# Electric Fields

```What is Electric Field??
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
What is Electric Field??
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
What is the value of an electric field?
Electric field = Electric force per unit charge
What is Electric Field??
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
What is the value of an electric field?
Electric field = Electric force per unit charge
q is NOT Q.
q is NOT the charge that creates the field!
q is a test charge, or second charge brought
into the field.
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.
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 always in the direction of the force that
would occur on a positive test charge.
Check for Understanding
1) How might you define electric field?
E=
𝑘𝑄
𝑟2
2) Which has a stronger electric field? A -4.0 μC charge or a 2.0 μC
charge?
3)The electric field of 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?
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)
Check for Understanding
E=
𝑘𝑄
𝑟2
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?
&frac14;E
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
11
(10-10)2 = 2.910 N/C
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.
Question?
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?
What is the direction of the electric field at point A?
1) Up
2) Down
3) Left
4) Right
5) Zero
A
x
Question?
What is the direction of the electric field at point B?
1) Up
2) Down
3) Left
4) Right
5) Zero
y
B
x
Question?
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
```