1. Electrostatics

2. 10.1 Properties of Electric Charges

3. 10.1 Properties of Electric Charges
Static electricity – not moving
Two types of charge
positive (+) when electrons are lost
negative (-) when electrons are gained
Objects can gain charges by rubbing
10.1 Properties of Electric Charges

4. 10.1 Properties of Electric Charges
Like charges repel
Unlike charges attract
Law of Conservation of electric charge – the net amount of electric charge produced in a process is zero
10.1 Properties of Electric Charges

5. 10.1 Properties of Electric Charges
Robert Millikan – charge is always
a multiple of a fundamental unit
Quantized – occurs in discrete
bundles
The discrete bundle is an electron
The charge on a single
electron is
10.1 Properties of Electric Charges

6. 10.2 Insulators and Conductors

7. 10.2 Insulators and Conductors
Conductors – outer
electrons of atoms
are free to move
through the
material
Insulator – electrons
tightly held, do
not move
10.2 Insulators and Conductors

8. 10.2 Insulators and Conductors
Semiconductors – conduct electricity under some circumstances, don’t under other conditions
Charges can be transferred by contact
Called Charging by Conduction
10.2 Insulators and Conductors

9. 10.2 Insulators and Conductors
Induction – charging without
contact
Object is brought near a
charged object
Electrons move
Object is grounded
An electroscope measures if
an object has a charge on it
10.2 Insulators and Conductors

10. 10.3 Coulomb’s Law

11. Electric charges apply forces to each other
From experiments
Force is proportional
to charge
Inversely proportional
to square of distance
10.3 Coulomb’s Law

12. Equation – gives magnitude of force
Opposite charges – force directed toward
each other
Like charges – force directed away from each
other
Charge is measured in Coulombs
10.3 Coulomb’s Law

13. 1 Coulomb is the amount of charge, that if placed 1 m apart would result in a force of 9x109 N
Charges are quantized – that is they come in discrete values
The constant k relates to the constant called the permittivity of free space
10.3 Coulomb’s Law

14. These are forces, so be sure to use vector math, draw free body diagrams
For multiple objects, require multiple free body diagram
10.3 Coulomb’s Law

15. 10.4 The Electric Field

16. Electrical forces act over distances
Field forces, like gravity
Michael Faraday
electric field – extends
outward from every charge
and permeates all of space
The field is defined by the force
it applies to a test charge
placed in the field
10.4 The Electric Field

17. The Electric field would then beOr
q is the test charge
We can also say that
Remember that E is independent of the test charge.
The electric field is also a vector (free body diagrams are probably a good idea)
10.4 The Electric Field

18. 10.5 Electric Field Lines

19. To visualize electric fields
Draw electric field lines
Direction of the lines is the
direction of force on a
positive test charge
The density of the lines
indicates relative
strength of the field
Note: the field density increase
as you get closer
10.5 Electric Field Lines

20. For multiple charges, keep in mind
Field lines indicate the direction of the field
The actual field is tangent to the field lines
The magnitude of the field is relative to the field line density
Fields start at positive and end at negatives
Field Lines
10.5 Electric Field Lines

21. If the field is produced by two closely spaced parallel plates
The field density is constant
So the electric field is
constant
Electric Diple – two
point charges of
equal magnitude
but oppsite sign
10.5 Electric Field Lines

22. 10.6 Conductors in Electrostatic Equilibrium

23. 10.6 Conductors in Electrostatic Equilibrium
Electrostatic Equilibrium – when no net motion of charge occurs within a conductor
The electric field is zero
everywhere inside a conductor
Any excess charge is on the
surface of a conductor
10.6 Conductors in Electrostatic Equilibrium

24. 10.6 Conductors in Electrostatic Equilibrium
The electric field just
outside a charged
conductor is
perpendicular to the
conductors surface
4. The charge accumulates
on areas of greatest
curvature
10.6 Conductors in Electrostatic Equilibrium

25. 10.7 Potential Difference and Electric Potential

26. 10.7 Potential Difference and Electric Potential
Electricity can be viewed in terms of energy
The electrostatic force is conservative because it depends on displacement
Now
We can calculate this value for a uniform electric field
10.7 Potential Difference and Electric Potential

27. 10.7 Potential Difference and Electric Potential
Positive test charge – increases when moved against the field
Negative test charge – increases when moved with the field
Electric Potential (Potential) – electric potential energy per unit charge
10.7 Potential Difference and Electric Potential

28. 10.7 Potential Difference and Electric Potential
Only difference in potential are meaningful
Potential Difference (Electric Potential Difference) – is measureable
Measured in volts (after
Alessandro Volta)
10.7 Potential Difference and Electric Potential

29. 10.7 Potential Difference and Electric Potential
If we want a specific potential value at a point, we must pick a zero point.
That point is usually either
A. The ground
B. At an infinite distance
10.7 Potential Difference and Electric Potential

30. 10.8 Electric Potential & Potential Energy

31. 10.8 Electric Potential & Potential Energy
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
Like Potential Difference, this value is a scalar So
10.8 Electric Potential & Potential Energy

32. 10.9 Potentials and Charged Conductors

33. 10.9 Potentials and Charged Conductors
All points on the surface of a charged conductor in electrostatic equilibrium are at the same potential.
The electric potential is a constant everywhere on the surface of a charged conductor in equilibrium.
The electric potential is constant everywhere inside a conductor and equal to its value at the surface.
10.9 Potentials and Charged Conductors

34. 10.10 Capacitance

35. Capacitor – device that stores electric charge
In RAM,
Camera
Flash,
10.10 Capacitance

36. Simple capacitors consist of
two plate
The symbol for a capacitor is
The symbol for a cell is
The symbol for a battery is
10.10 Capacitance

37. This process takes a short amount of time
When a potential difference is placed across a capacitor it becomes charged
This process takes a short amount of time
The charge on each plate is the same, but opposite charge
The amount of charge is proportional to the potential difference
A constant C (Capacitance) gives
Charging a Capacitor
Time for RC Circuit
10.10 Capacitance

38. Capacitance – Unit Farad
For a parallel plate capacitor, the capacitance depends on the area of the plates, the distance between the plates
10.10 Capacitance

39. 10.11 Combinations of Capacitors

40. 10.11 Combinations of Capacitors
Parallel – more than one pathway
For 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
10.11 Combinations of Capacitors

41. 10.11 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 potential
Since
And
We combine and get
10.11 Combinations of Capacitors

42. 10.11 Combinations of Capacitors
Series – components of a circuit are in one pathway
The magnitude of the charges is the same on each plate
10.11 Combinations of Capacitors

43. 10.11 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
10.11 Combinations of Capacitors

44. 10.12 Energy Stored in a Charged Capacitor

45. 10.12 Energy Stored in a Charged Capacitor
Capacitors store energy
Energy can be defined as change in work
Or the are under a plot of Q vs. V
10.12 Energy Stored in a Charged Capacitor

46. 10.13 Capacitors with Dielectrics

47. 10.13 Capacitors with Dielectrics
Most capacitors have an insulator between the plates
Called a Dielectric
Increases the
capacitance by
a factor K
Called the dielectric
constant
10.13 Capacitors with Dielectrics

48. 10.13 Capacitors with Dielectrics
Some Dielectric Constants
Material K
Paper
3.7
Glass 5
Rubber
6.7
Mica 7
Strontium Titanate
300
10.13 Capacitors with Dielectrics