Transcript Day 19: Electrostatic Potential Energy & CRT Applications

Day 19: Electrostatic Potential Energy
& CRT Applications
• The change in electric potential energy of
a charge moving through an electric field
• The electric potential energy of two
charges
• The electric potential energy of multiple
charges
• The Electron Volt
• Work Done to Dissemble a Hydrogen
Atom
• Cathode Ray Tube (CRT) Applications
Electric Potential Energy
• Suppose a point charge q is moved between
two points A & B, where the electric potential
due to the charge is VA & VB respectively. The
change in the electric potential energy of q in
the field is:
U  U B  U A  qVB  VA 
E
q
A
B
Electrostatic Potential Energy
• In isolation (ie: no electric field), a point charge has no
electric potential energy, because there is no force acting
on it.
• If a second charge is brought near the first charge, there
is an electrostatic force between them, then the electric
potential of the two charges is:
1 Q1Q2
U
4 0 r1, 2
• This represents the work done by an external force to
bring Q2 from ∞ to r1,2
Q1
Q2
r1,2

Electrostatic Potential Energy
• Now, in a system of three charges, the total potential
energy will be the work done to bring all three charges
together.
Q1
Q2
Q3
r1,2
r2,3
r1,3
1  Q1Q2 Q1Q3 Q2Q3 
U




4 0  r1, 2
r1,3
r2,3 
•
Systems of more charges requires n P2 
n!
terms
n  r !r!
The Electron Volt
• The Electron Volt is a measure of energy
to deal with small energies at the atomic
level.
• 1 eV is the energy required by a particle,
carrying a charge of q = e, moving through
a potential difference of 1.0 volt
if q  e  1.602 1019 C
and U  q  V
then 1 eV  (1.602 1019 C )(1.0V )  1.602 1019 J
The Work Done to Disassemble a
Hydrogen Atom
(-)
r = .529 x 10-10 m
(+)
• The work necessary to remove the electron is equal to
the change in total energy of the atom ( energy as an
atom vs. the energy of the atom infinitely far apart
• ΔE = -13.6 eV. This is, in fact, the ionization energy of
hydrogen
Cathode Ray Tube (CRT) Application
• A CRT is the basis of classic television receivers,
computer monitors, and oscilloscopes
• A CRT is based on a simple vacuum tube diode.
CATHODE
• A negatively charged cathode is heated by a 6.3 V PS to
1000°C, emitting electrons which are attracted to a
positive anode (plate), in that direction only.
Cathode Ray Tube (CRT) Application
•
A CRT is a device which is based upon the thermionic diode, in which
electrons emitted by the cathode are accelerated by a high voltage anode,
through a small hole in the anode – thus coins the term “electron gun”
• The electrons collide with the screen which has a phosphorescent
coating, illuminating the screen with a spot
Cathode Ray Tube (CRT) Application
• Two sets of deflection plates
(1-vertical, 1-horizontal) use an
applied voltage to steer the electron
beam into a sweeping pattern, called
raster.
• A control grid adjusts the brightness of the beam, using a
video signal voltage applied to it.
• An oscilloscope is a piece of electronic equipment using
a CRT to measure amplitude and frequency of voltages.