronger.
  • The amount of energy stored in the dielectric increases.
  • The capacitor is said to be charging.

When this electrostatic effect decreases the imbalance of electrons between the two plates: 

  • The electrostatic field grows weaker.
  • The amount of energy stored in the dielectic decreases.
  • The capacitor is said to be discharging.

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