Post on 18-Jan-2018
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S-113
A proton is placed 4 cm to the right of a 1.2 nC charge and 2 cm to the left of a -2.4 nC charge. What is the acceleration of the proton? Squirrel Jedi
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Four charges, each of 1.2 nC are placed on the corners of a square 2.5 cm per side. What is the magnitude and direction of the electric field on the upper right corner? What is the force on the charge in that corner?
Electric Potential
AP PhysicsChapter 17
Electric Charge and Electric Field
17.1 Electric Potential Energy and Potential Difference
Electric Potential Energy & Potential Difference
Electricity can be viewed in terms of energyThe electrostatic force is conservative because it depends on displacementNow
We can calculate this value for a uniform electric field 17.1
WPE FdPE qEdPE
Electric Potential Energy & Potential Difference
Positive test charge – increases when moved against the fieldNegative test charge – increases when moved with the field
Electric Potential (Potential) – electric potential energy per unit charge
17.1
qPEV
Electric Potential Energy & Potential Difference
Only difference in potential are meaningfulPotential Difference (Electric Potential Difference) – is measureable
Measured in volts (after Alessandro Volta)
17.1
qPEV qPEV
CJV111
Electric Potential Energy & Potential Difference
If we want a specific potential value at a point, we must pick a zero point.That point is usually either
A. The groundB. At an infinite distance
17.1
r
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A 1.2 mC charge is moved 3 cm into a 231 N/C electric field.
A. What is the change in potential energy?B. If the particle has a mass of 4.4 g, what will
be its velocity when it has moved back 3 cm (assume all energy is converted to kinetic energy)
Electric Charge and Electric Field
17.2 Relation between Electric Potential and Electric Field
Relation between Electric Potential & Electric Field
Work
It also is
So pew
17.2
)( ab VVqW VqW
qEdW
qEdVq EdV
Electric Charge and Electric Field
17.3 Equipotential Lines
Equipotential Lines
Equipotential Line or Equipotential Surface – one at which all points are at the same potential
17.3
Equipotential Lines
Must be perpendicular to the electric fieldConstant Field
17.3
Equipotential Lines
Must be perpendicular to the electric fieldPoint Charge
17.3
Equipotential Lines
Must be perpendicular to the electric fieldDipole
17.3
Electric Charge and Electric Field
17.4 The Electron Volt, a Unit of Energy
The Electron Volt
A unit of energy – for electrons, atoms, or moleculesElectron Volt (eV)
17.4
191 1.6 10eV x J
Electric Charge and Electric Field
17.5 Electric Potential Due to Point Charge
Electric Potential Due to Point Charge
Using calculus it can be shown that the electric potential a distance r from a single point charge q is
Assuming that potential is zero at infinity
17.5
qV kr
Electric Charge and Electric Field
17.7 Capacitance
Capacitance
Capacitor – device that stores electric chargeIn RAM, Camera Flash,
17.7
Capacitance
Simple capacitors consist of two plate
The symbol for a capacitoris
The symbol for a cell is
The symbol for a battery is
17.7
Capacitance
When a potential difference is placed across a capacitor it becomes chargedThis process takes a short amount of time
The charge on each plate is the same, but opposite chargeThe amount of charge is proportional to the potential differenceA constant C (Capacitance) gives
17.7
Charging a Capacitor
Time for RC Circuit
VQ CVQ
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Two electrons are moved from an infinite distance to a location 2 cm from each other.
A. What is the electric field produced by one at the location of the other?
B. What is the force between them?C. What is electric potential energy due to their
positions relative to each other?
cat gone to dark side
Capacitance
Capacitance – Unit Farad
For a parallel plate capacitor, the capacitance depends on the area of the plates, the distance between the plates
17.7
CVQ
VCF111
dAC o
Electric Charge and Electric Field
17.8 Dielectrics
Dielectrics
Most capacitors have an insulator between the platesCalled a DielectricIncreases the capacitance by
a factor KCalled the dielectric
constant
17.8dAKC o
Dielectrics
Some Dielectric Constants
17.8
Material K
Paper 3.7
Glass 5
Rubber 6.7
Mica 7
Strontium Titanate
300
Electric Charge and Electric Field
17.9 Storage of Electric Energy
Storage of Electric Energy
A charged capacitor stores energy
17.9
QVPE 21
221 CVPE
CQPE2
21
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A 1.7 F capacitor in a camera is charged across a 120 V socket. What is the energy stored in the capacitor?
Combinations of Capacitors
Parallel – more than one pathwayFor a parallel set of capacitors – the total charge is the sum of the individual charges
In all parallel circuits – the potential across each branch is the same as the total
nT QQQQ ..21
nT VVVV ..21
Combinations of Capacitors
The equivalent capacitance is the value of one capacitor that could replace all those in the circuit with no change in charge or potentialSince
AndWe combine and get
nT QQQQ ..21
CVQ nnTeq VCVCVCVC ..2211 neq CCCC ..21
Combinations of Capacitors
Series – components of a circuit are in one pathway
The magnitude of the charges is the same on each plate
nT QQQQ ..21
Combinations of Capacitors
The total potential is the sum of the potential drops across each capacitor
We then use that equation and the equation for capacitance
We get
nT VVVV ..21
CQV
n
n
eq
T
CQ
CQ
CQ
CQ ..
2
2
1
1 neq CCCC1..111
21
Combinations of Capacitors
3 10F capacitors are placed in a circuit with a 6 volt source of EMF.
A.What is the equivalent capacitance, potential drop and charge if they are in series?
B.What if they are in parallel?
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PeaceLoveHappiness
Test
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