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Physics 1214 — Chapter 18: Electric Potential and Capacitance
1 Electric Potential Energy
Essential Points from Chapter 7:
- when a constant force F~ acts on a particle that moves in a straight line through a displacement ~s from point a to point b, the work Wa→b done by the force is
Wa→b = F s cos φ
where φ is the angle between the force and displacement. Let x point in the direction of the particle’s motion.
- because the force field is conservative, the work that is done can always be expressed in terms of a potential energy U. When the particle moves from a point where the potential energy is Ua to a point where it is Ub, the work Wa→b done by the force is Wa→b = Ua − Ub.
- the work-energy theorem says that the change in kinetic energy ∆K = Kb − Ka during any displacement is equal to the total work done on the particle so if Wa→b = Ua − Ub is the total work, then Kb − Ka = Ua − Ub which is usually written as Ka + Ua = Kb + Ub.
Applied to electrical forces, a uniform electric field with magnitude, E, exerts a constant force on a positive test charge, q′, of F = q′E (for a positive test charge in a positive field) then for a distance, s:
Wa→b = F s = q′Es.
Potential Energy of Point Charges
For work in a non-constant field, such as the work, W , done on a test charge, q′, when it moves in an electric field caused by a single stationary point charge, q. (See graph on p. 585)
Wa→b = kqq′
a
b
and from this, potential energy
Ua =
kqq′ a
and Ub =
kqq′ b
It can be shown that the work Wa→b done on q′^ by the E~ field produced by q is the same for all possible paths from a to b. And the total work done in a roundtrip displacement (from a back to a) is zero. These are the characteristics of a conservative force.
Potential energy of point charges
The potential energy U of s system consisting of a point charge q′^ located in the field produced by a stationary point charge q, at a distance r from the charge, is
U = k qq′ r
Note: As the distance, r, goes to infinity, U goes to zero.
For a collection of charges,
U = kq′
q 1 r 1
q 2 r 2
q 3 r 3
Every electric field due to a static charge distribution is a conservative force field.
2 Potential
Electric Potential or Potential
The electric potential V at any point in an electric field is the electric potential energy U per unit charge associated with a test charge q′^ at that point:
V =
U
q′ or
U = q′V.
Potential energy and charge are both scalars, so potential is a scalar quantity. Units: The SI unit of potential, 1 J/C, is called one volt (1V).
1 V = 1 volt = 1 J/C = 1 joule/coulomb
voltage: the electric potential in electric circuits. potential difference: the difference in electric potential between two points in a system.
In a “work per unit charge” basis, Wa→b q′^
Ua q′^
Ub q′^ = Va − Vb
Potential of a point charge
When a test charge q′^ is a distance r from a point charge q, the potential V is
V =
U
q′^
= k
q r
where k is the same constant as in Coulomb’s law.
To find the potential V at a point due to any collection of point charges:
V =
U
q′^ = k
q 1 r 1
q 2 r 2
q 3 r 3
1V = 1
J
C
(1N)(1m) 1C and 1N/C = 1V/M
Physics 1214 — Chapter 18: Electric Potential and Capacitance
4 The Millikan Oil-Drop Experiment
- two parallel, horizontal metal plates, insulated and separated, maintained at a potential difference
- oil drops are sprayed from an atomizer and acquire a charge
- a few drops fall from a hole in the top plate and are observed with a telescope equipped with a scale allowing speed of the drops to be measured
- suppose a drop has a negative charge q
- with a downward electric field magnitude E (between the plates)
- the forces on the oil drop are the downward force F = mg and the upward force F = qE
- Millikan adjusted E such that mg = qE and the drop was in static equilibrium (floating!!!) thus q = mg/E
- E = Vab/d where d is the distance between the plates
- the mass of the oil drop can be found since the density, ρ, is mass per unit volume ρ = m/Volume
- the oil drop when in static equilibrium is a sphere due to surface tension (think of a drop of water floating in space) (volume of a sphere = 43 πr^3 )
m =
πr^3 ρ, E =
Vab d
thus q =
ρπr^3 gd Vab
The radius of the oil drop was too small to measure! Millikan found the radius by turning off ~E, and measuring the terminal speed (aka terminal velocity), νt. After thousands of drops, every drop had an integer value of e. See notes from Chapter 17 for the value of e.
electron volt (eV): derived from the change in potential energy, ∆U = q(Vb − Va) = qVba. If Vba = 1 V, then ∆U = (1. 602 × 10 −^19 C)(1V) = 1. 602 × 10 −^19 J = 1eV.
1eV = 1. 602 × 10 −^19 J
5 Capacitors
capacitor : a device that stores electric potential energy, U , and electric charge.
- in principle, a capacitor consists of any two conductors separated by vacuum or an insulating material
- there is a charge of opposite sign on each conductor, thus an ~E, between the conductors, thus a potential difference, V between them
- in most practical applications, the conductors have charges (Q and −Q) with equal magnitude thus net charge is zero
Capacitance
The capacitance, C of a capacitor is the ratio of the magnitude of the charge Q on either conductor to the magnitude of the potential difference Vab between the conductors:
C =
Q
Vab
Unit: The SI unit of capacitance is 1 farad (1 F).
1F = 1C/V
parallel-plate capacitor : two parallel conducting plates, each with area A, separated by a distance d that is small in comparison with the area.
- nearly all of the field is localized between the plates
- there exists some fringing of the field at the edges; generally neglected in our study
- the electric field, ~E, between the plates is uniform
surface charge density (σ): the electric charge per unit area. For a parallel-plate capacitor, the charge densities on the plates are σ = Q/A and σ = −Q/A.
The electric field magnitude:
E = σ 0
Q
0 A
and the potential difference (voltage) between the plates
Vab = Ed =
Qd A
Capacitance of a parallel-plate capacitor
The capacitance C of a parallel-plate capacitor in vacuum is directly proportional to the area A of each plate and inversely proportional to their separation d:
C =
Q
Vab
A
d
6 Capacitors in Series and in Parallel
series connection : two devices connected one after another between points a and b and a constant potential difference Vab is maintained. The total potential difference across all of the capacitors is the sum of the individual potential differences. parallel connection : two devices connected in parallel between points a and b. The upper plates of the capacitors are connected together to form an equipotential surface, and the lower plates form another. The potential difference is the same for both capacitors.
Equivalent capacitance of capacitors in series
When capacitors are connected in series, the reciprocal of the equivalent capacitance of a series combination equals the sum of the reciprocals of the individual capacitances:
1 Ceq
C 1
C 2
C 3
The magnitude of charge is the same on all of the plates of all the capacitors, but the potential differences across individual capacitors are, in general, different.
Equivalent capacitance of capacitors in parallel
When capacitors are connected in parallel, the equivalent capacitance of the combination equals the sum of the individual capacitances:
Ceq = C 1 + C 2 + C 3 + ...
