Download Electric Field

Electric Field
• We use Coulomb’s Law to find the
force between two charges, q1 and q2,
separated by a distance r:
•𝐹=𝑘
𝑞1 𝑞2
𝑟2
k=9x109 Nm2/C2
• Opposite charges attract, like charges
repel
Review…
• What is the magnitude of the force on
the proton due to the electron in
hydrogen?
•𝐹=𝑘
𝑞1 𝑞2
𝑟2
k=9x109 Nm2/C2
Qp=1.6x10-19C
Qe=-1.6x10-19C
+
• F= 2.3x10-8 N
• What is the direction?
You Try it…
-
r= 1x10-10 m
• Calculate the force on the +2μC charge
due to the other two charges.
•𝐹=𝑘
𝑞1 𝑞2
𝑟2
• k=9x109 Nm2/C2
You Try it…
• Charged particles create electric fields
in the space around them.
• Any other charged object that comes
into this space will interact with this
electric field.
• Direction of the E-field is the same as
for the force that a + charge would feel
at that location.
Electric Field
+
• The strength of the electric field is
measured by placing a positive “test
charge” at a spot near the source charge and
measuring the force on the “test charge”
•𝐹=𝑘
•𝐸=
𝐹
𝑞0
𝑄𝑝 𝑞0
𝑟2
Test charge
Source charge
q0
+
Qp
Electric Field
+
r= 1x10-10 m
F
• Electric Field is defined as the
electric force per unit charge.
•𝑬=
𝑭
𝑞0
=
𝑄
𝑘 2
𝑟
• Units are N/C
• E is a vector. The direction is taken as
the direction of the force it would
exert on a POSITIVE point charge.
Electric Field
• The electric field from multiple point
charges can be obtained by taking the
vector sum of the electric fields of the
individual charges.
Electric Field
• What is the direction of the electric
field at point A?
A
C
Your Turn…
• It is the surrounding charges that create
an electric field at a given point.
• Any charge (positive or negative) placed
at the point interacts with the field and
experiences a force.
• Note: positive charges placed in an Efield will move in the direction of the
field. Negative charges will move
in the opposite direction.
Electric Field
• Electric Force (F) is the force felt by a
charge at some location
• Electric Field (E) is found for a specific
location (any location) and tells what the
electric force would be if a positive charge
were located there
• F = Eq
• Both are vectors, with magnitude and
direction
Electric Force vs. Electric Field
• F = Eq
• Note, we can calculate the net E-field for all
the fixed charges and then use this value to
find the force on other charges
• Examples: ions/electrons in neurons, heart
tissue, and cell membranes.
Electric Force vs. Electric Field
• The electric field is radially outward from
a single positive point charge. Why?
• What direction would E be for a single
negative point charge?
• The circles represent where the magnitude
of E is the same (later we will see that
these are equipotential surfaces)
Electric Field
• We use Electric Field Lines to convey
information about an E-field
• Closeness (or density) of lines shows the field
strength. Note: lines NEVER cross
• The number of lines entering or leaving a
charge is proportional to the magnitude of the
charge.
• The arrow gives the direction of the E-field
(start on +, end on – charge)
Electric Field Lines
Electric Field Lines
Electric Field Lines
• Which charge is positive and which charge
is negative? How do you know?
Your turn…
• What is the ratio of charges? QA: QB
Your turn…
• Where is the E-field stronger: at point X or
point Y? How do you know?
Your turn…
Which one is correct?
p. 563 Focus #8, 9, 12, 13, 17
p. 564-565 Problems #14, 29, 30,
32, 34
Do Focus tonight. We will do the
Problems on white boards
tomorrow!
Assignment
• In a conductor, electrons are free to move.
• Therefore, if electrons feel an electric
force, they will move until the feel no
more force (F=0).
• Since F=Eq, if F=0 then E=0 inside of a
conductor. ALWAYS!
• For a conductor at equilibrium, any excess
charge resides on the surface of the
conductor.
E-Field inside a conductor
• A conductor shields any
charge within it from electric
fields created outside the
conductor.
E-Field inside a conductor