chapter 15 capacitance and rc circuits © goodheart-willcox co., inc.permission granted to reproduce...
TRANSCRIPT
![Page 1: Chapter 15 Capacitance and RC Circuits © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Define capacitance](https://reader035.vdocuments.site/reader035/viewer/2022062511/551c273a550346a34f8b5e88/html5/thumbnails/1.jpg)
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Chapter 15
Capacitance and RC Circuits
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Objectives
• Define capacitance and capacitor.
• Identify the many different types of capacitors.
• Describe the transient response of a capacitor.
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Objectives
• Explain how a capacitor behaves in a dc circuit.
• Discuss the effect of capacitance on an ac circuit.
• Describe the results of combining capacitance and resistance in a circuit.
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Capacitance and the Capacitor
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Capacitance and the Capacitor (Cont.)
• Dielectrics• Conductive plates
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Capacitance and the Capacitor (Cont.)
• Electrons from negative terminal of source flow to one plate of capacitor
• Capacitor blocks direct current
• Dielectric constants• Working voltage (WV)
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Calculating Capacitance
• Measured in farads (F)
• Determined by – Dielectric material– Area of plates– Distance between plates
E
QC
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Calculating Capacitance (Cont.)
• Capacity increases with area of plates or dielectric constant
• Capacity decreases when distance between plates increases
d
n–KAC
1)( 0.225
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Types of Capacitors
• Fixed paper capacitors
• Rectangular oil filled capacitors
• Can type electrolytic capacitors
• Tubular electrolytic capacitors
• Ceramic capacitors
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Types of Capacitors (Cont.)
• Mica capacitors
• Variable capacitors– Stators– Rotors
• Trimmer capacitors– Adjusted with alignment tools
• Tantalum capacitors
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Review
What is capacitance?
A property that opposes any change in voltage
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Review
What is a device that temporarily stores an electric charge?
A capacitor
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Review
What is the insulation separating the two plates in a capacitor?
Dielectric
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Review
What are the three factors that determine capacitance?
Material used as a dielectric, area of the plates, and distance between the plates
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Transient Response of the Capacitor
• Response of current and voltage immediately after change in applied voltage
• RC circuits
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Transient Response of the Capacitor (Cont.)
• Voltage builds up gradually with closed switch
• Current falls as capacitor charges
• Capacitor remains charged when switch opens
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Transient Response Example
• Short circuit in this RC circuit when switch closes
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Transient Response Example (Cont.)
• Discharge current rises to maximum and falls off
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RC Time Constant
• Time between charge and discharge of voltage and current in RC circuit
• Time constant = R × C• Five time constant periods needed for complete charge or
discharge
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RC Time Constant (Cont.)
• At ignition, or firing voltage, lamp glows
• Repetitive cycle, causes neon to flash
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Capacitors in Series
21
21T CC
CCC
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Capacitors in Series (Cont.)
N321T CCCCC
1
1
1
1
1
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Capacitors in Parallel
• CT = C1 + C2 + C3 … + CN
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Review
What does transient response refer to?
The response of current and voltage in a circuit immediately after a change in applied voltage
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Review
What is the amount of time needed for the capacitor to charge or discharge 63.2 percent?
The time constant of the circuit
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Capacitance in AC Circuits
• Power in capacitive circuits
• Resistance and capacitance in AC circuits
• Parallel RC circuits
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Capacitance in AC Circuits (Cont.)
• AC voltage causes plates to charge and discharge repeatedly
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Capacitance in AC Circuits (Cont.)
• Current begins at maximum
• Current leads voltage by 90° in capacitive circuit
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Capacitance in AC Circuits (Cont.)
• Current depends on rate of capacitor’s charge and discharge
• As ac frequency increases, current increases
• Capacitive reactance
fCX C 2
1
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Capacitance Example
• 10 µF capacitor
• 120-hertz frequency
fCX C 2
1
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Capacitance Example (Cont.)
F) 10 (10 Hz) (120 6.28
1
6-CX
132.7 0.007536
1 CX
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Power in Capacitive Circuits
• Wattless power• Apparent power = 100 V × 10 A = 1000 VA
powerApparent
power True cos (PF)factor Power
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Resistance and Capacitance in AC Circuits
• Capacitive reactance causes phase displacement
• Total resistance is vector sum of XC and R
• Impedance22 CXRZ
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Resistance and Capacitance Example
Z = 500 Ω
22 ) (400 ) (300 Z
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Resistance and Capacitance Example (Cont.)
• Angle between Z and R is phase displacement between current and voltage
cos = 0.6 = 53.1°
Z
R PF cos
500
300 cos
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Resistance and Capacitance Example (Cont.)
• Apparent power = I × E– Apparent power = 0.2 A × 100 V
Apparent power = 20 VA
• True power = I × E × cos – True power = 0.2 A × 100 V × cos 53.1°
True power = 12 W
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Parallel RC Circuits
• No phase difference among voltages
• Phase difference among total and branch currents
• Current in phase with resistive branch’s voltage
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Parallel RC Circuit Formulas
T
S
I
EZ
22 CRT III
R
C
I
I arctan
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Parallel RC Circuit Example
• Capacitive reactance
XC 80 Ω
• Branch currents
F) 10 (2 Hz) 10 (1
0.159
6-3 CX
A 0.2 100
V 20
RI
A 0.25 80
V 20
CI
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Parallel RC Circuit Example (Cont.)
• Phase angle
arctan 1.25 = 51.3°
• Total circuit current
• Impedance
A 0.2
A 0.25arctan
A 0.32 A) (0.25 A) (0.2 22 IT
62.5 A 0.32
V 20 Z
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Review
What is capacitive reactance?
The resistance resulting from capacitance
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Review
What is power called in a capacitive circuit?
Wattless power
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Review
What is the ratio of true power to apparent power in an ac circuit called?
Power factor (PF)
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For Discussion
• Research different types of capacitors, focusing on construction and various applications.
• Discuss the reasons a capacitor blocks dc while passing ac.