capacitors 12th class

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Chapter 12

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Summary The Capacitor Capacitors are one of the fundamental passive components. In its most basic form, it is composed of two plates separated by a dielectric. The ability to store charge is the definition of capacitance . Dielectric Conductors

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Summary The Capacitor Initially uncharged Charging Fully charged Source removed The charging process… A capacitor with stored charge can act as a temporary battery.

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Capacitance is the ratio of charge to voltage Rearranging, the amount of charge on a capacitor is determined by the size of the capacitor ( C ) and the voltage ( V ). Example If a 22 m F capacitor is connected to a 10 V source, the charge is 220 m C Capacitance

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Capacitance An analogy: Imagine you store rubber bands in a bottle that is nearly full. You could store more rubber bands (like charge or Q ) in a bigger bottle (capacitance or C ) or if you push them in more (voltage or V ). Thus,

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A capacitor stores energy in the form of an electric field that is established by the opposite charges on the two plates. The energy of a charged capacitor is given by the equation Capacitance where W = the energy in joules C = the capacitance in farads V = the voltage in volts

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The capacitance of a capacitor depends on three physical characteristics. Summary C is directly proportional to and the plate area . the relative dielectric constant C is inversely proportional to the distance between the plates Capacitance

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Summary Find the capacitance of a 4.0 cm diameter sensor immersed in oil if the plates are separated by 0.25 mm. The plate area is The distance between the plates is Capacitance Example 178 pF

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Summary Capacitor types Mica Mica capacitors are small with high working voltage. The working voltage is the voltage limit that cannot be exceeded.

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Summary Capacitor types Ceramic disk Ceramic disks are small nonpolarized capacitors They have relatively high capacitance due to high e r.

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Summary Capacitor types Plastic Film Plastic film capacitors are small and nonpolarized. They have relatively high capacitance due to larger plate area.

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Summary Capacitor types Electrolytic (two types) Symbol for any electrolytic capacitor Al electrolytic Ta electrolytic Electrolytic capacitors have very high capacitance but they are not as precise as other types and tend to have more leakage current. Electrolytic types are polarized.

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Summary Capacitor types Variable Variable capacitors typically have small capacitance values and are usually adjusted manually. A solid-state device that is used as a variable capacitor is the varactor diode; it is adjusted with an electrical signal.

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Capacitor labeling Capacitors use several labeling methods. Small capacitors values are frequently stamped on them such as .001 or .01, which have units of microfarads. Electrolytic capacitors have larger values, so are read as m F. The unit is usually stamped as m F, but some older ones may be shown as MF or MMF).

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A label such as 103 or 104 is read as 10x10 3 (10,000 pF) or 10x10 4 (100,000 pF) respectively. (Third digit is the multiplier.) Capacitor labeling When values are marked as 330 or 6800, the units are picofarads. Example What is the value of each capacitor? Both are 2200 pF.

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Summary Series capacitors When capacitors are connected in series, the total capacitance is smaller than the smallest one. The general equation for capacitors in series is The total capacitance of two capacitors is …or you can use the product-over-sum rule

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Summary Series capacitors Example If a 0.001 m F capacitor is connected in series with an 800 pF capacitor, the total capacitance is 444 pF

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Summary Parallel capacitors When capacitors are connected in parallel, the total capacitance is the sum of the individual capacitors. The general equation for capacitors in parallel is Example 1800 pF If a 0.001 m F capacitor is connected in parallel with an 800 pF capacitor, the total capacitance is

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Summary Capacitors in dc circuits When a capacitor is charged through a series resistor and dc source, the charging curve is exponential.

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Summary Capacitors in dc circuits When a capacitor is discharged through a resistor, the discharge curve is also an exponential. (Note that the current is negative.)

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Summary Capacitors in dc circuits The same shape curves are seen if a square wave is used for the source. V S V C V R Question What is the shape of the current curve? The current has the same shape as V R .

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Summary Universal exponential curves Specific values for current and voltage can be read from a universal curve. For an RC circuit, the time constant is Rising exponential Falling exponential

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Summary The universal curves can be applied to general formulas for the voltage (or current) curves for RC circuits. The general voltage formula is v =V F + ( V i - V F ) e - t/RC V F = final value of voltage V i = initial value of voltage v = instantaneous value of voltage The final capacitor voltage is greater than the initial voltage when the capacitor is charging, or less that the initial voltage when it is discharging. Universal exponential curves

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Summary Capacitive reactance Capacitive reactance is the opposition to ac by a capacitor. The equation for capacitive reactance is Example The reactance of a 0.047 m F capacitor when a frequency of 15 kHz is applied is 226 W

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Summary Capacitive phase shift When a sine wave is applied to a capacitor, there is a phase shift between voltage and current such that current always leads the voltage by 90 o .

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Summary Power in a capacitor Voltage and current are always 90 o out of phase. For this reason, no true power is dissipated by a capacitor, because stored energy is returned to the circuit. The rate at which a capacitor stores or returns energy is called reactive power . The unit for reactive power is the VAR (volt-ampere reactive). Energy is stored by the capacitor during a portion of the ac cycle and returned to the source during another portion of the cycle.

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Summary Switched capacitors Switched capacitors move charge in a specific time interval between two points that are different voltages. The switched capacitors emulate a resistor with a value of R =1/ fC . Switched capacitors are widely used in certain types of integrated circuits because they can be made very small, have virtually no drift, and do not dissipate heat.

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Selected Key Terms Capacitor Dielectric Farad RC time constant An electrical device consisting of two conductive plates separated by an insulating material and possessing the property of capacitance. The insulating material between the conductive plates of a capacitor. The unit of capacitance. A fixed time interval set by the R and C values, that determine the time response of a series RC circuit. It equals the product of the resistance and the capacitance.

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Capacitive reactance Instantaneous power ( p ) True power ( P true ) Reactive power ( Pr ) VAR (volt-ampere reactive) The value of power in a circuit at a given instant of time. The power that is dissipated in a circuit usually in the form of heat. The opposition of a capacitor to sinusoidal current. The unit is the ohm. Selected Key Terms The rate at which energy is alternately stored and returned to the source by a capacitor. The unit is the VAR. The unit of reactive power.

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Quiz 1. The capacitance of a capacitor will be larger if a. the spacing between the plates is increased b. air replaces oil as the dielectric c. the area of the plates is increased d. all of the above

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Quiz 2. The major advantage of a mica capacitor over other types is a. they have the largest available capacitances b. their voltage rating is very high c. they are polarized d. all of the above

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Quiz 3. Electrolytic capacitors are useful in applications where a. a precise value of capacitance is required b. low leakage current is required c. large capacitance is required d. all of the above

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Quiz 4. If a 0.015 m F capacitor is in series with a 6800 pF capacitor, the total capacitance is a. 1568 pF b. 4678 pF c. 6815 pF d. 0.022 m F

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Quiz 5. Two capacitors that are initially uncharged are connected in series with a dc source. Compared to the larger capacitor, the smaller capacitor will have a. the same charge b. more charge c. less voltage d. the same voltage

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Quiz 6. When a capacitor is connected through a resistor to a dc voltage source, the charge on the capacitor will reach 50% of its final charge in a. less than one time constant b. exactly one time constant c. greater than one time constant d. answer depends on the amount of voltage

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Quiz 7. When a capacitor is connected through a series resistor and switch to a dc voltage source, the voltage across the resistor after the switch is closed has the shape of a. a straight line b. a rising exponential c. a falling exponential d. none of the above

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Quiz 8. The capacitive reactance of a 100 m F capacitor to 60 Hz is a. 6.14 k W b. 265 W c. 37.7 W d. 26.5 W

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Quiz 9. If an sine wave from a function generator is applied to a capacitor, the current will a. lag voltage by 90 o b. lag voltage by 45 o c. be in phase with the voltage d. none of the above

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Quiz 10. A switched capacitor emulates a a. smaller capacitor b. larger capacitor c. battery d. resistor

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Quiz Answers: 1. c 2. b 3. c 4. b 5. a 6. a 7. c 8. d 9. d 10. d