Transcript Electric field

Advanced Physics
Chapter 16
Electric Charge and
Electric Field
Electric Charge and
Electric Field
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16-1 Static Electricity
16-2 Electric Charge in the Atom
16-3 Insulators and Conductors
16-4 Induced Charge: the Electroscope
16-5 Coulomb’s Law
16-6 Solving Problems Involving Coulomb’s
Law and Vectors
16-7 The Electric Field
16-8 Field lines
16-9 Electric Fields and Conductors
16-10 Electric Forces in Molecular Biology
(DNA)
Static Electricity
Static Electricity = ?
Electric charge = ?
• Types
• Interactions
• Giving an object a net electric
charge
Static Electricity
Law of conservation of electric
charge—the net amount of
electric charge produced in any
process is zero
• This means = ?
Electric Charge in the
Atom
• So what are the electric charges
in an atom?
• What happens if we “charge” an
atom?
• What is a polar molecule?
Insulators and Conductors
• What are insulators and
conductors?
• What are examples of insulators and
conductors?
• Can you charge insulators and/or
conductors?
– Explain how.
• What is a semiconductor?
Induced Charge: the
Electroscope
• What is an electroscope?
• Can you use an electroscope to
find the type of charge on an
object?
Induced Charge: the
Electroscope
• How do you charge an
electroscope by conduction?
• What charge does it get?
Induced Charge: the
Electroscope
• How do you charge an
electroscope by induction?
Induced Charge: the
Electroscope
• How do you charge an
electroscope by induction?
• What is grounding?
• What charge does it get?
Coulomb’s Law
• Where:
F = (kQ1Q2)/r2
– F = force between two charged objects
(1 & 2)
– k = Coulomb's constant (9 x 109 N*m2/C2)
– Q = net charges on objects (Coulomb)
• 1 electron has the charge of 1.6 x 10-19 C
– r = distance between objects
Coulomb’s Law
F = (kQ1Q2
2
)/r
• k can be represented by
k = 1/(40)
• Where 0 = permittivity of free
space
– 8.85 x 10-12 C2/N*m2
• You will see 0 again later
Coulomb’s Law
F = (kQ1Q2
2
)/r
• What is the relationship between F,
Q’s and r?
Coulomb’s Law
F = (kQ1Q2)/r2
• If the force is between two objects
with the same charge the force is
repulsive and positive
• If the force is between two objects
with the opposite charges the force
is attractive and negative
Solving Problems Involving
Coulomb’s Law and Vectors
• The net force acting on an object is
the sum of all the forces acting on it
• For charged objects, these forces
can be calculated using Coulomb’s
Law and remembering that force
vectors have direction
• It is usually better to ignore signs
and just deal with direction of force
when looking at net force acting on
an object
Solving Problems Involving
Coulomb’s Law and Vectors
Example
• Three charged particles Q1, Q2, and
Q3 are arranged in a straight line.
They have charges of –8.0C, 3.0C
and – 4.0C respectively. Q1 is 0.3 m
from Q2 and 0.5 m from Q3. Find the
net force on Q3
Answer
• 1.5 N toward Q1
Solving Problems Involving
Coulomb’s Law and Vectors
Example
• Three charged particles Q1, Q2, and
Q3 are arranged where Q1is 52 cm
east of Q2 and Q3 is 30 cm north of
Q2 They have charges of –86.0C,
50.0C and 65.0C respectively. Find
the net force on Q3
Answer
• 282.36 N at 64.6° north of east or
25.4° east of north
The Electric Field
Electric field—property of all
charged objects
• the vector sum of the force
exerted on a test charge when it
is by charged object divided by
the charge on the test charge
Test charge must be ______ and
_______. Why?
The Electric Field
Electric field
E = F/q'
• Why is the electric field not just
determined by the force on the test
charge?
• What is the unit for electric field
strength?
The Electric Field
Electric field
E = F/q'
So…F = Eq'
So…F = kQq‘/r2 = Eq'
So…E = kQ/r2 for an electric field
generated by charge Q
The Electric Field
Electric field
When the electric field is due to more
than one charge, the total field at
any point is the vector sum of the
individual fields (superposition
principle)
E = E1 + E2 + …..
Field Lines
Electric field
• How do you diagram electric field?
– Positive charge
• How do the lines tell an electric
field’s magnitude and direction?
Field Lines
Electric field
• How do you diagram electric field?
– Negative charge
Field Lines
Electric field
• How do you diagram electric field?
– Two negative charges
Field Lines
Electric field
• How do you diagram electric field?
– Two positive charges
Field Lines
Electric field
• How do you diagram electric field?
– One negative and one positive charge
Field Lines
Electric field
• How do you diagram electric field?
– Between two parallel plates of opposite
charge
Electric Fields and
Conductors
• The electric field inside a good
conductor is zero in the static
situation (when charges are at rest)
• Any net charge on a good conductor
distributes itself on the surface
• The direction of the electric field is
perpendicular to the surface of the
conductor
Electric Fields and
Conductors
• For a hollow sphere with a
charge in it; there is an electric
field due to net charges on
inside and outside surface so
there is an electric field inside
and outside but not within the
metal
Electric Fields and
Conductors
• For a hollow box inside an
electric field; there is no electric
field because of the
accumulation of charges along
its surface (safety in a car)
Electric Forces in
Molecular Biology (DNA)
• What are the four
bases in DNA?
– Thymine, cytosine,
adenine, and guanine
• How do they match up?
– Thymine and adenine
– Cytosine and guanine
• What holds the two
DNA strands together?
– Hydrogen bonds