electrostatics
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Electrostatics. 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. Like charges repel - PowerPoint PPT PresentationTRANSCRIPT
Electrostatics
10.1 Properties of Electric Charges
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
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
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
Cx 1910602.1
10.2 Insulators and Conductors
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
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
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
10.3 Coulomb’s Law
10.3 Coulomb’s Law
Electric charges apply forces to each other
From experiments
Force is proportional
to charge
Inversely proportional
to square of distance
1 22
q qF k
r
9 2 28.988 10 /k x Nm C
10.3 Coulomb’s Law
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
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
191.602 10e x C
12 2 20 8.85 10 /x C Nm
10.3 Coulomb’s Law
These are forces, so be sure to use vector math, draw free body diagrams
For multiple objects, require multiple free body diagram
10.4 The Electric Field
10.4 The Electric Field
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
The Electric field would then be
Or
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)
2r
kQE
q
FE
EqF
10.5 Electric Field Lines
10.5 Electric Field Lines
To visualize electric fieldsDraw electric field linesDirection of the lines is the direction of force on a positive test chargeThe density of the lines indicates relative strength of the fieldNote: the field density increase as you get closer
10.5 Electric Field Lines
For multiple charges, keep in mind
1. Field lines indicate the direction of the field
The actual field is tangent to the field lines
2. The magnitude of the field is relative to the field line density
3. Fields start at positive and end at negatives
Field Lines
10.5 Electric Field Lines
If the field is produced by two closely spaced parallel plates
The field density is constant
So the electric field is
constant
Electric Dipole – two
point charges of
equal magnitude
but oppsite sign
10.7 Potential Difference and Electric Potential
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
WPE FdPE qEdPE
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
q
PEV
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)
q
PEV
q
PEV
C
JV
1
11
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 r
10.8 Electric Potential & Potential Energy
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
qV k
r