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Chapter 23Electric Potential
Basics
0
parallels definition for ANY conservative force
, ,
Choice of where 0 is arbitrary
Typically, if charge distribution is finite a go
a
b
x y z
x y z
V a V b d
dV d E dx E dy E dz
V V VE
E
E Ex y z
V r
E
0
3
0
od chioce is
For a point charge :
' '1 or
4 '
i
i i
r
Q
r d r QQ QV r k V r k k
r r r r r r
The potential due to an electric dipole is just the sum of the potentials due to each charge, and can be calculated exactly. For distances large compared to the charge separation:
23-6 Electric Dipole Potential
23-7 E Determined from V If we know the field, we can determine the potential by integrating. Inverting this process, if we know the potential, we can find the field by differentiating:
This is a vector differential equation; here it is in component form:
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E
The potential due to an electric dipole is just the sum of the potentials due to each charge, and can be calculated exactly. For distances large compared to the charge separation:
Can always choose system such that P lies in x-y plane.
23-6 Electric Dipole Potential
ˆ x
ˆ
y
23-6 Electric Dipole Potential -- Field
3 22 23 22 2 2 2 2 2
3 22 2
1,
3
2
x xV x y kp kp kpx x y
x y x y x y
Vkp x y x
x 5 22 2 2
x y
2 2
2 2 2 2 2
5 22 2
5 2 5 22 2 2 23 2
3and
2
2
x
kpx
x
kp kpx y x y x
x y x y
VE
x
Vkpx
y
yx y
5 22 2 2
x y
5 22
5 22 2
23
3
y
xykp
x
xyy
y
VE
y y
kpx
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Summary of Chapter 23
• Electric potential is potential energy per unit charge:
• Potential difference between two points:
• Potential of a point charge:
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• Equipotential: line or surface along which potential is the same.
• Electric dipole potential is proportional to 1/r2.
• To find the field from the potential:
Summary of Chapter 23
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Chapter 24Capacitance, Dielectrics, Electric Energy Storage
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• Capacitors
• Determination of Capacitance
• Capacitors in Series and Parallel
• Electric Energy Storage
• Dielectrics
• Molecular Description of Dielectrics
Units of Chapter 24
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A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.
24-1 Capacitors
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Parallel-plate capacitor connected to battery. (b) is a circuit diagram.
24-1 Capacitors
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When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage:
The quantity C is called the capacitance.
Unit of capacitance: the farad (F):
1 F = 1 C/V.
24-1 Capacitors
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24-2 Determination of CapacitanceFor a parallel-plate capacitor as shown, the field between the plates is
E = Q/ε0A.
Integrating along a path between the plates gives the potential difference:
Vba = Qd/ε0A.
This gives the capacitance:
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24-2 Determination of CapacitanceExample 24-1: Capacitor calculations.
(a) Calculate the capacitance of a parallel-plate capacitor whose plates are 20 cm × 3.0 cm and are separated by a 1.0-mm air gap. (b) What is the charge on each plate if a 12-V battery is connected across the two plates? (c) What is the electric field between the plates? (d) Estimate the area of the plates needed to achieve a capacitance of 1 F, given the same air gap d.
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24-2 Determination of CapacitanceCapacitors are now made with capacitances of 1 farad or more, but they are not parallel-plate capacitors. Instead, they are activated carbon, which acts as a capacitor on a very small scale. The capacitance of 0.1 g of activated carbon is about 1 farad.
Some computer keyboards use capacitors; depressing the key changes the capacitance, which is detected in a circuit.
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24-2 Determination of Capacitance
Example 24-2: Cylindrical capacitor.A cylindrical capacitor consists of a cylinder (or wire) of radius Rb surrounded by a coaxial cylindrical shell of inner radius Ra. Both cylinders have length l which we assume is much greater than the separation of the cylinders, so we can neglect end effects. The capacitor is charged (by connecting it to a battery) so that one cylinder has a charge +Q (say, the inner one) and the other one a charge –Q. Determine a formula for the capacitance.
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24-2 Determination of CapacitanceExample 24-3: Spherical capacitor.
A spherical capacitor consists of two thin concentric spherical conducting shells of radius ra and rb as shown. The inner shell carries a uniformly distributed charge Q on its surface, and the outer shell an equal but opposite charge –Q. Determine the capacitance of the two shells.
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24-2 Determination of Capacitance
Consider two small (radius r) spheres a large distance (R) apart. What is their capacitance?
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24-2 Determination of Capacitance
Consider two small (radius r) spheres a large distance (R) apart. What is their capacitance?
22
22
1
1
1 1
1
R r R r
Qr r
R r
r
R r R rR r R r
r rr r
V E dx E x dx
k Q kQdx dx
x R x
dx dxkQ kQ
x x x RR x
kQr R
1 1 1
r R r R r R
0
2
22
kQ
r
Q rC r
V k
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Questions?
