Edexcel A2 Physics Unit 4 : Chapter 2.2: Capacitance

Prepared By: Shakil Raiman

2.2.1 Capacitor & Capacitance Capacitor is a device that stores electric charge. The amount of charge stored per unit potential

difference across the plates of a capacitor is called capacitance.

Its unit is Farad (F). Capacitance can be calculated from the equation

V

QC

2.2.2 Parallel Plate Capacitor, Circuit Symbol & Pictures

A Pair of plates with an insulator between them is called a parallel plate capacitor.

2.2.2 Parallel Plate Capacitor

Problem: 1

2.2.3 Energy Stored on a Charged Capacitor

A charged capacitor is a store of electrical potential energy. When the capacitor is discharged, this energy can be transferred into other forms.

The energy stored in a charged capacitor is given by,

QVE2

1

2.2.3 Energy Stored on a Charged Capacitor

But

Therefore,

or,

CVQ

2

2

1CVE

C

QE

2

2

1

Questions From Student’s BookPage No: 53

Questions From Student’s BookPage No: 53

2.2.4: Storing Charge

2.2.4: Storing Charge

In general charge need a complete circuit to flow. But is special case, charge can be made to flow in

an incomplete circuit. When the power supply is connected, the electric field created in the conducting wires causes electrons to flow towards the positive terminal. Since the electrons cannot cross the gap between the plates they build up on the plate connected to the negative terminal, which becomes negatively charged.

2.2.4: Storing Charge

Electrons in the plate connected to the positive terminal flow towards the positive terminal, resulting in a positive charge in the plate. The attraction between the opposite charges across the gap creates an electric field between the plates which increases until the pd across the plates is equal to the pd of the power supply.

2.2.5: Charging and Discharging Capacitors

2.2.5: Charging and Discharging Capacitors

For the same maximum pd, increasing the capacitance, C, will increase the charge stored, as Q=CV.

Alternatively, the charge would flow more slowly if the resistance, R, in the lamp circuit was greater.

2.2.6: Time constant

Time Constant: Time constant is a measure of how long it takes for a resistor-capacitor combination to discharge.

It is calculated by the equation: Unit: s (second) Time constant tells us how many seconds it

takes for the current to fall to 37% of its starting value.

RC

2.2.6: Time constant: Problem

Find the time constant in the given circuit.

2.2.7: Discharging Capacitor Equations:

Charging and discharging of a capacitor follows curved graphs in which the current is constantly changing, and so the rate of change of charge and pd are also constantly changing. These graphs are known as exponential curves.

RC

t

eQQ

0 RC

t

eII

0RC

t

eVV

0

2.2.7: Discharging Capacitor Problems:

1.

2.

2.2.7: Discharging Capacitor Problems:

3.

2.2.8: Capacitor Calculus:

2.2.8: Capacitor Calculus:

2.2.8: Capacitor Calculus:

2.2.9: The 37% Life:

Thank You All

Wish you all very good luck.