1
Electrostatics 2
Motion of Charged Particle in Uniform Electric Field;
Electric Potential Energy & Electric Potential
Motion of Charged Particle in an Electric
Field
•
if a particle of mass m and charge q is placed in an
electric field, it will feel a force
F
E
= q
E
, and
electric field, it will feel a force
q
, and
accelerate accordingly
•
thus q
E
= m
a
, or
a =
q/m
E
•
if
E
is
uniform
(constant in magnitude and
direction), then the particle’s acceleration is
constant
(kinematic equations apply !!)
–
if q is + for the particle, then it accelerates in the direction of
E
–
if q is -, its acceleration is opposite to the direction of
E
Positive Particle Moving in
Uniform Electric Field
•
consider simple electric field that
is
uniform (constant in
m gnit de nd di e tion)
like
magnitude and direction)
like
that between two charged plates
•
imagine a proton in that field
•
It will be repulsed from one
plate attracted to other : so net
plate, attracted to other : so net
force directed towards the
negative plate
A positive charge q, mass m, is released from rest in a
uniform electric field
E
directed along the x-axis as shown.
Describe its motion using kinematic equations.

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Clicker:
A positive point charge +
Q
is released from rest in an
general electric field. At any later time, the velocity of the
point charge
1. is in the direction of the
electric field at the position
of the point charge
2. is directly opposite the
direction of the electric
field at the position of the
point charge
3. is perpendicular to the
direction of the electric
field at the position of the
1
2
3
4
5
0%
0%
0%
0%
0%
0 of 5
point charge
4. is zero
5. not enough information
given to decide
20
Applying the kinematic equations for constant
acceleration in uniform
E
field for a charged
projectile (neglecting gravity) with
v
o
@
E
Applying the kinematic equations for constant
acceleration in uniform
E
field for a charged
projectile (neglecting gravity) with
v
o
@
E
Example: An electron enters the region of a uniform electric
field
with v
o
= 3.00 x 10
6
m/s and E = 200 N/C. The horizontal
length of the plates is L = 0.1 m. Find the acceleration of the
electron, and the length of time the electron is in the field.