Phy 1.5 S2

• Capacitor is a device that is capable of storing charges.
• Capacitor come in different shapes & sizes.
• Capacitor used in a variety of electric circuits.
• Capacitor consists of 2 conductors separated by a small air gap or insulator
• Insulator in capacitor called a dielectric could be made of mica, ceramics, paper or oil.
• the conductors could be rigid metal plates facing each other or a pair of insulated metal foil rowed into a cylindrical casing.
• The capacitance of a capacitor is defined as the ratio of the magnitude of the charge on either plate to the potential difference between them.
• Unit C is CV^-1 or F (Farad)
• Q = VC
• A capacitor with a large capacitance can hold more charges than one with a smaller capacitance for the same potential difference applied across them.
• C = (A . epsilon) / d

  
• If the gaps between the plates is not vacuum but filled with a dielectric:

  
• K = 7.0 for mica

  K = 3.7 for paper
• Dielectric is an insulating material such as rubber, plastic or waxed paper.
• The effect of a dielectric is to increase the capacitance by a factor of K.
• 1. A dielectric material is inserted between the plates of an isolated charged capacitor
• 2. When the material is in the capacitor, the dipoles orient themselves with the field, creates a reverse electric filed (E_{d})
• 3. The dipole field partially cancels the field due to the plate charges.
• 4. the net field (E) between the plates is reduced the voltage across the plates V=Ed) also decreases,
• Since the stored charge remains the same, the capacitance increases.
• C = KC⁰ V = V⁰ / K E = E⁰ / K

  
• When the switch is closed, charges begin to accumulate on the plates.

  A small amount of work (AW) is done in transferring a small amount of charge (Aq) from the battery to the capacitor.

  
• The total work (W) required to increase the accumulated charge from zero to Q is given by

  
• W = Q²/2C

  W = CV²/2

  W = QV/2