From last time

1
From last time
Electric fields
Electric field lines
2

• Exam 1 , Wed. Feb. 20, 530-7 pm

Alternate exam time for students with class
conflicts
Wed. Feb. 20, 230-4Wed. Feb. 20, 6-730
To attend alternate exam, you must email me with
1) your conflict, 2) which exam
Covers Chap. 21.5-7, 22-23,25-26 lecture, lab,
discussion, HW
8 1/2 x 11 handwritten note sheet (both sides)
allowed
Review Group/Quiz (solutions on website). Review
lab question sheets. Review sample exams on
website. Week5 HW (avail. this Thurs, due Fri.
next week) covers exam material.
3
Friday Guest Lecture

• Prof. Thad Walker
• UW Physics
• Holography

4
Superposition of electric fields

• Electric field from a point charge
• From a set of point charges
• Superpose E-fields from individual charges

5
Charge Densities

• Volume charge density when a charge is
  distributed evenly throughout a volume
• ? Q / V dq ? dV
• Surface charge density when a charge is
  distributed evenly over a surface area
• ? Q / A dq ? dA
• Linear charge density when a charge is
  distributed along a line
• ? Q / dq ? d

6
Infinite line of charge

• An infinite line of charge has a uniform charge
  density ? Coulombs / meter. What direction is the
  electric field at point x?

B.
C.
x
A.
D.
E.
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E-field infinite line of charge
y

• Charge density ??Coulombs/meter

8
Add all these up

• Units?

9
Finite line of charge
C

• What direction is the E-field above the end of
  the line charge?

D
B
E
A
10
Ring of uniform positive charge

• Which is the graph of the field on the axis?

y
A) B) C) D) E)
x
z
11
Quick quiz

• We have an infinite sheet of charge of uniform
  charge density ?. The electric field

1. increases with distance from plane
2. decreases with distance from plane
3. is independent of distance from plane
4. changes direction with distance from plane

12
E-field of plane of charge

• Text does this as adding up rings of charge.

13
Electric fields and forces

• Original definite of E-field was(Coulomb Force)
  / charge
• E-field produced force on charged particle.

14
Quick Quiz
A negative charge in outer space is in a uniform
E-field that points to the right. What is the
motion of the particle?
Fe
-
Uniform E?
A) Moves right at constant speed B) Moves left
at constant speed C) Remains stationary D)
Accelerates to the left E) None of the above
15
Force on charged particle

• Electric field produces force qE on charged
  particle
• Force produces an acceleration a FE / m
• Uniform E-field (directionmagnitude) produces
  constant acceleration
• Positive charge accelerates in direction of the
  field
• Negative charge accelerates in direction opposite
  the electric field

16
Electron in a Uniform Field

• The electron is projected horizontally into a
  uniform electric field
• The electron undergoes a downward acceleration
• It is negative, so the acceleration is opposite E
• Motion is parabolic while between the plates

17
Quick Quiz
A negative charge is released in a fluid with a
uniform E-field that points to the right. The
fluid imparts a force proportional to and
opposite to the velocity What is the motion of
the particle?
A. Accel left continuously B. Accel left, then
moves at const. speed C. Accel left, then slows
to stop D. Accel left, then turns around E. None
of the above
18
Motion of charged particle

• If no other forces, positive charge accelerates
  in direction of E-field.
• But many systems have drag forces (e.g.
  molecules in a liquid, etc)
• Drag force is complex, but usually depends on
  velocity.
• Particle reaches terminal velocity, determined
  by force balance

19
Application Gel Electrophoresis

• Charged macromolecules in gel with applied
  E-field
• Electric force FE qEDrag force FD -cv
• v qE / c
• Speed depends on charge and drag (molecule size)
• Sometimes too complex to interpret
• Protein electrophoresissoak in detergent to
  give proteinsall the same charge density.
• Result, small proteins move faster

Steady state FEFD0
20
Isoelectric focusing

• Separate macromolecules by charge, not size
• Macromolecules put in solution of linearly
  varying pH
• Electric field applied
• Charge on molecule depends on local pH
• Molecules move until no more force - at their
  isoelectric point

21
Quick Quiz

• An electric dipole is in a uniform electric field
  as shown. The dipole

Dipole

1. accelerates left
2. accelerates right
3. stays fixed
4. accelerates up
5. none of the above

22
Electric torque on dipoles

• Total torque has magnitude

• Dipole moment has magnitude

• Torque on dipole in uniform field

23
Dipole in non-uniform field

• A permanent dipole is near a positive point
  charge in a viscous fluid. The dipole will

1. rotate CW move toward charge
2. rotate CW move away
3. rotate CCW move toward
4. rotate CCW move away
5. none of the above