STPM PHYSICS CHAPTER 13 CAPACITORS.pdf

SMK SACRED HEART, SIBU 1.

2.

3.  

 



4.

STPM PHYSICS CHAPTER 13 CAPACITORS

Capacitance, C= = = Q = charge on both plates V = potential difference between the plates * Capacitance is always constant unless the physical factor varied (Temperature, properties of dielectric, area of metal plates and distance between two plates). * Thus, C of capacitor does not depend on the potential difference, V across it. ∑ *Since C is constant, Q V. = EA = A Parallel plate capacitors ∑ When Vcapacitor = e.m.f.battery,  The flow of electrons from the battery stops.  Charge on the metal plates reached to maximum.  No current flows through the capacitor.  Capacitor is fully-charged. Theory of dielectrics The molecules of dielectric are polarized by the electric field between the plates. The surface of the dielectric facing the positive plate is charged negative and vice versa. A reverse (polarized) electric field is set up in dielectric. Resultant electric field between the plates decreases. Eresultant = E – Epolarisation From Eresultant = , potential difference between the plates

6.

Capacitance energy, U = QV = CV2 =



Work done by applied voltage in charging capacitor, W = VQ



Energy stored in charged capacitor = QVacross the capacitor



Energy dissipated as heat by the same capacitor during

decreases. From C = , C is increased. Series and parallel circuit Series

8.

Current, I is constant. From Q = It, Q is constant. QT = Q 1 = Q 2 = Q 3 VT = V 1 + V 2 + V 3 )

Parallel

Voltage, V is constant Vtotal = V1 = V2 = V3 QT = Q 1 + Q 2 + Q 3 CTVT = C1V1 + C2V2 + C3V3 CTVT = VT ( C1 + C2 + C3 ) CT = C1 + C2 + C3

discharging = C 

Work done against electric field when capacitance is changed = C1

7.

– C

- C2

Charging and discharging of capacitor through a resistor Charging Discharging V = Vo (1 –

)

V = Vo (

)

Q = Qo (1 –

)

Q = Qo (

)

I = Io ( ) I = Io ( ) Vo = maximum voltage available relative to initial voltage

In STPM syllabus: (“-“ sign of discharging current have been considered earlier in the derivation)

They are positive or negative charge on either each plate of capacitor with same magnitude.

9.

Principle of conservation of charge = Electric charge can neither be created nor destroyed.  The magnitude of the net quantity of charge (of two capacitors) is the magnitude of algebraic sum of all the charge on one side where both capacitors are connected.  Algebraic sum of charge took into account of positive and negative terminals. 10. Clarification on series and parallel. C12F is series with C6F C8F is parallel with C12F + 6F C8F is not parallel with C12F C8F is not parallel with C6F

5. Simple formulation : Q = CV Answering question such as :

Connected to battery (Switch to A) V = constant =

= e.m.f.battery

Disconnected from battery (Switch to B) Q = constant C1V1 = C2V2= C3V3 = CtotalVtotal

If +ve sign of C1 connected to +ve sign of C2. C1 is parallel with C2. * Cnet = +C1 + (+C2) Qnet = +Q1 + (+Q1) If +ve sign of C1 connected to -ve sign of C2. C1 is series with C2 * Cnet = +C1 + (+C2) Qnet = +Q1 + (-Q2 )