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1 Capacitance and Dielectrics Chapter 26Capacitance and Dielectrics
2 IntroIn this chapter we will introduce the first of the 3 simple electric circuit elements that we will discuss in AP PhysicsCapacitorResistorInductorCapacitors are commonly used devices often as form of energy storage in a circuit.
3 26.1Capacitor- two conductors separated by an insulator called a dielectric.Often times the two conductors of a capacitor are called plates.If the plates carry a charges of equal magnitude and opposite sign, there will exist a potential difference (ΔV) or a voltage between the two.
4 26.1 How much charge can we store on the plates? Experiments have shown that the amount of charge stored increases linearly with voltage between the conductors.We will call the constant of proportionality Capacitance .
5 26.1Capacitance is defined as the ratio of the magnitude of the charge on either conductor to the magnitude of the potential difference between the conductors.Capacitance is a constant for a given capacitor and has units of C/V or F (farad)
6 26.11 Farad is a Coulomb of charge per Volt which is a huge capacitance.More commonly in the 10-6 to range or microfarads (μF) to picofarads (pF)Actual capacitors may often be marked mF (microfarad) or mmF (micromicrofarad)
7 26.1Consider two parallel plates attached to a battery (potential difference source)The battery creates an Electric fieldwithin the wires, moving electronsonto the negative plate.This continues until the negativeterminal, wire and plate areequipotential.
8 26.1The opposite occurs with the positive terminal, pulling electrons from the plate until the plate/wire/+ terminal are equipotential.Example-A 4 pF capacitor will be able to store 4 pC of charge for every volt of potential difference between the plates.If we attach a 1.5 V battery, one of the conductors will have a +6 pC charge, the other will have -6 pC.12 V battery?
10 26.2 Calculating Capacitance We can derive expressions for the capacitance of pairs of oppositely charged conductors by calculating ΔV using techniques from the previous chapters.The calculations are generally straightforward for simple capacitors with symmetrical geometry.
11 26.2 A single conductor Sphere (w/infinite imaginary shell) Since at the sphere, V = kQ/R, and at ∞, V = 0
12 26.2 Parallel Plate Capacitors For plates whose separation is much smaller than their size.From earlier (Ch 24) the E field between the plates isSo the potential difference is
13 26.2 And the capacitance is therefore The capacitance is proportional to area and inversely proportional to the plate separation.True in the middle of the plates, but not near the edges.
14 26.2 The capacitor stores electrical potential energy as well as charge due to the separationof the positive and negativecharges on the plates.Quick Quiz p. 800Examples
17 26.3 Combinations of Capacitors Often two or more capacitors are combined in electric circuits.We can calculate the equivalent capacitance of a circuit, based on how the capacitors are connected.We will use circuit diagrams (schematics) as pictorial representations of the circuit.
19 26.3Parallel Combination- two capacitors connected with their own conducting path to the battery.The potential difference acrosseach capacitor is the same, andits equal to the potential across thecombination.
20 26.3When the battery is connected electrons are removed from the positive plates and deposited on the negative plates.This flow of charge ceases when the potential across the plates reaches that of the battery.The capacitors are then at maximum charge Q1 and Q2, with a total charge given by
21 26.3Because the voltages across the capacitors are the same the charges areIf we wanted to replace the two capacitors with a single equivalent capacitor the total charge stored must be
22 26.3 Therefore the equivalent capacitance must be The equivalent capacitance for any number of parallel combination of capacitors is
24 26.3Series Combination- two or more capacitors connected along the same conducting path
25 26.3As the battery charges the capacitors the electrons leaving the positive plate of C1 end up on the negative plate of C2.The electrons from the positive plate of C2 move to the negative plate of C1.All capacitors hold the same charges.
26 26.3 The voltage of the battery is split across the capacitors. The total potential difference across a series combination of capacitors is the sum of the potential difference across each individual capacitor.
27 26.3If we wanted to find one capacitor equivalent to the series combination, the total potential difference isAnd each individual is
31 26.4 Energy Stored in a Charged Capacitor To determine the energy in a capacitor, we’re going to look at an atypical charging process.We’re going to imagine moving the charge from one plate to the other mechanically, through the space in between.
32 26.4Assume we currently have a charge q on our capacitor, giving the current potential difference to be ΔV = q/C.The work it will take to move a small increment of charge across the gap is
33 26.4The total work W, required to charge the capacitor from q = 0 to q = Q is
34 26.4The work done in charging the capacitor is the Electrical Potential Energy stored and applies to any capacitor regardless of geometry.Energy increases as both Charge and Voltage increase, within a limit. At high enough potential difference, discharge will occur between the plates.
35 26.4We can describe the energy as being stored in the electric field between the plates.For parallel plate capsTherefore
36 26.4We use this expression to derive a new quantity called Energy Density (uE)Since the volume occupied by the field is Ad, the energy U per unit volume is (U/Ad)The energy density of an E-field is proportional the square of the magnitude of the E-field at a given point.
38 26.4Defibrillation- Capacitors store 360 J of energy at a potential difference that will deliver the energy in a time of 2 ms.Circuitry allows the capacitor to be charged (to a much higher voltage than the battery) over several seconds.Similar technology to camera flashes
39 26.5 Capacitors and Dielectrics A dielectric is a non-conducting material (rubber/glass/waxed paper) that can be placed between the plates of a capacitor to increase its capacitance.If the space is entirely filled with the dielectric material, C will increase by a dimensionless factor κ, the dielectric constant of the material.
40 26.5 The new capacitance voltage and charge will be given by (for constant charges)(for constant voltage)Or specifically for a parallel plate capacitor
42 26.5 Again we see that capacitance still increases with decreasing d. In practice though, there is a lower limit to d before discharge across the plates will occur for a given voltage.So for any given capacitor of separation d, there is a maximum voltage limit.
43 26.5 This limit depends on a factor called the Dielectric Strength. This is the maximum Electric Field (V/m) that the material can withstand before its insulating properties break down and the material becomes a conductor.Similar in concept to the spark touching a doorknob and also corona discharge.
44 26.5For parallel plates, the maximum voltage (AKA “working voltage,” “breakdown voltage,” and “rated voltage” is determined byWhere Emax is the dielectric strength
45 26.5Table of Dielectric Constants/Strengths p. 812
46 26.5 Types of Commercial Capacitors Tubular Capacitors- metallic foil interlaced with wax paper/mylar, rolled into a tube.
47 26.5High voltage Capacitors- interwoven metallic plates immersed in an insulating (silicon) oil.
48 26.5 Electrolytic Capacitors Designed for large charges at low voltages.One conducting foil immersed in an conducting fluid (electrolyte)The metal forms a thin insulating oxide layer when voltage is applied.
49 26.5Variable Capacitors- interwoven sets of plates with one set fixed and one set able to be rotated.Typical used for tuning dial circuits (radios, power supplies etc)