Download Revisiting moving electric charges

Revisiting moving electric charges
A 20 μF parallel plate capacitor stores 300 μC of charge. The
plates are 6 cm apart. An electron enters the region between the
plates along the midpoint with a speed of 1 x 106 m/s as shown.
What is the magnitude and direction of the force that acts on the
electron (Neglect gravity). Sketch the path the electron follows on the
diagram. (Neglect fringing).
What direction is the electric field between the plates? down
fringing
It looks like this
How can we neglect gravity?? –
what is the weight of the electron?
C = 20µF = 20 x 10-6 F
Q= 300μC = 300 x 10-6 C
Q
C= V
d = 6 cm
electron
m = 9.1 x 10-31 kg
q = -1.6 x 10-19 C
v = 1 x 106 m/s
F=Eq
F = 250 (1.6 x 10-19)
F = 4.0 x 10-17 N
20 =
300
V
V = 15 V
V=Ed
15 = E (.06)
E = 250 V/m down
4.0 x 10-17 N up
We can cause this particle to pass through the region between the plates undeflected
by introducing a magnetic field in that region.
Determine the magnitude and direction of the magnetic field necessary for this to occur.
What is the electric force?
4.0 x 10-17 N up
What is the magnetic force
necessary to cause the electron
4.0 x 10-17 N
to pass through the field
down
undeflected?
palm - down
fingers - into
thumb
left
page
F= qvB
(negative)
-17
-19
6
4.0 x 10 = (1.6 x 10 )(1.0 x 10 )(B)
B = 2.5 x 10-4 T
into page
direction?? RHRII
This is the magnitude of the field
palm - up
thumb - left
fingers -
F= qvB
F = (1.6 x 10-19)(1.0 x 107)(17x10-4)
out of page
2.72 x 10-15 N
We can cause this particle to pass through the tube undeflected by introducing an electric
field in the region of the magnetic field.
Determine the magnitude and direction of the electric field necessary for this to occur.
F=Eq
E = 1.7 x 104 N/C
2.72 x 10-15 = E (1.6 x 10-19)
NEVER USE RHR TO DETERMINE THE DIRECTION
OF AN ELECTRIC FIELD OR FORCE
What direction does the electric
down
force need to be?
Where would a positive plate have
to be placed to force the electron
beneath it
down?
Where would a negative plate have
to be placed to force the electron
above it
down?
What direction is the electric field from positive to
between parallel plates
negative (up)
Electric field is up