Transcript Charges

Announcements
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Homework:
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Test 1:
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Webassign HW due on SUNDAY at 11:59pm
No Hand-in Homework
Feb 17th, 6-7:30 pm
Location: SMG 105
Chapters: 21-24
Practice Exams posted on WebCT
Review Sessions by discussion TFs
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Friday 3-5pm in SCI 115 (Eric Pinnick)
5-7 pm in CAS 313 (Maggie Geppert)
Summary: Electric Potential
• Electric Potential
– Reference point: V=0 at an infinite distance (r=)
– Electric field lines point in the direction of decreasing
electric potential.
• Potential due to a set of charges:
• Properties of Conductors:
– All points on the conductor (surface + bulk ) are at the
same potential
– Charge concentrates on pointy surfaces.
Example
•
A spherical drop of water carrying a charge of
30pC has a potential of 500V at its surface (with
V=0 at infinity).
1. What is the radius of the drop?
2. If two such drops of the same charge and radius
combine to form a single drop, what is the
potential at the surface of the new drop?
E-fields from V
• Potential:
• OR
Example: E from V
• Compute the Electric field in a region where the
potential is:
Equipotential surfaces
• Equipotentials connect points of the
same potential.
– Similar to contour lines on a topographical
map, which connect points of the same
elevation, and to isotherms (lines of
constant temperature) on a weather map.
• No net work is done by the E-field when a charge
moves from one point to another on the equipotential
surface.
Equipotential Surfaces
• point charge: family of concentric spheres.
• Uniform electric field: family of planes perpendicular to the field
• What are equipotentials good for?
– make it easy to determine how much work is needed to move a charge
from one place to another.
– It takes no work to move a charge along an equipotential.
• As E is perpendicular to the displacement
Equipotential Surfaces
• point charge: family of concentric spheres.
• Uniform electric field: family of planes perpendicular to the field
• What are equipotentials good for?
– make it easy to determine how much work is needed to move a charge
from one place to another.
– It takes no work to move a charge along an equipotential.
– The more equipotential lines are crossed, the more work is associated
with the trip.
Equipotentials
1. Which direction is the E-field?
2. In which case is the E-field strongest?
Case 2
3. If a particle with charge +q moves from a to b, in which
situation does it experiences the largest change in
potential energy? Case 2
b
a
Case 1
V
Case 2
V
10
20
4
8
-6
-12
E fields
Equipotential Surfaces
Lightning
Storms
Moro Rock in California's
Sequoia National Park
Equipotentials
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Three points A, B, and C are shown in the vicinity of a
positive point charge. Which takes more work, moving a
negative charge from A to C, OR from B to C.
C
B
1. Moving from A to C takes more work
2. Moving from B to C takes more work
3. Neither, the work required is the same for both cases.
A
Example: Equipotential Surfaces
• A metal sphere carries a charge Q=0.50 C. Its
surface is at a potential of 15000 V. Equipotential
surfaces are to be drawn for 100V intervals
outside the sphere.
Determine the radius for 1st, 10th and 100th
equipotential from the surface.
Quiz Time?
Quiz Solution
Two charged conducting spheres of different radii are connected by a conducting wire.
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[ X] The sphere with the smaller radius has higher surface charge density.
Comparing points A, B, C, and D only, at which of those points is the magnitude of the electric
field largest?
[
] A
[X]B
[ ]C
[ ]D
[ ] all four points would be on the same field line,
so the magnitude of the field would be equal at those points
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If you draw field lines you’ll see that they are closest together at B, at least for those 4
points.
Comparing points B and E only, at which point is the magnitude
of the electric field largest?
[ ] B [ X ] E [ ] both points are on the same equipotential,
so the magnitude of the field would be equal at both points
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The magnitude of the field is proportional to how quickly
the potential changes with distance. |E| = dV/dr, and to
achieve the same dV from point E requires a smaller
distance, so |E| is larger at point E.
sketch the electric field line passing through point E.
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field lines are perpendicular to equipotentials, and that
the direction of the field is in the direction of decreasing potential.
Chapter 24: Capacitance
• Capacitors (or condensors)
• Device for storing charge/energy
– Camera flashes, circuit applications (radio tuners), computer key
boards.
• Capacitance C:
– the amount of charge a capacitor can store for a given potential
difference
– For a capacitor with a charge of +Q on one plate and -Q on the
other: Q = C V
(C > 0)
– Unit of Capacitance is Farad (F)
(1F = 1C/1V)
• For a parallel-plate capacitor
• The energy stored in a capacitor is
Playing with a Capacitor
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Take a parallel-plate capacitor and connect it to a power supply. The
power supply sets the potential difference between the plates of the
capacitor.
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The distance between the capacitor plates can be changed. While
the capacitor is still connected to the power supply, the distance
between the plates is increased. When this occurs, what happens to
C, Q, and V?
Q = C V
1.
2.
3.
4.
5.
C decreases, Q decreases, and V stays the same
C decreases, Q increases, and V increases
C decreases, Q stays the same, and V increases
All three decrease
None of the above
Playing with a Capacitor
•
Take a parallel-plate capacitor and connect it to a power supply. The
power supply sets the potential difference between the plates of the
capacitor.
Now the capacitor is charged by the power supply and then the
connections to the power supply are removed. When the distance
between the plates is increased now, what happens to Q, C, and  V?
Q = C V
1.
2.
3.
4.
5.
C decreases, Q decreases, and V stays the same
C decreases, Q increases, and V increases
C decreases, Q stays the same, and V increases
All three decrease
None of the above
Capacitance…
• Will the changes below cause the capacitance of a parallelplate capacitor to increase, decrease, or stay the same.
• Increase the area of each plate:
 C INCREASES
• Double the charge on each plate:
 C stays the same
• Increase the potential difference across the capacitor:
 C stays the same
• Increase the distance between the plates:
 C DECREASES
Multiple Capacitors in circuits
• Devices in parallel: same potential difference across them.
Ceq = C1 + C2 + ...
• Charge on the equivalent capacitor = sum of the charges
on each capacitor.
• Devices in series: all of them have the same charge.
• Total potential difference across the chain = sum of the
potential differences across each one of them.
• Read, Read, Read ….
Chapter 25