Medical Physics General Physics Chapter 15 Electric Forces and Electric Fields.
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Transcript of Medical Physics General Physics Chapter 15 Electric Forces and Electric Fields.
Medical Physics
General Physics
Chapter 15Chapter 15Electric Forces andElectric Forces and
Electric FieldsElectric Fields
Medical Physics
General Physics
Gauss’s LawGauss’s Law
Sections 7-9Sections 7-9
Medical Physics
General Physics
What is the difference between What is the difference between Electric Field (E) and Force (F)?Electric Field (E) and Force (F)?
25% 25%25%25%
45
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1. E factors out the magnitude of the test charge, it only depends on the other charges
2. E is defined everywhere,F only at one point
3. Both 1) and 2)
4. Neither 1) or 2)
Medical Physics
General Physics
ReviewReview• Corn Field: corn at regularly space intervals• Vector Field: a vector at each point in space
• Scalar Field: a number at each point in space– Eg: Temperature
Medical Physics
General Physics
ReviewReview
• Electric Field
• Field lines:an easy way to visualize fields– Tangent to E– Density of lines
proportional to E
field
• begin at (+), end at (-)• # of lines proportional to q• lines don’t cross
Medical Physics
General Physics
ReviewReview
• Conductors & Electrostatic equilibrium– E=0 inside a conductor– excess charge lies on outer surface– E is perpendicular to surface– charge is greatest at highest curvature
Medical Physics
General Physics
Why are you safe inside a car Why are you safe inside a car during a thunder storm?during a thunder storm?
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1. The rubber tires insulate the car from the ground
2. Charge cannot come inside the metal shell of the car
3. The rounded car has a lower charge density
4. It’s not safe!
Medical Physics
General Physics
Millikan Oil-Drop ExperimentMillikan Oil-Drop Experiment
• Millikan Oil-Drop Experiment– Found every charge had an integral multiple of e (q = n e)– Measured the elementary charge, e = 1.610-19 C
Active Figure: The Millikan Oil-Drop Experiment
Equilibrium Drag -- Terminal velocity
Medical Physics
General Physics
Van de GraaffVan de GraaffGeneratorGenerator
• An electrostatic generator designed and built by Robert J. Van de Graaff in 1929
• Charge is transferred to the dome by means of a rotating belt
• Limited by ionization of air, which carries charge away
• Eventually an electrostatic discharge takes place
MU28T33
Medical Physics
General Physics
Museum of Science, BostonMuseum of Science, Boston
Medical Physics
General Physics
Holifield Radioactive Ion Beam FacilityHolifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory 25.5 MeV
Medical Physics
General Physics
Electric FluxElectric Flux
• Field lines penetrating an area A perpendicular to the field
• The product of EA is the flux, Φ (the number of
field lines!)• In general:
ΦE = E A cos θ
Medical Physics
General Physics
Electric Flux, cont.Electric Flux, cont.
– When the area is constructed such that a closed surface is formed, use the convention that flux lines passing into the interior of the volume are negative and those passing out of the interior of the volume are positive
• ΦE = E A cos θ– The perpendicular to
the area A is at an angle θ to the field
Medical Physics
General Physics
Gauss’ LawGauss’ Law
• Gauss’ Law: the electric flux through any closed surface is proportional to the net charge Q inside the surface
– εo=8.85 x 10-12 C2/Nm2 is the permittivity of free space– The area in Φ is an imaginary surface, a Gaussian surface,
it does not have to coincide with the surface of a physical object
Active Figure: Electric Flux Through an Arbitrary Closed Surface
surface
insideE
QEA
0
cos
MU29T17
Medical Physics
General Physics
Electric Field of a Charged Thin Electric Field of a Charged Thin Spherical ShellSpherical Shell
• The calculation of the field outside the shell is identical to that of a point charge
• The electric field inside the shell is E = 0
2eo
2 r
Qk
r4
QE
MU29T23
Medical Physics
General Physics
Electric Field of a Nonconducting Plane Electric Field of a Nonconducting Plane Sheet of ChargeSheet of Charge
• Total charge Q is uniformly distributed over surface A
• Charge density σ = Q/A• Use a cylindrical Gaussian
surface• The flux through the ends
is EA, no field through the curved part of the surface
• Note, the field is uniform
o
E2
Medical Physics
General Physics
Electric Field of a Nonconducting Plane Electric Field of a Nonconducting Plane
Sheet of Charge, cont.Sheet of Charge, cont.
• The field must be perpendicular to the sheet
• The field is directed either toward or away from the sheet
Medical Physics
General Physics
Parallel Plate CapacitorParallel Plate Capacitor
• The device consists of plates of positive and negative charge
• The total electric field between the plates is given by
• The field outside the plates is zero
o
E
Medical Physics
General Physics
What is the total electric flux through What is the total electric flux through the surface of the cubethe surface of the cube
in a constant electric field E?in a constant electric field E?20% 20% 20%20%20%
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1. 0
2. E L2
3. 6 E L2
4. 2 E L2
5. 6 E
Medical Physics
General Physics
Maxwell’s Equation #1Maxwell’s Equation #1
or
They all mean the same thing.
or
SPACE
TIM
E