7. CAPACITOR & DIELECTRICS.pptx follow this

Physics capacitors

1. Phenomenon of polarization of charges in a body, when a charged body is present near it, is called electrostatic induction. In this process bodies are charged without touching them. ELECTROSTATIC INDUCTION

2. CHARGING BY INDUCTION

3. A charged object will induce a charge on a nearby conductor. In this example, a negatively charged rod pushes some of the negatively charged electrons to the far side of a nearby copper sphere because like charges repel each other. The positive charges that remain on the near side of the sphere are attracted to the rod. If the sphere is grounded so that the electrons can escape altogether, the charge on the sphere will remain if the rod is removed.

4. CAPACITOR AND CAPACITANCE A capacitor is a two-terminal electrical device that can store energy in the form of an electric charge. It consists of two electrical conductors that are separated by a distance.  The space between the conductors may be filled by vacuum or with an insulating material known as a dielectric.  The ability of the capacitor to store charges is known as capacitance.

5. Capacitance of a conductor is defined as the charge required to raise its potential through one unit. SI Unit of capacitance is ‘farad’ (F). Symbol of capacitance:

6. Behavior of Conductors in the Electrostatic Field: Net electric field intensity in the interior of a conductor is zero. Electric field just outside the charged conductor is perpendicular to the surface of the conductor. Net charge in the interior of a conductor is zero. The charges are temporarily separated. The total charge of the system is zero.

7. 4. Charge always resides on the surface of a conductor Electric potential is constant for the entire conductor. Surface charge distribution may be different at different po i nt

8. Generally, a non-conducting medium or insulator is called a ‘dielectric’. Precisely, the non-conducting materials in which induced charges are produced on their faces on the application of electric fields are called dielectrics. E.g. Air, H2, glass, mica, paraffin wax, transformer oil, etc. Dielectrics:

9. Polarization of Dielectrics When a non-polar dielectric slab is subjected to an electric field, dipoles are induced due to separation of effective positive and negative centers. E0 is the applied field and Ep is the induced field in the dielectric. The net field is E N = E – Ep i.e. the field is reduced when a dielectric slab is introduced. The dielectric constant is given by

10. Polarization Vector The polarization vector measures the degree of polarization of the dielectric. It is defined as the dipole moment of the unit volume of the polarized dielectric. If n is the number of atoms or molecules per unit volume of the dielectric, then polarization vector is SI unit of polarization vector is C m -2 .

11. Dielectric Strength: Dielectric strength is the maximum value of the electric field intensity that can be applied to the dielectric without its electric break down. Its SI unit is V m -1 .

12. Capacitance of Parallel Plate Capacitor: Parallel plate capacitor is an arrangement of two parallel conducting plates of equal area separated by air medium or any other insulating medium such as paper, mica, glass, wood, ceramic, etc….

13. If the space between the plates is filled with dielectric medium of relative permittivity ε r , then Capacitance of a parallel plate capacitor is (i) directly proportional to the area of the plates and (ii) inversely proportional to the distance of separation between them

14. Series Combination of Capacitors: In series combination, i) Charge is same in each capacitor ii) Potential is distributed in inverse proportion to capacitances

15. The reciprocal of the effective capacitance is the sum of the reciprocals of the individual capacitances. Note: The effective capacitance in series combination is less than the least of all the individual capacitances .

16. Parallel Combination of Capacitors: In Parallel connection

17. Capacitance of Parallel Plate Capacitor with Dielectric Slab: The charge on a parallel plate capacitor – q Area of slab – A The distance between the parallel plate – d The dielectric constant of the slab of a material – K The thickness of the material –t The vacuum between the plates – (d-t) The surface charge density on the plates – 

18. V = E (d – t) + E t The electric field in the air between the plates is E = = The electric field in the dielectric material E = The potential difference between the plates Now substitute the values of E & E in the above equation

19. Since K>1, the effective distance between the plates becomes less than d and so the capacitance increases.

20. Application of Capacitors Flash Capacitors used in digital cameras for taking photographs. It is used in defibrillator. It is used in automobile engines. It is used to reduce power fluctuations in power supplies. It is used in energy storage. RF Coupling and Decoupling.

21. The process of charging a capacitor is equivalent to transferring charges from one plate to the other of the capacitor. The moment charging starts, there is a potential difference between the plates. Therefore, to transfer charges against the potential difference some work is to be done. This work is stored as electrostatic potential energy in the capacitor. If dq be the charge transferred against the potential difference V, then work done is Energy Stored in a Capacitor:

22. SI unit of energy density is J m -3 . Energy Density Energy density is generalized as energy per unit volume of the field.

23. Energy Stored in a Series Combination of Capacitors Energy Stored in a Parallel Combination of Capacitors