1. Physics 1202: Lecture 4 Today’s Agenda
Announcements:
Lectures posted on:
HW assignments, solutions etc.
Homework #1:
On Masterphysics: due this coming Friday
Go to the syllabus and click on instructions to register (in textbook section).
Make sure to input oyur information to google form
Labs: Begin this week 1

2. Today’s Topic : Chapter 20: Electric energy & potential
Review of electric potential & Equipotentials
Capacitors
Effect of dielectrics
Energy storage
Chapter 21: Electric current & DC-circuits
Electric current

3. 20-Electric Potential Definitions Examples C B r A r q equipotentialsV Q
4pe0 r
4pe0 R
Definitions
Examples C B r B A r q A
equipotentials
path independence

4. - - - - - - - - - - - - - - - - - - - - - - - - - -
Electric potential V
By analogy with electric field Þ
We have + F Þ
Therefore

5. Þ Point charges For a point charge, the formula is: For N charges r1 xq1 q3 q2
For N charges
simply the algebraic sum of the potential due to each charge separately. Þ

6. 20-4: Equipotentials
Defined as: The locus of points with the same potential.
Example: for a point charge, the equipotentials are spheres centered on the charge.
GENERAL PROPERTY:
The Electric Field is always perpendicular to an Equipotential Surface.
Why??
Along the surface, there is NO change in V (it’s an equipotential!)
So, there is NO E component along the surface either… E must therefore be normal to surface

7. Equipotential Surfaces: examples
For two point charges:
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8. Conductors Claim Why?? Note+
Claim
The surface of a conductor is always an equipotential surface
(in fact, the entire conductor is an equipotential)
Why??
If surface were not equipotential, there would be an Electric Field component parallel to the surface and the charges would move!!
Note
Positive charges move from regions of higher potential to lower potential (move from high potential energy to lower PE).
Equilibrium means charges rearrange so potentials equal.

9. Charge on Conductors?
How is charge distributed on the surface of a conductor?
KEY: Must produce E=0 inside the conductor and E normal to the surface .
Spherical example (with little off-center charge):
E outside has spherical symmetry centered on spherical conducting shell. +
charge density induced on outer surface uniform
E=0 inside conducting shell. +q -
charge density induced on inner surface non-uniform.

10. A Point Charge Near Conducting Plane+ a q -
V=0

11. A Point Charge Near Conducting Planeq + a
The magnitude of the force is -
Image Charge
The test charge is attracted to a conducting plane

12. Equipotential Example
Field lines more closely spaced near end with most curvature .
Field lines ^ to surface near the surface (since surface is equipotential).
Equipotentials have similar shape as surface near the surface.
Equipotentials will look more circular (spherical) at large r.

13. Equipotential & Electric Field
An ideal conductor is an equipotential surface
2 conductors at same V, the more curved one has a larger electric field around it
Also true for different parts of the same conductor
Explains why more charges at edges

14. Applications: human body
There are electric fields inside the human body
the body is not a perfect conductor, so there are also potential differences.
An electrocardiograph plots the heart’s electrical activity
An electroencephalograph measures the electrical activity of the brain:

15. Definitions & Examples
Capacitance
20-5
Definitions & Examples a b L d A

16. Capacitance Q=CV Q + + + + DV or V - - - - -
A capacitor is a device whose purpose is to store electrical energy which can then be released in a controlled manner during a short period of time. - +
A capacitor consists of 2 spatially separated conductors which can be charged to +Q and -Q respectively.
The capacitance is derived from the capacity to carry a charge Q when a voltage V is applied
DV or V Q
Q=CV
The capacitance is defined as the ratio of the charge on one conductor of the capacitor to the potential difference between the conductors.

17. Capacitance A capacitance C of the device Q=CV
Should not depend on Q nor V !
That means should depend on how it is made
Material, geometry, dimensions
Should be intrinsic to the capacitor.
DV or V Q
Q=CV
Is this a "good" definition? Does the capacitance belong only to the capacitor, independent of the charge and voltage ?

18. Example: Parallel Plate Capacitor
Calculate the capacitance. We assume +s, - s charge densities on each plate with potential difference V:
Need Q:
Need V: recall
where Dx = d so or

19. Recall:Two infinite planes
Same charge but opposite
Fields of both planes cancel out outside
They add up inside
Perfect to store energy !

20. Example: Parallel Plate Capacitord A
Calculate the capacitance:
Assume +Q,-Q on plates with potential difference V. Þ
As hoped for, the capacitance of this capacitor depends only on its geometry (A,d).

21. Dimensions of capacitance
C = Q/V Þ [C] = F(arad) = C/V = [Q/V]
A Farad is very large
Often will see µF or pF d A
Example: Two plates, A = 10cm x 10cm
d = 1cm apart
Þ C = Ae0/d =
= 0.01m2/0.01m * 8.852e-12 C2/Jm
= X F = pF

22. Lecture 4 – ACT 1 A + + + + d - - - - - A + + + + d1 - - - - -
Suppose the capacitor shown here is charged to Q and then the battery disconnected.
Now suppose I pull the plates further apart so that the final separation is d1. d1 > d d1 A
If the initial capacitance is C0 and the final capacitance is C1, is …
A) C1 > C0 B) C1 = C0 C) C1 < C0

23. Example : Isolated Sphere
Can we define the capacitance of a single isolated sphere ?
The sphere has the ability to store a certain amount of charge at a given voltage (versus V=0 at infinity) +
Need DV: V = 0
VR = keQ/R
So, C = R/ke

24. Dielectrics Empirical observation:
Inserting a non-conducting material between the plates of a capacitor changes the VALUE of the capacitance.
For the same charge Q0
Lowers the potential difference
Increases the capacitance

25. Dielectrics
A dielectric is an insulating material that, when placed between the plates of a capacitor, increases the capacitance
Why ? polarization of the material.

26. An Atomic Description of Dielectrics
Polarization of the material
alignment of small dipoles create a small electric field Epol
Epol is in the opposite direction
Ratio without/with dielectric
The field is smaller: k > 1

27. Parallel Plate Example
Charge a parallel plate capacitor filled with vacuum (air) to potential difference V0.
An amount of charge Q = C0 V0 is deposited on each plate.
Now insert material with dielectric constant k .
Charge Q remains constant
Electric field decreases also: + -
Voltage decreases from V0 to
But E and V are related by
So…, C = k C0

28. Dielectrics Definition:
The dielectric constant of a material is the ratio of the capacitance when filled with the dielectric to that without it. i.e.
k values are always > 1 (e.g., glass = 5.6;
water = 78)
They INCREASE the capacitance of a capacitor (generally good, since it is hard to make “big” capacitors
They permit more energy to be stored on a given capacitor than otherwise with vacuum (i.e., air)

29. 20-5 Capacitors and Dielectrics
If the electric field in a dielectric becomes too large
it can tear the electrons off the atoms, thereby enabling the material to conduct. This is called dielectric breakdown; the field at which this happens is called the dielectric strength.
© 2017 Pearson Education, Inc.

30. Applications of Capacitors Camera Flash
The flash attachment on a camera uses a capacitor
A battery is used to charge the capacitor
The energy stored in the capacitor is released when the button is pushed to take a picture
The charge is delivered very quickly, illuminating the subject when more light is needed

31. Applications of Capacitors Computers
Computers use capacitors in many ways
Some keyboards use capacitors at the bases of the keys
When the key is pressed, the capacitor spacing decreases and the capacitance increases
The key is recognized by the change in capacitance

32. Energy of a Capacitor
How much energy is stored in a charged capacitor?
Calculate the work provided (usually by a battery) to charge a capacitor to +/- Q:
Calculate incremental work DW needed to add charge Dq to capacitor at voltage V:
But DW is also the change in potential energy DU
The total U to charge to Q is shaded triangle: Q q Dq Vq
Vq=q/C V
In terms of the voltage V:

33. Lecture 4 – ACT 2 A + + + + d - - - - - A + + + + d1 - - - - -
The same capacitor as last time.
The capacitor is charged to Q and then the battery disconnected.
Then I pull the plates further apart so that the final separation is d1. d1 > d
If the initial energy is U0 and the final capacitance is U1, is …
A) U1 > U0 B) U1 = U0 C) U1 < U0 A d1

34. Where is the Energy Stored?
Claim: energy is stored in the Electric field itself. Think of the energy needed to charge the capacitor as being the energy needed to create the field.
To calculate the energy density in the field, first consider the constant field generated by a parallel plate capacitor:
The Electric field is given by: Þ
The energy density u in the field is given by:
Units: J/m3

35. Summary A + + + + d - - - - - Q: W: C: V: E:
Suppose the capacitor shown here is charged to Q and then the battery disconnected.
Now suppose I pull the plates further apart so that the final separation is d1.
How do the quantities Q, W, C, V, E change? Q: W: C: V: E:
remains the same.. no way for charge to leave.
increases.. add energy to system by separating
decreases.. since energy ­, but Q remains same
increases.. since C ¯, but Q remains same
remains the same.. depends only on chg density
How much do these quantities change?.. exercise for student!!
answers:

36. 21
Electric Current e R I
 = R I

37. Fig 27-CO These power lines transfer energy from the power company to homes and businesses. The energy is transferred at a very high voltage, possibly hundreds of thousands of volts in some cases. Despite the fact that this makes power lines very dangerous, the high voltage results in less loss of power due to resistance in the wires. (Telegraph Colour Library/FPG)

38. Overview Charges in motion How charges move in a conductor
mechanical motion
electric current
How charges move in a conductor
Definition of electric current

39. Charges in Motion Up to now we have considered
fixed charges on isolated bodies
motion under simple forces (e.g. a single charge moving in a constant electric field)
We have also considered conductors
charges are free to move
we also said that E=0 inside a conductor
If E=0 and there is any friction (resistance) present
no charge will move!

40. Is there a contradiction?
Charges in motion
We know from experience that charges do move inside conductors - this is the definition of a conductor E E
Is there a contradiction? no V1 V2
Up to now we have considered isolated conductors in equilibrium.
Charge has nowhere to go except shift around on the body.
Charges shift until they cancel the E field, then come to rest.
Now we consider circuits in which charges can circulate if driven by a force such as a battery.

41. Analogy with fluids Consider a hose filled with water
Need a difference of potential for fluid to flow
Same is true for electric charges

42. Current Definition + E
Consider charges moving down a conductor in which there is an electric field.
If we take a cross section of the wire, over some amount of time Dt we will count a certain number of charges (or total amount of charge) DQ moving by.
We define current as the ratio of these quantities,
Iavg = DQ / Dt
Units for I, Coulombs/Second (C/s) or Amperes (A)
Note: This definition assumes
the current in the direction of
the positive particles,
NOT in the direction of the electrons!

43. How charges move in a conducting material
Electric force causes gradual drift of bouncing electrons down the wire in the direction of -E.
Drift speed of the electrons is VERY slow compared to the speed of their bouncing motion, roughly 1 m / h !
(see example later)
Good conductors are those with LOTS of mobile electrons.

44. How charges move in a conducting material
DQ is the number of carriers in some volume times the charge on each carrier (q).
Let n be the carrier density, n = # carriers / volume.
The relevant volume is A * (vd Dt). Why ?
So, DQ = n A vd Dt q
And Iavg = DQ/Dt = n A vd q
More on this later …
vd = Δx/ Δt

45. Drift speed in a copper wire
The copper wire in a typical residential building has a cross-section area of 3.31e-6 m2. If it carries a current of 10.0 A, what is the drift speed of the electrons? (Assume that each copper atom contributes one free electron to the current.) The density of copper is 8.95 g/cm3, its molar mass 63.5 g/mol.
Volume of copper (1 mol):
Because each copper atom contributes one free electron to the current, we have (n = #carriers/volume)

46. Drift speed in a copper wire, ctd.
We find that the drift speed is
with charge / electron q
Thus

47. What makes charges move ?
Need to create DV
recall W = -DU
A battery uses chemical reactions to produce a potential difference V1 V2
Fluid analogy: person lifting water causing it to flow through the paddle wheel and do work.

48. Electromotive “force”
Electric potential difference between the terminals of a battery is called the electromotive force or emf:
Remember—despite its name, the emf is an electric potential, not a force.
The amount of work it takes to move a charge ΔQ from one terminal to the other is: + - e

49. © 2017 Pearson Education, Inc.
Electric current
The direction of current flow—from the positive terminal to the negative one—was decided before it was realized that electrons are negatively charged. Therefore, current flows around a circuit in the direction a positive charge would move; electrons move the other way. However, this does not matter in most circuits.
© 2017 Pearson Education, Inc.

50. Recap of today’s lecture
Chapter 20: Electric energy & potential
Review of electric potential & Equipotentials
Capacitors
Effect of dielectrics
Energy storage
Chapter 21: Electric current & DC-circuits
Electric current
Homework #1 on Mastering Physics
From Chapter 19
Due this Friday
Labs started this week