SAL TECHNICAL CAMPUS SAL Institute of Technology Engineering

SAL TECHNICAL CAMPUS SAL Institute of Technology & Engineering Research CAPACITORS Guided By: Mrs. Megha Panicker Prepared By: T 13 EE 02 Rohan Alwani T 13 EE 12 Kumavat Sunilkumar T 13 EE 29 Patel Ketankumar B.

CAPACITORS

Introduction • Electrostatics is a branch of science dealing with electricity at rest. • Capacitors come under electrostatics. • Two conducting surfaces separated by a layer of insulating medium is called a capacitor. • Capacitor is also called as “condenser”. • Capacitance is the property of body to store electric charge. Conducting surface 1 Conducting surface 2 Insulating material

Structure of capacitor • The two conducting surfaces of capacitor are known as “capacitor plates”. • The insulating material between the plates is called as the “dielectric”. • Various dielectrics are : - air, mica, glass, waxed paper, ceramic etc. • The shape of conducting surfaces may be circular, spherical or cylindrical.

Working of Capacitor • • When switch ‘S’ is closed, Free electrons from plate A are attracted by “+ve” terminal of battery. Deficiency of electrons on plate A creates a “+” charge on it. The electrons are transferred from + terminal of battery to – terminal of battery. The “-ve” terminal repels these electrons towards plate B, creating an excess of electrons there. Thus a potential difference is established between plates A and B. The movement of electrons will continue till the potential difference between the two plates becomes exactly equal to the emf of battery(i. e V volts). Hence the charge(Q) is accumulated between the plates of capacitor. The charge is found to be directly proportional to the applied voltage(V).

Contd… • • • Q QαV Or, Q/V = C Where, = charge on the capacitor plates in coloumb V = potential difference between the plates in volts. C = Capacitance in farads. Thus, Capacitance is defined as the ratio of the charge on capacitor plates to the potential difference across its plates. + + + _ _ -

Parallel plate capacitor with uniform Dielectric medium • The capacitance ‘C’ of a parallel plate capacitor with a single dielectric medium is given by. . d +Q -Q A C = er eo A/d Where, er= relative permittivity of the medium. eo= Absolute permittivity. V

Parallel plate capacitor with composite dielectric medium • The capacitance ‘C’ of a parallel plate capacitor with three different dielectric medium is given by, E 1 E 2 E 3 εr 1 d 1 v 1 εr 2 d 2 v 2 εr 3 d 3 v 3 V

Capacitor in series • Q is same in all the three capacitors • V is divided in all the three capacitors • V = V 1 + V 2 + V 3 • Hence, total capacitance is given by: - C 1 V 1 C 2 V C 3 V 3

Capacitor in parallel • V is same in all the three capacitors. • Q is divided in all the three capacitors. • Q = Q 1 + Q 2 + Q 3 • Hence, total capacitance is given by: - V Q 1 C 1 Q 2 C 2 Q 3 C 3

Energy stored in capacitor • During charging of capacitor, the voltage source has to spend some energy. • This energy is stored in the electrostatic field of charged capacitor and is given by the equation.

Charging of capacitor • When capacitor is connected across a voltage supply(E), it gradually charges till the voltage across capacitor(Vc) is equal to supply voltage(E) • The rate of capacitor charging is given by: Vc = E(1 – e-t/RC) Where RC = λ(Time constant) • λ is the time taken by the voltage across capacitor to reach 63. 2 % of its final value

Discharging of capacitor • When capacitor is removed from voltage supply(E) and connected across any load, the capacitor gradually discharges till the voltage across capacitor(VR) reaches zero volts. • The rate of capacitor discharging is given by: VR = Ee-t/RC Where RC = λ(Time constant) • λ is the time taken by the voltage across capacitor to decay to 37% of its initial maximum value.

Applications Capacitors are used in many applications like 1. Timing circuits. 2. Electronic flash lamps for camera 3. Line conditioners 4. Electronic power supplies 5. Regulators.

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