LAB THIS WEEK! Pre-lab due at start of lab.
camera signal broadcast
EM waves  into electrical signals  into as radio wave image.
image to electrical signal object
Learning goals:
•Be able to explain the basic features of a color TV.
•Be able to design system for controlling energy and pointing of electron beam.

Reading Quiz on TV’s:
1) Which of the following best describes what is happening inside a TV? a. atoms are flying towards the screen through empty space, collide with the backside of the TV screen, and light is produced b. a whole bunch of small wires are connected up to each pixel on the screen, a current flows through each wire, and light is produced c. electrons are flying towards the screen through empty space, collide with the back-side of the TV screen, and light is produced d. electromagnetic waves are directed from the back of the TV tube toward the screen, they hit the back-side of the TV screen, and light is produced
2) A color TV emits a. all colors of visible light b. only red, green, and blue light c. all colors of visible light as well as IR and UV light d. only yellow, blue, and red light
3) Which one of the following physics principles is true and is employed in a TV?
a. Magnetic fields can produce light by oscillating back and forth b. Electrons are attracted to each other c. Magnetic fields can apply a force on electrons moving through empty space d. UV light can be used to excite atoms and produce light of a different color.

camera signal broadcast
EM waves  into electrical signals  into as radio wave image.
image to electrical signal object
TV is possible because: the human eye is crummy.
Only need to send a little information to make viewer think is real color moving picture.
Eye deficiencies: a. fuzzyleave out almost all detail from TV picture, just use bunch of dots.
b. slowjust flash dots up, then change.
c. poor color sense (3 sensors)just use three colors to trick into thinking full color range (red, green, and blue).

1. Turning a picture into electrical signals and vice-versa.
Take advantage of “fuzziness” of eye  making a picture with dots.
Black and white first.
Physics 2000 website: TV’s
This is image of a. Albert Einstein, b. bunch of black, white, and grey dots c. a cow do experiment- change scale
Is both a and b. How do you decide when something is one or the other?
All a question of what eye can resolve. If dots are big, eye sees as separate dots. As long as small enough so eye cannot easily tell the dots apart, eye sees as part of larger image.
So easiest (least information required) way to make picture is to just build up from just barely small enough grey dots.

Physics 2000 #2.
What is this? a. dot moving across page, b. an image of a person, c. image of a moose.
do demo.
How decide?
If dots illuminated fast enough, happens faster than eye can follow, looks like solid stationary image.
How fast is that? About 1/50 of sec.
So can make image by just painting a bunch of grey dots one after the other, just be sure to repaint at least every 0.02 sec.
Eye thinks is solid permanent image!

Energetic beam of electrons hits a phosphor on screen.
Why electrons? Because easy to get and can move them around really fast ! Phosphor gives off light.
Phosphors on screen
Electron beam…
Physics 2000 demo,
Oscilloscope no sweep demo.
What happens to atom in phosphor when electron hits it?
a. kinetic energy of beam electron converted into thermal energy.
b. kinetic energy of beam electron converted directly into light, c. kinetic energy of beam electron excites phosphor atom electron into higher energy level.
d. potential energy of beam electron turns into light.

c. kinetic energy of beam electron excites electron in the phosphor atom into higher energy level
Phosphor
Phosphor atom has bunch of extra energy. How does it get rid of it?
a. sends it off as light, b. flies to pieces, c. turns it into heat, d. spits out an electron.

c. kinetic energy of beam electron excites phosphor atom electron into higher energy level.
Phosphor
Phosphor atom has bunch of extra energy. How does it get rid of it?
a. sends it off as light, b. flies to pieces, c. turns it into heat, d. spits out an electron.
sends it off as light .

To change color of light given off by phosphor you could a. hit with higher intensity (#/sec) electrons b. hit with higher energy electrons, c. heat up phosphor, d. change type of atom in phosphor d. change type of atom .-
Color set by atom energy levels,
Color/wavelength of light corresponds to energy of jump.
Energy of photon = h x speed of light / wavelength
E = hc/ l
Black and white TV: just one phosphor, bluish white light.
Oscilloscope: one type of phosphor, green light.
Color TVs:
3 phosphors, blue, red, green.

Heart of TV- cathode ray tubemakes beam of electrons which hits screen to make light.
Most important physics of TV:
The electron beam and how to move it around.
Forces on electrons to create and steer beam:
1) Coulomb interactions of charge particles,
2) magnetic force.

Heart of TV- cathode ray tubemakes beam of electrons which hits screen to make light.
Forces on electrons to create electron beam:
1) Coulomb interactions of charge particles
(likes repel, opposites attract)
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1) Heat up cathode , boil off electrons
2) Anode – lots and lots of excess positive charges, attracts electrons speeds them up.
3) Grid – “control valve” … use more or less excess negative charges to control # of electrons making it into the beam.

Electron beam demomagnetic forces on moving electrons.
make electron beam accelerate it with voltage (excess positive charge) wide beam hits phosphor screen at gradual angle.
Electron
Beam
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Why not see beam before it hits phosphor screen?
nothing to bump into.
Electrons need to be accelerating to give off EM radiation!!
(bending around tight corner, jumping between levels in atom, moving up and down rapidly in radio antenna).
What would happen if there was air in the tube?
a. nothing, would act just like before, b. electrons would bump into air, make air glow but electrons would still get down the tube the same. c. electrons would bump into air and make glow but lose their energy and so not get down the tube.
c. bump into air and make glow but lose their energy and so not get down the tube.

What will happen when I bring up magnet from side?
Electron
Beam
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Magnetic Field from bar magnet
Electron beam will a. deflect up or down, b. not move, c. deflect towards or away from magnet do experiment
N
S

What will happen when I bring up magnet from side?
Electron
Beam
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Electron beam will a. deflect up or down
, beam bends down . do experiment
What will happen if I turn magnet around?
a. will bend the same way as before. b. will bend in opposite direction, c. will deflect toward magnet b. bends in opposite direction
What if use bigger magnet?
a. bend less, b. bend more, c. bend the same.
b. bends more, even off the screen
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Magnetic Field from bar magnet
N
S

what will happen when I bring up magnet from top?
N
Electron
Beam
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S
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++ beam will a. move toward side b. not move, c. deflect towards or away from magnet do exper. a. deflects to side
Deflected towards you!
Deflected away from you!

Magnetic fields put force on moving electrons!
Electron velocity
B
(magnetic field) force
Electrons feel force perpendicular to the direction they are moving and perpendicular to the magnetic field!
Bigger magnet  beam bends more.
Turn up voltage  velocity of electrons- what happens?
Size of force increases with B (magnetic field strength) and v (velocity).
written as equation: (won’t need)
F = charge x velocity X magnetic field
NOTE: If magnetic field and velocity are parallel…
NO FORCE ON ELECTRONS
TV use electromagnets, not bar magnets.
can change magnetic field faster. P2K demos