Electric Field
Physics
Dr Manjunatha S
Charles Coulomb
French physicist (1736–1806)
Analogy
The electric field is the space
around an electrical charge
just like
a gravitational field is the space
around a mass.
Electric field
The electric field can be defined as the region of
the space in which a charged body experience a
force.
Electric Field
 Space around a charge.
What is the difference?
Van de Graaff Generator
 Builds up static electric
charges.
Electric Field Vector, E
 E = F/ qo
 F is electrostatic force
 qo , positive test charge
 E is a vector quantity
 Unit: N/C
 E is analogous to the gravitational
field, g, where g=F/m
Example 1
 A charge of 3µC is used to test the electric
field of a central charge of 6C that causes a
force of 800N. What is the magnitude of the
electric field?
 Answer: 2.7 x108 N/C
Electric Field Lines
Electric Field Lines of two Positive
Charges
Electric Field Lines (Electric
lines of force)
Electric line of force is an imaginary line in
the electric field, along which a unit
positive charge would tend to move freely
to do so.
 Lines that indicate the strength and
direction of the electric field.
 The more dense the lines, the stronger the
field.
 Electric field vectors are tangent to the
curve.
Conductors and Electric Fields
(under electrostatic conditions)
 “The electric field is zero inside a charged
conductor”.
 “Excess charge on an isolated conductor
resides on the surface”.
 “Excess charge accumulates on sharp points”.
 Electric field lines meet the conductor
perpendicular to the surface of the conductor.
Where are you safe during a
thunderstorm?
A) In a car
or
A) Outdoors
Why can you not get radio
reception in a tunnel or in a steel
bridge?
Which field is stronger?
A
B
Electric Field for a Point Charge
Using E=F/qo and Coulomb’s Law prove:
E=k Q
______
r2
where Q is the central charge.
Example 2
 A test charge of +3µC is located 5m to the
east of a -4µC charge.
 A) Find the electric force felt by the test
charge.
 B) Find the electric field at that location.
 Answer: 4.32x10-3 N, 1.44 x 103 N/C along the –x
axis.
Example 3
 If a test charge is moved to a location three
times as far as its original location, how does
the electric field change?
Example: 4
Two equal positive charges, each of 2 µC interact
with a third positive charge of 3 µC situated is as
shown in the figure. Calculate the magnitude and
direction of the force on the 3 µC charge.
Ans: 3.456x10-3 N
Example: 5
A water particle of mass 10 mg and having a
charge of 1.5x10-6 C stays suspended in a
room. What is the magnitude of electric field
in the room? What is its direction?
(F = q E =mg)
Ans : E=65.3 N/C
Example: 6
 Calculate the electric field felt by a test charge
located half way between a charge of +1C and
a charge of -3C, that are 2m apart.
 Answer: 1.8 x 1010 N/C
Example 7
 A charged particle has a mass 10-16 carries a
charge of 4.9 Χ 10 -19 C. Calculate the
intensity of electric field to be applied on it in
vertical direction so as to keep it at rest.

Answer
: 2000N C -1
Example 8
 Two point charges 5 μ C and 8μC are kept at
a distance of 0.4 m form each other find the
field midway between them.
 Answer: 6.75Χ 105 N / C
Example 9
 The force experienced by a charge of 1μC is
2 Χ 10-3 N. when placed in a field. Find the
intensity of electric field.

Answer
: 2000 N C-1