Capacitor

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CHAPTER 1
Capacitor & Dielectric

Miss Nor Amalina Ahmad 2012

1.1 1.2 1.3 1.4 1.5

Capacitance Capacitor in series & parallel Energy stored in a charged capacitor Charging & discharging of capacitors Capacitors with dielectric

Miss Nor Amalina Ahmad 2012

1.1 Capacitance
Learning Outcomes :
At the end of this topic, students should be able to:  Define capacitance  Use the capacitance formula  Determine capacitance of parallel plate

Miss Nor Amalina Ahmad 2012

 A capacitor is electric component to store electric charge  It consists two parallel conductors plates, one positively charged and the other is negatively charged  The two plates are placed parallel with each other at a small distance apart

Miss Nor Amalina Ahmad 2012

Symbol

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 The conducting plates could be in the shape of : i. cylindrical ii. spherical iii. parallel plate

cylindrical

spherical

Parallel plate

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Charging & Discharging
 When a capacitor is connected to a circuit with Direct Current (DC) source, two processes, which are called “charging” and “discharging” the capacitor will happen i. Charging process



The capacitor is connected to the DC power supply and current flows through the circuit

Charging process

Miss Nor Amalina Ahmad 2012



Both plates get the equal and opposite charges and an increasing potential difference, vc, is created while the capacitor is charging Once the voltage at the terminals of the capacitor, vc, is equal to the power supply voltage, vc = V, the capacitor is fully charged and the current stops flowing through the circuit



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ii.

Discharging process

 When the capacitor disconnected from the power supply, the capacitor is discharging through the Resistor RD and the voltage between the plates drops down gradually to zero, vc = 0

Discharging process

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 Figure below shows a simple way of charging each plate by using a battery  If the battery is disconnected the charge remain on the plates  Electric field exists between the plates which indicate the existence of potential difference (voltage) between the plates

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 Energy is now stored and ready to be used
 Dielectric materials are usually used to fill the gap between plates, not only to stop the plates from collapsing but the materials have properties which can enhance the characteristics of the capacitors

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Ch a r g i n g p r o ce ss

capacitor

+

-

battery

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Use s o f ca p a ci t o r s
 Capacitors are commonly used in many electronic circuits  Examples : i. photoflash unit – capacitor store large amount of energy which can be released in a flash when triggered

ii. Radios, televisions, computers

Miss Nor Amalina Ahmad 2012

Capacitance
Definition : the amount of charge stored on each plate per unit voltage applied across the plates C=Q V symbol : C unit : Coulomb = Farad (F) Volt E +Q -Q

+

-

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Exercise 1 1. When a potential difference of 24V is applied across a capacitor, the charge on the capacitor is 120 μC. What is the capacitance of the capacitor? [Ans : 5x10-6] 2. What is the charge on a 300 pF capacitor when it is charged to a voltage of 1.0 kV? [Ans : 3x10-7] 3. What is the potential difference across a capacitor with 235 μC and 526 nF? [Ans : 446.8]

Miss Nor Amalina Ahmad 2012

Pa r a l l e l - p l a t e Ca p a ci t o r s
 Consider a two parallel plates capacitor of equal area A are separated by a distance d and the space between plates is vacuum/air : d + +Q + + + V

-

A
-Q

E

 When the capacitor is charged, its plates have charges of equal magnitudes but opposite signs: + Q and −Q then the potential difference V across the plates is produced Miss Nor Amalina Ahmad 2012

 Since...
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