Download Electric Field Lines

Jan 26, 2017
Quick Review:
What do we already know about the
electrostatic force?
The electrostatic force is the force between stationary
charges.
Opposite charges attract.
Same charges repel.
We also know that this force is fairly strong and that it can
act over a distance (but that the strength of this force
decreases with the square of distance)
Today, we will learn how to quantify the electrostatic force.
Calculating Electrostatic force:
Coulomb’s Law
Coulomb’s Law allows us to
Notice the similarities
between
thebetween
equations
fortwo
calculate
the force
any
electrostatic force and gravity:
charged objects.
m1m2
F G
r2
q1q2
F k
2
r
What do the variables represent?
In both law
cases,
the force
increases
withk the
8.99size
10 N  m / C
k is “Coulomb’s
constant,
and it has
a value of
or charge) of each object and force
q1 and q(mass
2 are the amount of charge (measured in C) of each object
r is the decreases
distance between
thedistance
objects (measured
m) object
as the
betweenin the
9
increases.
2
Don’t forget: Force is a vector, so it has direction. You can reason out the
direction of the force by remembering the opposites attract and like repel.
2
We do: Calculating Electrostatic Force
q1q2
F k 2
r
k  8.99 109 N  m 2 / C 2
1) A charge of +2 mC and a charge of -3 mC are separated
by 0.1m. What is the force between them? If the
charges are each 0.001kg, what will be their
acceleration?
F = 8.99 X 109 X 2X10-6 X -3 X 10-6 / (0.1)2
F = -5N (attraction!)
a = F / m = -5 N / 0.001 kg = 5000 m/s2
2) Two charges are separated by a certain distance. If the
distance between them is halved, how will the force
change?
The force will be quadrupled (four times bigger)
You Do– Calculating Electrostatic Force
1. A woman accumulates a charge of
2.0 x 10-5 C when sliding out of the
seat of a car. A man has
accumulated a charge – 8.0 x 10-5 C
while waiting in the wind.
What is the force between them if they are 6.0 m apart?
F k
q1q2
 0.40 N
2
r
(“-“ = attractive force)
2. A positive charge is 0.5 m away from a negative charge.
If the size of the positive charge is doubled and the distance
is between them is also doubled, how will the force between
the two charges change?
The force will be halved.
Electrostatic force when there are
multiple charges
Calculating the force between two charges is easy.
What do you do if there are multiple charges?
You can find the total force on any charge adding the forces from each
other charge. Use Coulomb’s law to find the force from each individual
charge, and make a free body diagram!
Example: Charge Q is in between two other charges, arranged in a line.
What is the total force on Q?
d = 0.05 m
q1 = -5.0 mC
F1
Q
Q = -2.0 mC
q2 = 4.5 mC
Use Coulomb’s Law
to find F1 and F2
Draw FBD for Q
F2
d = 0.08 m
F1 = 36 N
F2 = 13 N
Since both forces
are pointing in the
same direction, they
can just be added
Total F = 49 N
What is a “force” field anyway?
In physics, a ‘force’ field describes an area where objects
experience a force-at-a-distance such as gravity, electrostatic
force, or magnetism.
In each case, the field is generated by a certain type of
particle:
• Gravity by a mass
• Electrostatic by a charge
• Magnetism by a moving charge
The particle changes the space around it so that other
particles of the same type will experience a force if they come
into that area.
Watch me!
Electric Field Lines
Electric field lines show the direction of the force
on a positive ‘test charge’
Electric field lines point
towards negative charges
Electric field lines point
away from positive
charges
Electric Field Lines
The density of lines shows the relative strength
of the electric field
This has two implications:
1) larger charges have more field lines radiating
2) As you move farther away from the charge, the
strength of the field (density of lines) decreases
Net Electric Field Lines
If we have two or more charges creating an
electric field, we add the vectors from each
charge.
NOTE: Electric field lines can never cross! That
would mean that a test charge would go in two
directions at once.
Play with me!
Question 1 - show with fingers
B
a) What is the charge of q1?
Thumbs up for +, down for -
b) What is the charge of q2?
Thumbs up for +, down for -
A
c) Which charge is larger?
d) Where will a positive charge at
position A move?
e) Where will an electron at position
B move?
Question 1 - show with fingers
B
A
a) What is the charge of q1?
Thumbs up for +, down for negative b/c field lines going
towards the charge
b) What is the charge of q2?
Thumbs up for +, down for negative b/c field lines going
towards the charge
c) Which charge is larger?
2 – field lines are denser
d) Where will a positive charge at
position A move?
Up
e) Where will an electron at position
B move?
Up
Question 4
What is the direction of the electric field at point C?
1) Left
2) Right
3)
Zero
Away from positive charge (right)
Towards negative charge (right)
y
Net E field is to right.
C
x
Question 5
What is the direction of the electric field at point A?
1) Up
2) Down
3) Left
4) Right
5) Zero
A
x
Question 6
What is the direction of the electric field at point B?
1) Up
2) Down
3) Left
4) Right
5) Zero
y
B
x
Question7
What is the direction of the electric field at point A, if the
two positive charges have equal magnitude?
1) Up
2) Down
3) Left
4) Right
5) Zero
A
x