January 14-15 2016
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
Watch me!
Electric Fields
Let's take a single electric charge,
Q, and put it somewhere.
The space around Q is altered – it
has the ability to cause a force on
any other charge that approaches
it. This space contains an electric
field.
Q
An electric field is an area where another charge
could feel a force. To actually experience the force,
we need to introduce a second charge, q.
An electric field can be measured by
the force it exerts on another charge.
E has units of N / C
Magnitude and Direction of Electric Field
The strength (magnitude) of electric fields depends just
on distance from the charge Q and on the value of the
charge.
It does NOT depend on the test charge.
Magnitude and Direction of Electric Field
The strength (magnitude) of electric fields depend just on
distance from the charge Q and on the value of the
charge.
It does NOT depend on the test charge.
𝐹 𝑘𝑞𝑄 𝑘𝑄
E= = 2 = 2
𝑞
𝑟𝑞
𝑟
E=
𝑘𝑄
𝑟2
Magnitude and Direction of Electric Field
The strength (magnitude) of electric fields depend just on
distance from the charge Q and on the value of the
charge.
It does NOT depend on the test charge.
𝐹 𝑘𝑞𝑄 𝑘𝑄
E= = 2 = 2
𝑞
𝑟𝑞
𝑟
E=
𝑘𝑄
𝑟2
Electric field is defined as having the same direction as
the force that would occur on a positive test charge.
Check for Understanding
1) How might you define electric field?
an area around a charge that can exert a force on other charges / the
force per unit charge
2) Which has a stronger electric field? A -4.0 μC charge or a 2.0 μC
charge?
-4.0 μC charge b/c E is proportional to charge
3)The electric field of a charge has a value E at 1 m away from the charge.
What will be the strength of the electric field at 2 m away from the charge?
¼E
4) What is the value of the electric field 0.1 m away from an electron?
(HINT: 1 electron has a charge of 1.6 X 10-19 C)
E=
9109
1.610-19 = 1.4 X 10-7 N/C
(10-1)2
5) A 5 X 10-5 C charge experiences a 15 N force when it is 0.5 m away from
a charged sphere. What is the strength of the electric force at that location?
What is the charge on the sphere?
E = 3 X 106 N/C
Q = 8 μC
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