Transcript Electric Field

lab
• Levitating toys
Objectives
1. Calculate electric field strength.
2. Draw and interpret electric field lines.
3. Identify the four properties associated with a
conductor in electrostatic equilibrium.
objectives
Know:
– Electric fields: Exist near charges, originate on positives and
end on negatives, never cross
– Electric field intensity equations.
Understand:
– Relationship between field strength, distance, force and
charge
– Behavior of charges between charged plates
Be able to:
– Use the electric field equation to solve for unknown variables.
– Draw and recognize electric field diagrams for:
Point charges
Systems of charges
Parallel plates
Electric force is a field force
Gravitational force - the mass
of the Earth exerted an influence,
affecting other masses which were in the
surrounding neighborhood.
electrical force – The charges
exerts an influence over a distance
affecting other charges which were in
the surrounding neighborhood
The Electric Field and Gravitational Field
Concept
• How can an apple reach across space
and falls toward Earth?
• The massive Earth creates a
Gravitational field. Other masses in that
field would feel its effect in the space.
Whether a massive object enters that
space or not, the gravitational field
exists.
• Similarly, a charged object creates an
electric field. Other charges in that
field would feel its effect in the space.
Whether a charged object enters that
space or not, the electric field exists.
E = Fe / q
g = Fg / m
Electric Field magnitude
E 
Fe
q
(
E 
kQq
2
d
q
)

kQ
d
2
• Electric field strength (E) is the force per charge ratio. The unit for
electric field is N/C
• q is the test charge – in Coulombs
• Fe is the force on the test charge q – in Newton
• k: constant, k = 8.99 x 109 Nm9/C2
• Q: source charge – in Coulombs
e
E 
kQ
d
2
• Note that there are two charges here - the source charge and
the test charge. Electric field is the force per quantity of charge
on the test charge.
• The electric field strength is not dependent upon the quantity
of charge on the test charge.
• The electric field strength is dependent upon the quantity of
charge on the source charge Q and the distance of separation
d from the source charge.
An Inverse Square Law
• Electric field strength is location dependent, and its
magnitude decreases as the distance from a location to the
source increases. And by whatever factor the distance is
changed, the electric field strength will change inversely by
the square of that factor.
E
E=
d
k∙Q
d2
example
• What is the magnitude of the electric force
acting on an electron located in an electric
field with an intensity of 5.0 x 103 N/C?
E = 5.0 x 103 N/C q = 1.6 x 10-19 C
F =? N
E = F/q
F = Eq
F = (5.0 N x 103 N/C) x (1.6 x 10-19 C)
= 8.0 x 10-16 N
example
• What is the magnitude of an electrostatic
force experienced by one elementary charge
at a point in an electric field where the electric
field intensity is 3.0 × 103 N/C?
E = 3.0 x 103 N/C q = 1.6 x 10-19 C
F =? N
E = F/q
F = Eq
F = (3.0 N x 103 N/C) x (1.6 x 10-19 C)
= 4.8 x 10-16 N
example
• The diagram above represents a uniformly
charged rod. Which graph below best
represents the relationship between the
magnitude of the electric field intensity (E)
and the distance from the rod as measured
along line AB?
A
B
C
D
example
•
Suppose that two equally charged spheres attract
each other with a force of -0.5 N ("-" means
attractive) when placed a distance of 30. cm from
each other. Determine the charge of the spheres.
PSYW
Sample problem 17D
• A charge q1 = +7.00 µC is at the origin, and a charge q2 = -5.00
µC is on the x-axis 0.300 m from the origin, as shown. Find the
electric field strength at point P, which is on the y-axis 0.400 m
from the origin.
E1
P
E2
0.400 m
q1+
0.500 m
q
0.300 m 2
Class work
• Page 647 - Practice 17D
Electric Field – direction
• is determined by using a POSITIVE test charge
• 1 – place test charge
• 2 – draw arrow in direction pushed (net effect)
• 3 – this is the field direction
+
+
-
+
Lines are directed away from positively charged source charges and toward negatively
charged source charges
Electric Field Maps
+
-
Electric Field Maps
+
+
Rules for Drawing Electric Field Patterns
1.
2.
The lines must begin on positive charges and terminate on
negative charges
Surround more charged objects by more lines.
The electric field is greatest at locations closest to the surface of
the charge and least at locations further from the surface of the
charge.
3.
draw the lines of force ___________________
perpendicular to the
surfaces of objects at the locations where the lines connect
to object's surfaces.
•
The electric force, and thus the electric field, is always
directed perpendicular to the surface of an object. There
are never any component of force parallel to the surface.
4.
never cross
Electric field lines should _____________________.
•
Every single location in space has its own electric field
strength and direction associated with it; consequently, the
lines representing the field cannot cross each other at any
given location in space.
• Examples of electric field lines
example
•
1.
2.
3.
4.
The diagram shows the electric field in the vicinity of two
charged conducting spheres, A and B. What is the static
electric charge on each of the conducting spheres?
A is negative and B is positive.
A is positive and B is negative.
Both A and B are positive.
Both A and B are negative.
example
• Two small metallic spheres, A and B, are separated by a
distance of 4.0 × 10-1 meter, as shown. The charge on each
sphere is +1.0 × 10-6 coulomb. Point P is located near the
spheres. Which arrow best represents the direction of the
resultant electric field at point P due to the charges on spheres
A and B?
1
2
3
4
Fields between two oppositely charged parallel
plates
• If the distance separating two
oppositely charged parallel
E is constant
plates is small compared to
their area, the electric field
between the plates is
uniform.
• The field lines are from
positive plate to the negative
plate.
force is the same
• Since E=F/q, the __________________________
on a charged
particle everywhere inside the plates.
• A charged particle will accelerate toward the plate with the
opposite charge.
• Ex: negative charge accelerates to positive plate, and positive
charge accelerate to negative plate.
++++++++++++++++++++++++++++++++++++++++++++++
┼
─
example
•
As an electron moves
between two charged parallel
plates from point B to point A,
as shown in the diagram, the
force of the electric field on
the electron
1. decreases
2. increases
3. remains the same
example
•
1.
2.
3.
4.
In the diagram, proton p, neutron n, and electron e are
located as shown between two oppositely charged plates.
The magnitude of acceleration will be greatest for the
neutron, because it has the greatest mass
neutron, because it is neutral
electron, because it has the smallest mass
proton, because it is farthest from the negative plate
Electric field and conductors
conductor is material which allows electrons to
• A _______________
move relatively freely from atom to atom.
• Electrostatic equilibrium is the condition established by
charged conductors in which the excess charge has optimally
distanced itself so as to reduce the total amount of repulsive
forces. Once a charged conductor has reached the state of
electrostatic equilibrium, there is no further motion of charge
about the surface.
-
+
+
+
+
-
-
Four properties of conductor in electric
equilibrium
1.
the electric field anywhere beneath the surface of a
charged conductor is zero.
•
This principle of shielding is commonly utilized today as we
protect delicate electrical equipment by enclosing them in
metal cases.
2.
Any excess charge on an isolated conductor resides entirely
on the conductor’s outer surface.
3.
the electric field on the surface of the conductor is directed
entirely perpendicular to the surface.
4.
A forth characteristic of conducting objects at electrostatic
equilibrium is that the electric fields are strongest at
locations along the surface where the object is most
curved.
example
•
1.
2.
3.
4.
A metallic sphere is positively charged. The
field at the center of the sphere due to this
positive charge is
positive
negative
zero
dependent on the magnitude of the charge
Millikan’s oil-drop experiment
• In 1909, Robert Millikan performed the oil-drop
experiment to measure the elementary electric
charge. The experiment entailed balancing the
downward gravitational force with the upward
electric forces on tiny charged droplets of oil
suspended between two metal plates.
Fe
Fg
Fg = Fe
m∙g = E∙q
q = mg / E
• Milliken measured the forces on charged oil drops in a uniform
electric field.
• He found no drop with a charge less than 1.60 x 10-19 coulomb.
The charges on other drops were integral multiples of this value.
• This finding demonstrated that there is a ______________ unit of
fundamental
charge. This elementary charge of 1.60 x 10-19 coulomb
is called
the charge on a single electron.
example
•
What did Milliken conclude after performing his oildrop experiment?
1. The charge on an electron is 1.0 C.
2. The mass of an electron is 1.7 × 10-27 kg.
3. The charge on any oil drop is an integral multiple of
the charge on an electron.
4. The charge on an oil drop may have any value
larger than 1.6 × 10-19 C.
example
• The diagram, which illustrates the Milliken oil drop
experiment, shows a 3.2 × 10-14-kilogram oil drop with a
charge of -1.6 × 10-18 coulomb. The oil drop was in
equilibrium when the upward electrical force on the drop was
equal in magnitude to the gravitational force on the
drop. What was the magnitude of the electric field intensity
when this oil drop was in equilibrium?
Fnet = Fe - Fg = 0
Fe = Fg
E∙q = m∙g
E(-1.6x10-18C) = 3.2x10-14kg(9.81 N/kg)
E = 1.96 x 105 N/C = 2.0 x 105 N/C
example
• An object with a net charge of 4.80 × 10-6 coulomb
experiences an electrostatic force having a magnitude of 6.00
× 10-2 newtons when placed near a negatively charged metal
sphere. What is the magnitude and direction of electric field
strength at this location? [show all work including substitution
with units]
Given: q = 4.8 x 10-6 C
F = 6.00 x 10-2 N
Unknown: E = ? N/C
Solve: E = F / q = 6.00 x 10-2 N / 4.8 x 10-6 C
E = 1.25 x 104 N/C directed toward the sphere.
lightning
• Perhaps the most known
and powerful displays of
electrostatics in nature is
a lightning storm.
• What is the cause and
mechanism associated
with lightning strikes?
• How do lightning rods
serve to protect buildings
from the devastating
affects of a lightning
strike?
Static Charge Buildup in the Clouds
• The precursor of any lightning
strike is the polarization of
positive and negative charges
within a storm cloud. The tops of
the storm clouds are known to
acquire an excess of positive
charge and the bottom of the
storm clouds acquire an excess of
negative charge.
• When a thunderhead passes over the
ground, electrons on Earth's outer
surface are repelled by the negatively
charged cloud's bottom surface. This
creates an opposite charge on the
Earth's surface. Buildings, trees and
even people can experience a
buildup of static charge as electrons
are repelled by the cloud's bottom.
• The electric field between the cloud
and the Earth is similar to the electric
field between two oppositely
charged plates.
• When the difference in negative
and positive charges between
ground and cloud gets large
enough, a lightning bolt begins.
The excess electrons on the
bottom of the cloud start a
journey through the conducting
air to the ground at speeds up to
60 miles per second.
• As electrons travel close to the
Earth, it encounters the positive
charges traveling upward, when
the two types of charges meet,
lightning begins.
• The enormous and rapid flow of charge along this pathway
between the cloud and Earth heats the surrounding air, causing
it to expand violently. The expansion of the air creates a
shockwave which we observe as thunder
Lightning Rods and Other Protective
Measures
• Tall buildings, farm houses and
other structures susceptible to
lightning strikes are often
equipped with lightning rods.
• the lightning rod serves to safely
divert the lightning to the
ground in event that the cloud
discharge its lightning via a bolt.
Check Your Understanding
1. TRUE or FALSE:
The presence of lightning rods on top of buildings
prevents a cloud with a static charge buildup from
releasing its charge to the building.
2. TRUE or FALSE:
If you place a lightning rod on top of your home but
failed to ground it, then it is unlikely that your home
would be struck by lightning.
Class work
• Charge conversions and electric force practice