1. A device which is used to store charge or used to store electrical energy is called a capacitor. ► A capacitor is constructed of two parallel conducting plates separated by a small distance. ► The medium between the two plates is air or a sheet of some insulator. ► This medium is known as dielectric.

2.  When a charge is transferred to one of the plate due to electrostatic induction it would induce charge Q on the inner surface of the other plate.  The capacitor is commonly charged by connecting its plate with opposite terminal of the battery.

3. Capacitor  Charge +Q and –Q appear on the plates.  Mutual attraction between the charge keep them bound on the inner surface of the two plates.  Thus the charge remains stored in the capacitor even after the removal of the battery.

4. Capacitance is a measure of a capacitor’s ability to store charge on its plates. When a charge Q is transferred on one of the plate of a capacitor, the potential difference V b/w the plates also increases.  Q ∝ V  Q=CV  C=Q/V C is capacitance.

5. The unit of capacitance is farad and defined as: The capacitance of a capacitor is 1F which stores a charge of one coulombs having the potential difference of 1 volt b/w plates of the capacitor. As C=Q/V C(units) =1Coulombs/1Volt 1F=1CV -1 Sub multiple of farad are 1 μF= 10 -6 F and 1pF= 10 -12 F

6. Capacitance depends upon:  Area of the plates.  The distance b/w the plates.  The medium (dielectric) b/w the plates.

7.  The figure below depicts a parallel plate capacitor. We can see two large plates placed parallel to each other at a small distance d. The distance between the plates is shown by the dotted array. The two plates carry an equal and opposite charge. Then strength of electric field b/w the plates is ……… (1)

8. The charge density is the total charge per unit area. By using Gauss’s law the electric field intensity “E” b/w the plates is

10. When a dielectric is placed b/w plates of a capacitor, then it is seen that the charge storing capacity of a capacitor enhanced by the dielectric which permit it to store ∈ r times more charge for the same potential difference. ∈ = ∈ r ∈ 0

11. The ratio of capacitance of a capacitor with a given material filling the space b/w the conductors to the capacitance of the same capacitor when the space is evacuated is the relative permittivity of the material

12. The relationship between voltage difference and charge is normally linear Different capacitors for the same voltage across their plates will acquire greater or lesser amounts of charge on their plates, hence the capacitors have greater or lesser capacitance.

13.  The purpose of the dielectric is to create an electric field to oppose the electric field setup by free charges on the parallel plates. Common types of capacitors are: Mica Ceramic Plastic film

14. A “more typical” geometry is two large, closely spaced, parallel conducting plates Area A, separation d.

15. Permittivity – The ratio of the flux density to the electric field intensity in the dielectric. A measure of how easily the dielectric will “permit” the establishment of flux lines within the dielectric. “The ratio of the capacitance of a parallel plate capacitor with an insulating substance as medium between the plates to its capacitance with vacuum (or air) as medium between them”

16. Consider an atom. We know that it is electrically neutral. Furthermore, the centre of the negative charge of the electrons coincides with the positive nuclear charge, which means that the atom has no net dipole moment.

17. However, when this atom is placed in an external electric field, the centre of the positive charge is displaced along the field direction while the centre of the negative charge is displaced in the opposite direction. When a dielectric material is placed inside an electric field, such dipoles are created in all the atoms inside.

18. The ideal capacitor does not dissipate any energy supplied to it. It stores the energy in the form of an electric field between the conducting surfaces. 0 Q V The area under the graph gives the energy stored in the capacitor.

19. A d Energy density is energy over volume We can associate the energy with the electric field itself

20. When a capacitor is connected across a battery, electrons flow from the negative terminal of the battery to a plate of the capacitor connected to it. At the same rate, electrons flow from the other plate of the capacitor to the positive terminal of the battery. This gives a flow of current as the capacitor is being charged.

21. As charges accumulate on the plates of the capacitor, electric potential built across the plates. This hinders further accumulation of charges and makes the charge up current decreasing. When the potential difference across the plates equals that of the battery, the current becomes zero. Discharging of Capacitors R R

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