s-113 define these terms a.charge b.potential difference (voltage) c.current (amps) d.resistance
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S-113
Define these terms
A. Charge
B. Potential Difference (Voltage)
C. Current (Amps)
D. Resistance
Objective
I can define electric field in terms of the force on the test charge
I can calculate the magnitude and direction of the force on a positive or negative charge placed in a specified field
Electric Charge and Electric Field
AP Physics
Chapter 16
Electric Charge and Electric Field
16.1 Static Electricity
16.1 Static Electricity
Static electricity – not moving
Two types of charge
positive (+) when electrons are lost
negative (-) when electrons are gained
Objects can gain charges by rubbing
16.1
16.1 Static Electricity
Like charges repel
Unlike charges attract
Law of Conservation of electric charge – the net amount of electric charge produced in a process is zero
16.1
Electric Charge and Electric Field
16.2 Electric Charge in the Atom
16.2 Electric Charge in the Atom
Atoms are made of
positive protons
negative electrons
Electrons move from
one object to another when the objects
are rubbed
Eventually charges bleed off
objects because water
molecules are polar 16.2
Electric Charge and Electric Field
16.3 Insulators and Conductors
16.3 Insulators and Conductors
Conductors – outer
electrons of atoms
are free to move
through the
material
Insulator – electrons
tightly held, do
not move
16.3
16.3 Insulators and Conductors
Semiconductors – conduct electricity under some circumstances, don’t under other conditions
16.3
Electric Charge and Electric Field
16.4 Induced Charge; the Electroscope
16.4 Induced Charge; the Electroscope
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
it16.4
Electric Charge and Electric Field
16.5 Coulomb’s Law
16.5 Coulomb’s Law
Electric charges apply forces to each other
From experiments
Force is proportional
to charge
Inversely proportional
to square of distance
16.5
1 22
q qF k
r
9 2 28.988 10 /k x Nm C
16.5 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
16.5
S-114
A mad scientist charges two objects that are 2.45 cm apart. If the measured force between them is 120 N, toward each other, what is the magnitude of the charges (assume they are both the same charge)
Objective
I can determine the force that acts between specified point charges.
Homework Problem
Practice Problem
A 10C charge is placed so that x=0m and a second charge of -20C is placed so that it x=5m. Where must a third charge of 5C be placed so that the net force on the charge is zero.
16.5 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
16.5
191.602 10e x C
12 2 20 8.85 10 /x C Nm
16.5 Coulomb’s Law
AP tends to write k as
We will assume that charges are point charges - size is negligible
16.5
0
1
4k
Electric Charge and Electric Field
16.6 Solving Problems Involving Coulomb’s Laws and Vectors
16.6 Coulomb’s Law and Vectors
Principle of Superposition – Electrostatic forces are added just like any other force
As vectors
16.6
Practice Problem
Three charges are placed in space. The first charge has a value of 5nC. The second is placed 3 m above the first and has a value of -8nC. The third has a value of 2nC and is placed 4 m to the right of the first. What is the net force on the 5nC charge?
16.6
Practice Problem
Using the same charges as the previous problem, what is the net force on the -8nC charge?
16.6
Practice Problem
4 charges, each with a value of 5C are placed at the corners of a square 2 m on a side. What is the net force on the charge in the upper right corner?
16.6
Practice Problem
4 charges, each with a value of 5C are placed at the corners of a square 2 m on a side. What is the net force on the charge in the upper right corner?
16.6
Electric Charge and Electric Field
16.7 The Electric Field
16.7 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 field16.7
16.7 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)
16.7
2r
kqE
q
FE
EqF
S-112
A 3 mc charge is on the right, and a -2 mc charge on the left. They are 0.8 m apart. What is the field strength .2 m from the right?
Electric Charge and Electric Field
16.8 Field Lines
16.8 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
16.8
16.8 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
16.8
Field Lines
16.8 Field Lines
If the field is produced by two closely spaced parallel plates
The field density is constant
So the electric field is
constant
16.8
Electric Charge and Electric Field
16.9 Electric Fields and Conductors
16.9 Electric Fields and Conductors
For a conductor
1. The electric field inside a
conductor is zero (static)
2. Any net charge is distributed on the surface of a conductor
3. Electric field is always
perpendicular to the surface of a conductor
2. Charges concentrate at the
area of greatest
curvature 16.9
S-113
Four charges, each of 1.2 nC are placed on the corners of a square 2.5 cm per side. What is the magnitude and direction of the electric field on the upper right corner? What is the force on the charge in that corner?
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