chapter 10 capacitors and capacitance. 2 capacitance capacitor –stores charge –two conductive...
TRANSCRIPT
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Chapter 10
Capacitors and Capacitance
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Capacitance• Capacitor
– Stores charge– Two conductive plates separated by insulator– Insulating material called dielectric– Conductive plates can become charged with
opposite charges
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Definition of Capacitance• Amount of charge Q that a capacitor can
store depends on applied voltage
• Relationship between charge and voltage given by
Q = CV or C = Q/V (Similar to Ohm’s Law)
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Definition of Capacitance• C is capacitance of the capacitor
• Unit is the farad (F)
• Capacitance of a capacitor – One farad if it stores one coulomb of charge– When the voltage across its terminals is one
volt
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Effect of Area• Capacitance is directly proportional to
amount of charge
• Larger plate will be able to hold more charge
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Effect of Area• Capacitance is directly proportional to
plate area
• If plate area is doubled, capacitance is doubled
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Effect of Spacing• As plates are moved closer together
– Force of attraction between opposite charges is greater
• Capacitance– Inversely proportional to distance between
plates
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Effect of Spacing• Double the distance between plates
– Capacitance becomes half as much
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Effect of Dielectric• If a dielectric other than air is used
between the plates– More charge can build up on the plates
• The factor by which the capacitance increases– Dielectric constant or the relative permittivity
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Effect of Dielectric• Permittivity
– How easy it is to establish electric flux in a material
– Represented by ε (Greek letter epsilon)
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Capacitance of a Parallel-Plate Capacitor
• Directly proportional to plate area
• Inversely proportional to plate separation
• Dependent on dielectric
• A farad is a very large unit d
AC =∈
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Electric Flux
• Electric fields – Force fields in region surrounding charged
bodies
• Direction of this field is direction of force on a positive test charge
• Field lines never cross
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Electric Flux
• Density of lines indicate field strength
• Electric field lines are indicated by (Greek letter psi)
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Electric Fields
• Strength of an electric field is force that field exerts on a small test charge– E = F/Q
• Electric flux density = total flux/area– D = /A
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Electric Fields• Flux is due to the charge Q
• The number of flux lines coming from a charge is equal to the charge itself = Q
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Field of a Parallel-Plate Capacitor
• To move a charge from the negative plate to the positive plate requires work
• Work = Force × distance
• Voltage = Work/charge
• E = V/d
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Field of a Parallel-Plate Capacitor
• Electric field strength between plates– Equal to voltage between them – Divided by distance between them
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Voltage Breakdown• If voltage is increased enough, dielectric
breaks down
• This is dielectric strength or breakdown voltage
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Voltage Breakdown• Breakdown can occur in any type of
apparatus where insulation is stressed
• Capacitors are rated for maximum operating voltage
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Nonideal Effects• Leakage current
• Equivalent Series Resistance
• Dielectric Absorption
• Temperature Coefficient
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Fixed Capacitors
• Ceramic Capacitors– Values change little with temperature, voltage,
or aging
• Plastic Film Capacitors
• Mica Capacitors– Low cost, low leakage, good stability
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Fixed Capacitors
• Electrolytic Capacitors– Large capacitance at low cost– Polarized
• Surface Mount Capacitors
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Variable Capacitors• Used to tune a radio
• Stationary plates and movable plates– Combined and mounted on a shaft
• A trimmer or padder capacitor is used to make fine adjustments on a circuit
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Capacitors in Parallel• Total charge on capacitors is sum of all
charges
• Q = CV
• CTE = C1V1 + C2V2 + C3V3
• All voltages are equal
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Capacitors in Parallel• CT = C1 + C2 + C3
• Total capacitance of capacitors in parallel– Sum of their capacitances (like resistors in
series)
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Capacitors in Series• Same charge appears on all capacitors
• Total V – Sum of individual voltages (like resistors in
parallel)
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Capacitors in Series
321
321
1111
CCCC
C
Q
C
Q
C
Q
C
QC
QV
++=
++=
=
T
T
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Capacitor Voltage• Voltage across a capacitor does not
change instantaneously
• Voltage begins at zero and gradually climbs to full voltage
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Capacitor Voltage• Full voltage is source voltage
• May range from nanoseconds to milliseconds – Depending on the resistance and
capacitance
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Capacitor Current • During charging
– Electrons move from one plate to another
• Current lasts only until capacitor is charged
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Capacitor Current • Current
– Large initial spike to zero
• No current passes through dielectric
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Energy Stored in a Capacitor
• A capacitor does not dissipate power
• When power is transferred to a capacitor– Stored as energy
2
2
1CV=Energy
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Capacitor Failures and Troubleshooting
• Reasons for capacitor’s failure– Excessive voltage, current, or temperature, or
aging
• Test with an ohmmeter– Good capacitor will read low, then gradually
increase to infinity
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Capacitor Failures and Troubleshooting
• Capacitor short– Meter resistance will stay low
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Capacitor Failures and Troubleshooting
• If capacitor is leaky– Reading will be lower than normal
• If open– Stays at infinity