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Capacitors Chapter 12

Capacitors Chapter 12. Definition. Capacitance – the ability of a component to store energy by accumulating charge A capacitor is a circuit component designed to store charge

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Capacitors Chapter 12

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  1. CapacitorsChapter 12

  2. Definition • Capacitance – the ability of a component to store energy by accumulating charge • A capacitor is a circuit component designed to store charge • Practical applications with capacitors:Camera flash – Charges up and then quickly dischargesPower storage – Solar collectors charge up capacitors so that energy can be used after dark

  3. Capacitor Construction • 2 Plates • Separated by a Dielectric

  4. Variable Capacitors • Interleaved-Plate Capacitors

  5. Fixed Value Capacitors • Polarized Electrolytic Capacitors • Most electrolytic capacitors are polarized

  6. Capacitance • Amount of charge that a capacitor can store per unit volt applied where C = the capacitance of the component, in Coulombs per Volt defined as Farad (F) [C] = [Q]/[V]=C/V = F. Q = the total charge stored by the component V = the voltage across the capacitor corresponding to the value of Q

  7. Capacitance Examples C = C = C =

  8. Unit of Measure – farad (F) = 1 coulomb per volt (C/V) • Typical ranges • Most capacitors fall in the picofarad (pF) to microfarad (F) range • Tolerance • Usually fairly poor • Variable capacitors used where exact values required

  9. Capacitor Value Codes • Physically large capacitors usually have their values printed directly on the case • Smaller capacitors are generally labeled using a code: • 2-digit code: the number represents the value of the component in pF Example: 15 = 15 pF • 3-digit code: the code is interpreted like the first three digits of a resistor code Example: 473 = 47 x 103pF = 47 nF • The numbers 6 and 7 are not used as multiplier values • The numbers 8 and 9 are decoded as follows: 8 = 0.01 and 9 = 0.1 Example: 158 = 0.15 pF

  10. A Physical Characteristics of Capacitors d where C = the capacity of the component, in farads (F) e = permittivity of the dielectric A = the area of either plate, in square meters (m2) d = the distance between the plates, in meters (m) What are the units of e ?

  11. Comparison to Resistance • For resistance, R = rL/A • For capacitance, C = eA/d • As r increases, R increases; as e increases, C increases • As L increases, R increases; as d increases, C decreases • As A increases, R decreases; as A increases, C increases

  12. Permittivity • Permittivity of a capacitor dielectric is  = o x r - Permittivity of a vacuum: eo = 8.85x10-12 F/m MULTIPLIED BY - The relative permittivity of the material r e.g.: Materialerair 1paper 2.5mica 5glass 7.5

  13. Team Activity 1 • If you have a capacitor with the following parameters, what is its capacitance? • Plate cross-sectional area = 1cm2Dielectric material = airdistance between plates = 2cm • What happens to the capacitance if you change the dielectric to oil and the distance between plates to 1cm? • For the original dielectric material and plate distance, what would the cross-sectional area need to be to create a 1 F capacitor?

  14. Series Capacitors Where CT = the total series capacitance Cn = the highest-numbered capacitor in the circuit

  15. Team Activity 2Determine the total capacitance

  16. Parallel Capacitors A1 A2 where Cn = the highest-numbered capacitor in the parallel circuit

  17. Team Activity3Determine the total capacitance

  18. Demonstration http://www.howstuffworks.com/framed.htm?parent=capacitor.htm&url=http://micro.magnet.fsu.edu/electromag/java/capacitor/

  19. Relationship between Capacitor Voltage and Current Capacitor Current vc _ i + where i = the instantaneous value of capacitor current C = the capacity, in farads = the instantaneous rate of change in capacitor voltage

  20. Team Activity 4 • If the voltage across a 2F capacitor is what is the current through the capacitor?

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