chapter 17 electric potential and electric energy; capacitance
TRANSCRIPT
Chapter 17 Electric Potential and Electric Energy; Capacitance
Electric Potential and Electric Energy; Capacitance
Review of Chapter 1617.1 Electric Potential and Potential Difference17.2 Relationship between Electric Potential and Electric
Field17.3 Equipotential Lines17.4 The Electron Volt, a Unit of Energy17-5 Electric Potential Due to Point Charges17-6 Electric Dipoles17-7 Capacitance17-8 Dielectrics17-9 Storage of Electric Charge17-10 Cathode Ray Tube17-11 The Electrocardiogram
Important stuff from Chapter 16:Coulomb’s Law: F = kQ1Q2/r
2
where:k = 9.0 x 109 Nm2/C2 Q1 & Q2 are two charges (coulomb)
r = distance between two charges
Important stuff from Chapter 16:Electric Field (E)—force (F) exerted on a
positive test charge divided by the magnitude of the charge (q, coulombs)
E = F/q (units N/C) electric field goes from positive to
negative (the path of a positive test charge)
Important stuff from Chapter 16:Electric Field due to a Point Charge:
E = kQ/r2
Important stuff from Chapter 16:Electric potential energy—the energy
stored in a charged objects when its in an electric field
positive when the two charges are the same (repulsive) and negative when the two charges are opposite (attractive)
Electric Potential and Potential DifferenceTo move an charge in an electric field work
must be done.
Electric Potential and Potential Difference change in electric potential
energy (PEa – PEb) when a charge, q, moves from point b to point a is the negative work done by the electric force to move the charge from b to a
PE of a charge is the largest when it is closest to the plate with the same charge
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b a
Electric Potential and Potential Difference
Electric Potential (potential)—the potential energy per unit charge (V)
Va = PEa/q --for a test charge, q, at point a in an electric field
Where is the test charge’s electric potential the most, at point a or b?Positive plate has higher potential
than negative (by definition, why?)
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b a
Electric Potential and Potential Difference
Can only measure differences in PE; so can measure the potential difference (difference in potential) between two points
Since potential difference (PEa – PEb) = W then
Vab = Va Vb = Wba/q
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b a
Electric Potential and Potential Difference
Vab = Va Vb = Wba/q Unit; volt (1 V = 1 J/C) Voltage = potential
differenceZero for voltage is
arbitrary since we can only measure PE
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b a
Electric Potential and Potential Difference Since electric potential (V) =
PE/q then PE = PEb PEa = qVba
if an object with charge q moves through a potential difference Vba its potential energy changes by an amount qVba
electric potential difference is a measure of how much energy an electric charge can acquire in a situation and also a measure of how much work a charge can do
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b a
Electric Potential and Potential Difference
Accelerating a charge; PE = qV = KE so
v = (2qV/m) since KE = ½ mv2
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b a
Relationship between Electric Potential and Electric FieldIn a uniform electric field (parallel plates) to
move a charge:W = qV = Fd = qEd (since F = Eq) so V = Ed or E (electric field) = V/d
Equipotential Lines Equipotential lines—graphic
representation of electric potential Potential the same on lines so it takes no
work to move charges along the lines Always perpendicular to field lines
(diagram p.507) Continuous lines, never end A conductor must be entirely at the same
potential in the static case or electrons would accumulate at its surface
Equipotential Lines
Equipotential lines—graphic representation of electric potential
Always perpendicular to field lines (diagram p.507)
The Electron Volt; Unit of Energy Electron Volt (eV)—used to measure very
small energies (electrons, atoms, molecules, etc.)
Energy acquired by a particle carrying the charge of one electron as a result of moving through a potential difference of 1 volt
1 eV = 1.6 x 1019 J (qe = 1.6 x 1019 C)
electrons accelerated through potential difference of 10V loses 10V of PE and gains 10V of KE
Electric Potential due to Point Charges V = kQ/r where Q = point charge, r =
distance between point and test charge and k = ?
V represents the absolute potential since the V at r = equals zero
So V Q and V 1/r but E1/r2 (remember E = kQ/r2)
Electric Dipoles electric dipole--two equal point charges (Q)
of opposite sign separated by a distancedipole moment--the product of charge times
length (Ql)polar molecules—molecules that have a
dipole moment
Capacitance capacitor—a device that can store electric
charge consists of two conducting objects placed
near each other but not touching widely used in electronic: camera flash,
surge protectors, energy backups, memory for binary code (RAM)
often consists of two parallel plates (of area A, and separation d) rolled together with an insulating material between them
symbol : —||—
Capacitance
CapacitorsLeydon Jar
Capacitance capacitor—a device that can store electric
charge (diagram)
Capacitance Amount of charge acquired by a given
capacitor
Q = CV where:
Q = amt. of charge (C)V = potential difference (V)C = capacitance of capacitor (constant of
proportionality dependent on properties of capacitor)--units farad (F) = Coulombs/Volts
Capacitance For a parallel plate capacitor: C = oA/d where:
o = permittivity of free space = 8.85 x 1012 C2/Nm2 (remember)
Dielectrics Dielectric—the insulating sheet between the
plates of a capacitorServes several purposes:
Allows higher voltages to be applied without charge passing the gap, dielectrics break down less readily than air
Allows plates to be closer together, the closer the plates are the larger the capacitance of the capacitor (WHY?)
Dielectrics For a parallel plate capacitor: C = KoA/d where:
K = dielectric constant (Table 17-3) Since C = oA/d then
= Ko where:
= the permittivity of the material
Dielectrics Molecular Description of dielectricsWith air between plates; only plates of
capacitor have a potential difference
Dielectrics Molecular Description of
dielectricsWith a dielectric;
molecules of dielectric can line up in electric field of capacitor plates causing a net negative side by positive plate and a net positive by negative plate (even though charges do not move in dielectric material—insulator)
Dielectrics
Molecular Description of dielectrics
A positive test charge within the dielectric does not feel the full force of the electric field of the capacitor so it takes a greater potential difference between the two plates of the capacitor to cause it to move in the dielectric
Storage of Electric EnergyA charged capacitor stores electric energyThe net effect of charging a capacitor is to
remove charge from one plate and add it to another (using a source of electricity—battery)
Storage of Electric EnergyA capacitor is not charged instantly—it
requires time and work to do this and this increases with increasing charge on plates (?)
If the work were constant then the work required to charge a capacitor would be W = QV
But since it is not we deal with the average voltage (1/2 of Vf + VI) so
W = Q Vf/2 (why?)
Storage of Electric EnergyInternal energy stored in a capacitor is: U = 1/2QV where:V is potential difference between plates Since Q = CV then: U = ½ Q2/Cand since C = oA/d & V = Ed
then U = 1/2oE2 A/d
Storage of Electric EnergyEnergy density (u)—energy per unit volume u = energy/volume = 1/2 oE
2
Cathode Ray TubeRead section 17-10 and know this:What is a CRT?What is thermionic emission?What is a cathode?What is an anode?Explain how a cathode ray tube works.What is an oscilloscope?Explain how an electrocardiogram measures
heart function.Extra Credit: Find out about LCD, LED and
Plasma screens (for TV’s)
Cathode Ray Tube
Explain how a cathode ray tube works.
Cathode Ray Tube
What is an oscilloscope?
The ElectrocardiogramExplain how an electrocardiogram measures
heart function.