Transcript The electric force in an electric field

Electric fields
Objectives
•
Interpret electric field diagrams.
•
Describe and calculate the relationship between electric
force and electric field for a point charge.
•
Identify examples of electric fields in everyday life.
Physics terms
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electric force
•
electric field
•
electric field lines
•
Faraday cage
Equations
Think
Where would you be safer in a lightning storm?
in a car? or under a tree?
Why?
Lightning safety
Where would you be safer in a lightning storm?
in a car? or under a tree?
Why?
The answer to this question lies in an understanding
of the nature of electric fields and conductors.
What is the electric field?
Imagine that a positively-charged sphere is placed
in a region of space.
This charged sphere creates an electric field at ALL
points in space in this region.
If you bring another charged particle into this
region, it will feel an electric force of attraction or
repulsion due to this electric field.
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+ + +
+ +
What is the electric field?
The electric field has units of newtons per coulomb.
Imagine that the electric field at some point near this
charged sphere has a strength of 500 N/C.
• What is the force on a 1 C charge placed at that point?
• What is the force on a 0.5 C charge placed that that
point?
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+ + +
+ +
What is the electric field?
The electric field has units of newtons per coulomb.
Imagine that the electric field at some point near this
charged sphere has a strength of 500 N/C.
• What is the force on a 1 C charge placed at that point?
500 N
• What is the force on a 0.5 C charge placed that that
point? 250 N
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+ + +
+ +
Electric field diagrams
How can we represent the electric field
if it exists at ALL points in space?
Electric field diagrams
How can we represent the electric field
if it exists at ALL points in space?
The electric field can be represented
with electric field lines, similar to
magnetic field lines.
Electric field lines trace the direction
of the force on a positive test charge
at all points in space.
Electric field lines
Electric field lines . . .
• always point away from positive charges
• always point toward negative charges
• are close together where the force is strong
• are far apart where the force is weak
Electric vs. gravitational fields
Gravitational force
field diagram for Earth
The force of gravity is
always attractive.
Electric vs. gravitational fields
Gravitational force
field diagram for Earth
The force of gravity is
always attractive.
Electric force field diagrams:
Positive charge
+ test charge
is repelled
Electric vs. gravitational fields
Gravitational force
field diagram for Earth
The force of gravity is
always attractive.
Electric force field diagrams:
Positive charge
Negative charge
+ test charge
is repelled
+ test charge
is attracted
The electric field is a vector
The symbol for electric field:
At every point in space, the electric field has both a
magnitude and a direction.
The direction of the electric field is the direction of the force
that a positive test charge would experience at that point.
Electric field of two charges
How about the electric field around two or more charges?
Electric field of two charges
How about the electric field around two or more charges?
is the vector sum of the electric fields from each charge.
two positive charges
two negative charges
two opposite charges
A positive test charge will feel a force from each charge creating the field.
Like charges
A positive (+) test charge experiences
equal and opposite forces which sum to
zero here.
Unlike charges
A positive (+) test charge is
repelled by positive charge,
attracted by negative charge.
Unlike charges
A positive (+) test charge is
repelled by positive charge,
attracted by negative charge.
Where lines are
close together,
the field is
strong.
Where lines are far
apart, the field is
weak.
Electric charge on a conductor
Like charges repel one another.
If a conductor has excess charge,
those charges will move as far apart
as possible – to the surface of the
conductor.
Electric charge on a conductor
Like charges repel one another.
If a conductor has excess charge,
those charges will move as far apart
as possible – to the surface of the
conductor.
This is true regardless of the shape
of the conductor.
Electric field of a conductor
The electric field inside a
conductor in electrostatic
equilibrium is always zero.
E=0
Electric field of a conductor
The electric field inside a
conductor in electrostatic
equilibrium is always zero . . .
even if the conductor is placed
in an external electric field!
The Faraday cage
English scientist Michael Faraday
invented the Faraday cage in 1836.
If a cage is made of metal,
anything inside the cage is
shielded from outside electricity
and electric fields.
The Faraday cage
In a lightning storm, the metal
frame of a car acts as a
Faraday cage.
Even if lightning strikes the
outside of the car, electric
charges will redistribute
themselves to keep the electric
field inside the car at zero.
Shielding effect of conductors
Some electrical devices create
electric fields that can interfere
with the operation of other
sensitive devices.
Shielding effect of conductors
Some electrical devices create
electric fields that can interfere
with the operation of other
sensitive devices.
One way to prevent interference
is to enclose the device in a
conducting material to block
outside electric fields.
Standard RJ45 cable
has conducting metal
mesh on the outside.
Calculating force and field
Electric field is a vector quantity, so it has both magnitude and direction.
The relationship between the
electric force and electric field is:
Calculating force and field
Electric field is a vector quantity, so it has both magnitude and direction.
The relationship between the
electric force and electric field is:
The electric field is a property
of a point in space.
It tells you the force that will
be exerted per coulomb of
charge placed at that point.
Electric field and g
The force of gravity near
Earth’s surface:
g is the force per unit
mass. It can be
measured in N/kg!
Electric field and g
The force of gravity near
Earth’s surface:
The electric force in an
electric field:
g is the force per unit
E is the force per unit
mass. It can be
measured in N/kg!
charge, measured in N/C.
Electric field of a point charge
Two charges, q and q2, exert
repulsive forces on each other:
Electric field of a point charge
Two charges, q and q2, exert
repulsive forces on each other:
The electric field created by q
at the location of q2 is:
Electric field of a point charge
Two charges, q and q2, exert
repulsive forces on each other:
The electric field created by q
at the location of q2 is:
E has this value at this point
in space even if q2 is not there
to “feel” it.
Test your knowledge
What is the strength of the electric field 1.00 meter away
from a point charge of q = +2.0 x 10-6 C?
Test your knowledge
What is the strength of the electric field 1.00 meter away
from a point charge of q = +2.0 x 10-6 C?
Assessment
1. Two charged particles are arranged as shown.
Which statement below is true?
A. Both charges must be positive.
B. Both charges must be negative.
C. The upper charge is positive and the
lower charge is negative.
D. The upper charge is negative and the
lower charge is positive.
Assessment
1. Two charged particles are arranged as shown.
Which statement below is true?
A. Both charges must be positive.
A. Both charges must be negative.
A. The upper charge is positive and the
lower charge is negative.
A. The upper charge is negative and the
lower charge is positive.
Assessment
2. The proton in the nucleus of a hydrogen atom has a charge of
1.6 x 10-19 C while the electron, located 5 x 10-11 m away, has a
charge of −1.6 x 10-19 C.
a. Calculate and describe the electric force between them.
b. What is the strength of the electric field at the location of the
electron if the electron is not there to “feel” it?
Assessment
2. The proton in the nucleus of a hydrogen atom has a charge of
1.6 x 10-19 C while the electron, located 5 x 10-11 m away, has a
charge of −1.6 x 10-19 C.
a. Calculate and describe the electric force between them.
attractive
b. What is the strength of the electric field at the location of the
electron if the electron is not there to “feel” it?
Assessment
3. If you find yourself outside in a lightning storm, would you be safer
standing under a tree or getting into a car? Why?
Assessment
3. If you find yourself outside in a lightning storm, would you be safer
standing under a tree or getting into a car? Why?
answer: you would be safer in the car. The car acts as a Faraday cage,
shielding you from outside electricity and electric fields.
Uniform fields (advanced)
Point charges create electric fields that are non-uniform.
These fields are strong close to the charges and weak far
from the charges.
Question: Is it possible to create a uniform electric field—a
field that has the same strength at all points?
Capacitors (advanced)
Capacitors are electrical
components that CAN create a
uniform electric field.
A capacitor consists of two
metal plates separated by an
insulating material called a
dielectric.
Capacitors (advanced)
When the capacitor is
connected to a voltage
source, the two plates
become oppositely
charged.
A uniform electric field is
created in the region
between the two plates.
Uses of capacitors (advanced)
Capacitors are one of the
most common circuit
elements, and have many
uses.
One of these uses is to
store and quickly release
energy. So they are used
in flash photography and
in defibrillators.
Uses of capacitors (advanced)
Capacitors can be combined
with resistors into RC circuits,
which are useful as timing
devices.
Capacitors can be combined
with resistors and inductors to
create LRC circuits, which can
be tuned to specific
frequencies, such as for radio
reception.