iclicker A metal ball of radius R has a charge q. Charge is changed q >  2q. How does it s capacitance changed?


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2 iclicker A metal ball of radius R has a charge q. Charge is changed q >  2q. How does it s capacitance changed? q A: C>2 C0 B: C> C0 C: C> C0/2 D: C> C0 E: C>2 C0 2
3 iclicker A metal ball of radius R has a charge q. Charge is changed q >  2q. How does it s capacitance changed? q A: C>2 C0 B: C> C0 A: C> C0/2 D: C>4 C0 A: C>8 C0 3
4 Physics of a spark +q q V d E V/d 4
5 Physics of a spark V d1 E V/d 1 E 0 +q q V d E V/d 4
6 Physics of a spark V E V/d 1 E 0 d1 λ e E +q q V d E k Eλ 1eV E V/d 4
7 Physics of a spark V E V/d 1 E 0 d1 λ e E +q q V d E V/d E k Eλ 1eV λ 1µm E spark MeV/m 4
8 Energy Stored in Capacitors Capacitors store electric energy U = 1 2 qv q = CV (V created by q s, selfinteraction) U = 1 2 CV 2 q 2 U = 1 2 C or 37
9 Energy Density in Capacitors (1) We define the energy density, u, as the electric potential energy per unit volume Taking the ideal case of a parallel plate capacitor that has no fringe field, the volume between the plates is the area of each plate times the distance between the plates, Ad Inserting our formula for the capacitance of a parallel plate capacitor we find 38
10 Energy Density in Capacitors (2) Recognizing that V/d is the magnitude of the electric field, E, we obtain an expression for the electric potential energy density for parallel plate capacitor This result, which we derived for the parallel plate capacitor, is in fact completely general. This equation holds for all electric fields produced in any way The formula gives the quantity of electric field energy per unit volume. 39
11 Example: Isolated Conducting Sphere (1) An isolated conducting sphere whose radius R is 6.85 cm has a charge of q=1.25 nc. Question 1: How much potential energy is stored in the electric field of the charged conductor? Answer: Key Idea: An isolated sphere has a capacitance of C=4πε 0 R (see previous lecture). The energy U stored in a capacitor depends on the charge and the capacitance according to and substituting C=4πε 0 R gives 40
12 Example: Isolated Conducting Sphere (2) An isolated conducting sphere whose radius R is 6.85 cm has a charge of q = 1.25 nc. Question 2: What is the field energy density at the surface of the sphere? Answer: Key Idea: The energy density u depends on the magnitude of the electric field E according to q so we must first find the E field at the surface of the sphere. Recall: 41
13 What is the total energy in Efield? U tot = R udv = 1 4π R 2 0E 2 r 2 dr = 2 1 q 2 2π 0 4π 0 r 4 r2 dr = R 1 q 2 2 4π 0 R = 1 2 qv 10
14 What is the total energy in Efield? U tot = R udv = dv = dφ sin θdθr 2 dr =4πr 2 dr 1 4π R 2 0E 2 r 2 dr = 2 1 q 2 2π 0 4π 0 r 4 r2 dr = R 1 q 2 2 4π 0 R = 1 2 qv 10
15 What is the total energy in Efield? U tot = R udv = 1 4π R 2 0E 2 r 2 dr = 2 1 q 2 2π 0 4π 0 r 4 r2 dr = R 1 q 2 2 4π 0 R = 1 2 qv 10
16 What is the total energy in Efield? U tot = R udv = 1 4π R 2 0E 2 r 2 dr = 2 1 q 2 2π 0 R 4π 0 r 4 r2 dr = 1 q 2 2 4π 0 R = 1 qv Yes! 2 10
17 Example: Thundercloud (1) Suppose a thundercloud with horizontal dimensions of 2.0 km by 3.0 km hovers over a flat area, at an altitude of 500 m and carries a charge of 160 C. Question 1: What is the potential difference between the cloud and the ground? Question 2: Knowing that lightning strikes require electric field strengths of approximately 2.5 MV/m, are these conditions sufficient for a lightning strike? Question 3: What is the total electrical energy contained in this cloud? 42
18 Example: Thundercloud (2) Question 1: What is the potential difference between the cloud and the ground? Answer: We can approximate the cloudground system as a parallel plate capacitor whose capacitance is The charge carried by the cloud is 160 C V = 1 2 q C 720 million volts =
19 Example: Thundercloud (3) Question 2: Knowing that lightning strikes require electric field strengths of approximately 2.5 MV/m, are these conditions sufficient for a lightning strike? Answer: We know the potential difference between the cloud and ground so we can calculate the electric field E is lower than 2.5 MV/m, so no lightning cloud to ground May have lightning to radio tower or tree. 44
20 Example: Thundercloud (4) Question 3: What is the total electrical energy contained in this cloud? Answer: The total energy stored in a parallel place capacitor is 45
21 Electric circuits 15
22 Circuit diagram Lines represent conductors The battery or power supply is represented by The capacitor is represented by the symbol Battery provides (a DC) potential difference V 16
23 Charging/Discharging a Capacitor (2) Illustrate the charging processing using a circuit diagram. This circuit has a switch (pos c) When the switch is in position c, the circuit is open (not connected). (pos a) When the switch is in position a, the battery is connected across the capacitor. Fully charged, q = CV. (pos b) When the switch is in position b, the two plates of the capacitor are connected. Electrons will move around the circuita current will flowand the capacitor will discharge. c c 8
24 demo 18
25  V + 19
26  V + 19
27 I  V + 19
28 I  V + 19
29 I   V
30   V
31   V + V + 19
32   V + V + 19
33   V + V + 19
34 I   V + + V 19
35 I   V + + V 19
36   V + V + 19
37 Capacitors in Circuits A circuit is a set of electrical devices connected with conducting wires Capacitors can be wired together in circuits in parallel or series Capacitors in circuits connected by wires such that the positively charged plates are connected together and the negatively charged plates are connected together, are connected in parallel Capacitors wired together such that the positively charged plate of one capacitor is connected to the negatively charged plate of the next capacitor are connected in series
38 Capacitors in Parallel (1) Consider an electrical circuit with three capacitors wired in parallel Each of three capacitors has one plate connected to the positive terminal of a battery with voltage V and one plate connected to the negative terminal. The potential difference V across each capacitor is the same... key point for capacitors in parallel We can write the charge on each capacitor as 26
39 Capacitors in Parallel (2) We can consider the three capacitors as one equivalent capacitor C eq that holds a total charge q given by We can now define C eq by A general result for n capacitors in parallel is If we can identify capacitors in a circuit that are wired in parallel, we can replace them with an equivalent capacitance 27
40 Capacitors in Series (1) Consider a circuit with three capacitors wired in series The positively charged plate of C 1 is connected to the positive terminal of the battery The negatively charge plate of C 1 is connected to the positively charged plate of C 2 The negatively charged plate of C 2 is connected to the positively charge plate of C 3 The negatively charge plate of C 3 is connected to the negative terminal of the battery The battery produces an equal charge q on each capacitor because the battery induces a positive charge on the positive place of C 1, which induces a negative charge on the opposite plate of C 1, which induces a positive charge on C 2, etc... key point for capacitors in series 28
41 Capacitors in Series (2) Knowing that the charge is the same on all three capacitors we can write We can express an equivalent capacitance C eq as We can generalize to n capacitors in series If we can identify capacitors in a circuit that are wired in series, we can replace them with an equivalent capacitance 29
42 Review The equivalent capacitance for n capacitors in parallel is = The equivalent capacitance for n capacitors in series is = 31
43 iclicker Three capacitors, each with capacitance C, are connected as shown in the figure. What is the equivalent capacitance for this arrangement of capacitors? a) C/3 b) 3C c) C/9 d) 9C e) none of the above
44 iclicker Three capacitors, each with capacitance C, are connected as shown in the figure. What is the equivalent capacitance for this arrangement of capacitors? a) C/3 b) 3C c) C/9 d) 9C e) none of the above
45 Example: System of Capacitors (1) Question: What is the capacitance of this system of capacitors? Method: Find the equivalent capacitance Analyze each piece of the circuit individually, replacing pairs in series or in parallel by one capacitor with equivalent capacitance 32
46 Example: System of Capacitors (2) We can see that C 1 and C 2 are in parallel, and that C 3 is also in parallel with C 1 and C 2 We find C 123 = C 1 + C 2 + C 3 and make a new drawing 33
47 Example: System of Capacitors (3) We can see that C 4 and C 123 are in series We find for the equivalent capacitance: and make a new drawing 34
48 Example: System of Capacitors (4) We can see that C 5 and C 1234 are in parallel We find for the equivalent capacitance and make a new drawing 35
49 Example: System of Capacitors (5) So the equivalent capacitance of our system of capacitors 36
50 Capacitors with Dielectrics (1) So far, we have discussed capacitors with air or vacuum between the plates. However, most reallife capacitors have an insulating material, called a dielectric, between the two plates. The dielectric serves several purposes: Provides a convenient way to maintain mechanical separation between the plates (plates attract!) Provides electrical insulation between the plates Allows the capacitor to hold a higher voltage Increases the capacitance of the capacitor Takes advantage of the molecular structure of the dielectric material 46
51 Capacitors with Dielectrics (2) Placing a dielectric between the plates of a capacitor increases the capacitance of the capacitor by a numerical factor called the dielectric constant, κ We can express the capacitance of a capacitor with a dielectric with dielectric constant κ between the plates as where C air is the capacitance of the capacitor without the dielectric Placing the dielectric between the plates of the capacitor has the effect of lowering the electric field between the plates and allowing more charge to be stored in the capacitor. 47
52 Parallel Plate Capacitor with Dielectric Placing a dielectric between the plates of a parallel plate capacitor modifies the electric field as The constant ε 0 is the electric permittivity of free space The constant ε is the electric permittivity of the dielectric material 48
53 Microscopic Perspective on Dielectrics (1) Let s consider what happens at the atomic and molecular level when a dielectric is placed in an electric field There are two types of dielectric materials Polar dielectric Nonpolar dielectric Polar dielectric material is composed of molecules that have a permanent electric dipole moment due to their molecular structure e.g., water molecules Normally the directions of the electric dipoles are randomly distributed: 53
54 Microscopic Perspective on Dielectrics (2) When an electric field is applied to these polar molecules, they tend to align with the electric field 54
55 Microscopic Perspective on Dielectrics (2) Nonpolar dielectric material is composed of atoms or molecules that have no electric dipole moment 54
56 Microscopic Perspective on Dielectrics (3) These atoms or molecules can be induced to have a dipole moment under the influence of an external electric field This induction is caused by the opposite direction of the electric force on the negative and positive charges of the atom or molecule, which displaces the center of the relative charge distributions and produces an induced electric dipole moment 55
57 Microscopic Perspective on Dielectrics (3) These atoms or molecules can be induced to have a dipole moment under the influence of an external electric field This induction is caused by the opposite direction of the electric force on the negative and positive charges of the atom or molecule, which displaces the center of the relative charge distributions and produces an induced electric dipole moment E 55
58 Microscopic Perspective on Dielectrics (3) These atoms or molecules can be induced to have a dipole moment under the influence of an external electric field This induction is caused by the opposite direction of the electric force on the negative and positive charges of the atom or molecule, which displaces the center of the relative charge distributions and produces an induced electric dipole moment  + E 55
59 Induced Electric field E 40
60 Induced Electric field E E 40
61 Induced Electric field E E E 40
62 Induced Electric field E E Against the external field! E 40
63 Microscopic Perspective on Dielectrics (4) In both the case of the polar and nonpolar dielectric materials, the resulting aligned electric dipole moments tend to partially cancel the original electric field E0 The electric field inside the capacitor then is the original field minus the induced field = E κ 56
64 Microscopic Perspective on Dielectrics (4) In both the case of the polar and nonpolar dielectric materials, the resulting aligned electric dipole moments tend to partially cancel the original electric field E0 Ed The electric field inside the capacitor then is the original field minus the induced field = E κ 56
65 Dielectric Strength The dielectric strength of a material measures the ability of that material to withstand voltage differences If the voltage across a dielectric exceeds the breakdown potential, the dielectric will break down  a spark  and begin to conduct charge between the plates Reallife dielectrics enable a capacitor to provide a given capacitance and withstand the required voltage without breaking down Capacitors are usually specified in terms of their capacitance and rated (i.e., maximum) voltage 51
66 Dielectric Constant The dielectric constant of vacuum is defined to be 1 The dielectric constant of air is close to 1 and we will use the dielectric constant of air as 1 in our problems The dielectric constants of common materials are 52
67 Capacitor with Dielectric (1) Question 1: Consider a parallel plate capacitor with capacitance C = 2.00 µf connected to a battery with voltage V = 12.0 V as shown. What is the charge stored in the capacitor? Question 2: Now insert a dielectric with dielectric constant κ = 2.5 between the plates of the capacitor. What is the charge on the capacitor? The additional charge is provided by the battery. 57
68 Capacitor with Dielectric (2) We isolate the charged capacitor with a dielectric by disconnecting it from the battery. We remove the dielectric, keeping the capacitor isolated. Question 3: What happens to the charge and voltage on the capacitor? The charge on the isolated capacitor cannot change because there is nowhere for the charge to flow. Q remains constant. The voltage on the capacitor will be V increases The voltage went up because removing the dielectric increased the electric field and the resulting potential difference between the plates. 58
69 46
70 Example: Dielectric Constant of Wax An airfilled parallel plate capacitor has a capacitance of 1.3 pf. The separation of the plates is doubled, and wax is inserted between them. The new capacitance is 2.6pF. Question: Find the dielectric constant of the wax. Answer: Key Ideas: The original capacitance is given by Then the new capacitance is Thus rearrange the equation: 59
71 Example: Dielectric Material Given a 7.4 pf airfilled capacitor. You are asked to convert it to a capacitor that can store up to 7.4 µj with a maximum voltage of 652 V. Question: What dielectric constant should the material have that you insert to achieve these requirements? Answer: Key Idea: The capacitance with the dielectric in place is given by C=κC air and the energy stored is given by So, 60
72 49
73 iclicker For a circuit with three capacitors in series, the equivalent capacitance must always be a) equal to the largest of the three individual capacitances. b) equal to the smallest of the three individual capacitances. c) larger than the largest of the three individual capacitances. d) smaller than the smallest of the three individual capacitances.
74 Review  So Far The capacitance of a spherical capacitor is r 1 is the radius of the inner sphere r 2 is the radius of the outer sphere The capacitance of an isolated spherical conductor is R is the radius of the sphere 30
75 Energy Stored in Capacitors A battery must do work to charge a capacitor. We can think of this work as changing the electric potential energy of the capacitor. The differential work dw done by a battery with voltage V to put a differential charge dq on a capacitor with capacitance C is The total work required to bring the capacitor to its full charge q is This work is stored as electric potential energy 37
76 Review  So Far The electric potential energy stored in a capacitor is given by The field energy density stored in a parallel plate capacitor is given by The field energy density in general is 49
77 Review (2) Placing a dielectric between the plates of a capacitor increases the capacitance by κ (dielectric constant) The dielectric has the effect of lowering the electric field between the plates (for given charge q) We also define the electric permitivity of the dielectric material as 50
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