Download Electric Forces and Fields

Electrostatics
Forces and Fields
The study of stationary
charges and their interactions
Electric Charge





Two types of charge – “positive” and “negative”.
Charge is measured in “coulombs” - C
“e” – “elementary charge” – the amount of
charge on a proton or electron
e = 1.602 x 10-19 C (+ for proton;- for electron)
“q” or “Q” is the variable used for charge

Law of Charges -

Law of Conservation of Charge –
Opposite charges
attract, like charges repel
charge cannot be created or destroyed only
transferred. So the net amount of charge
produced in any process is zero.

Conductors allow electrons to move freely,
Insulators do not!
Methods of Charging

Charging by induction (conductors only)



Charge distribution is changed due to influence of
nearby charged object
A grounding path is provided for charge
movement
Charging by contact

(conductors and insulators)
Actual contact by the charged object
Polarization

Polarization: a
“separation of
charge” is
induced in an
insulator by the
influence of a
charged object
Electroscopes – a device to detect charge
“Sharing” charge - charged objects will
transfer charge to come to a better
equilibrium
If two charged conductors are connected (such as by a
conducting wire) they will share charge to achieve a
more balanced (shared) charge.
-8mC
6mC
-1mC
-1mC
Electric Forces – Coulomb’s Law
+q1
+
kq1q2
F 2
r
F
r
+
F
+q2
Coulomb’s constant
k = 9 x 109 Nm2/C2
Force interactions – “superposition”

Linear – charges on an axis
q1
Find Fnet on q3
q2
-
30 cm
-8.0mC
+
+3.0mC
q3
20 cm
-4.0mC
Fon3by1 = 1.2N
Fnet = -F32 + F31 = -2.7N + 1.2 N = -1.5N
Fon3by2 = 2.7N
The Fnet is 1.5N and points to the left
(Note: found magnitude only)

Angles – use components
F31 = 140N
y
Find Fnet on q3
F32 = 330N
Find x and y components of each force
acting on q3 – be careful of direction
F32
Use Pythagorean theorem and tan-1
SEE NEXT SLIDE
+
q3 = 65mC
F31
30cm
30o
+
q2 = 50mC
52cm
-
x
q 3 = -86mC
Equilibrium
Two charges are on the x axis as shown. At what point must a third charge,
q3 (2mc), be placed so the net force on it is zero?
Set distance 3 to 2to “r”
q1 15 mC
Set distance from 1 to 3 as 2 -r
r = 2.0 m
F3,1 = F3,2
kq1q3
r2
=
q2 6 mC
Electric Fields ( N/C)
Electric fields: vector quantity
Electric fields: represented by “field lines”
Definition: force per unit charge



F
E
q
Its action on a charge leads to
a force on that charge

F  Eq
A charge, Q, produces a field in space. This exerts a force (N) on a small “test”
charge “q” placed in the field.
F
E
q
+Q
q
r
Note: Q can
be either + or –
but the test charge
(small q) is always +
The E-field can also be calculated using the strength of the charge “Q”
and the distance to a particular point in the field.
kQ
E 2
r
E-Field lines
1. Field lines drawn away from positive and toward negative
2. Lines intersect surface at right angles
3. Number of lines is proportional to charge strength
Interaction of field lines between charges of
equal magnitude and opposite sign