Download Motion of charge particles in a uniform electric field

Motion of charge particles in a
uniform electric field
1
Dr. Loai Afana
Motion of charge particles in a uniform electric field
An electron moving horizontally passes between two horizontal planes, the upper
plane charged negatively, and the lower positively. A uniform, upward-directed
electric field exists in this region. This field exerts a force on the electron. Describe
the motion of the electron in this region.
- - - - - - - -
E0
-
+ + + + + + + + + +
2
Dr. Loai Afana
Q: A positive point charge +q of mass m is released from rest in a uniform electric
field E directed along the x-axis as shown in figure , describe its motion.
•
Ex  0
Horizontally:
– No electric field
– No force
– No acceleration
– Constant horizontal velocity
Fx  0
ax  0
+
vx  vo
x  vo t
+

E y  Eo
Vertically:
 Constant electric field
 Constant force
 Constant acceleration
 Vertical velocity increase linearly
with time.
Fy  eE o
ay  eE o / m e
v y  eE o t / m e
1
y  eE o t 2 / m e
2
Dr. Loai Afana
1 eE 0 x 2
y
.
2 m0 v 0 2
3
t1
t2
t1=t2
Conclusions:

The charge will follow a parabolic path downward.

Motion similar to motion under gravitational field only except the downward acceleration is
now larger.
4
Dr. Loai Afana
A charge q its mass m, fixed in uniform electric field.
Describe the motion of the charge after releasing it.
y
Dr. Loai Afana
5
Cathode Ray Tube (CRT)
+Q
+Q
-Q
-e
Phosphor
Screen
-e
x
-Q
This device is known as a
cathode ray tube (CRT)
6
Dr. Loai Afana
Discussion
Dr. Loai Afana
7
Example
An electron is released from rest in Uniform electric field: E= 1x106N/C .
Find:
1- its acceleration (a) .
2- the time taken for an electron to arrive the velocity of one tenth of
Velocity of light.
8
Dr. Loai Afana
Example
A uniform electric field E= 4x103 N/C exists in the region between two oppositely charged
plane parallel plates. An electron is released from rest at the surface of the negatively charged
plate and strikes the surface of the opposite plate 2x10-8 s later.
what is the separation between the plates?
9
Dr. Loai Afana
Example
A uniform electric field exists in a region between two oppositely charged plates.
An electron is released from rest at the surface of the negatively charged plate and strikes the
surface of the opposite plate, 2.0cm away, in a time 1.5*10-8s.
(a) What is the speed of the electron as it strikes the second plate?
(b) What is the magnitude of the electric field .
10
Dr. Loai Afana
Example
An electron is projected with an angle θ= 450 to the horizontal and With an initial velocity
V0=6x106 m/s in an uniform electric field E=2000 N/C ,
if the distance between the tow plates of the capacitor is d=2 cm and their length is L=10cm.
Show if the electron collides the upper plate of the capacitor ,
and if the answer is yes find The collision position.
- - - - - - - E0
v0
-
45
+ + + + + + + + + +
10cm
Dr. Loai Afana
11