Electrons and Photons - JEE Main Physics by ednexa.com

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Electrons and Photons
Electrons
Electron
Experiments with a Crookes tube first demonstrated the particle
nature of electrons. In this illustration, the profile of the Maltesecross-shaped target is projected against the tube face at right by a
beam of electrons.[1]
Elementary particle[2]
Composition
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Generation
First
Interactions
Gravity, Electromagnetic, Weak
Symbol
e−, β−
Antiparticle
Positron (also called antielectron)
Theorized
Richard Laming (1838–1851),[3]
G. Johnstone Stoney (1874) and
others.[4][5]
Discovered
J. J. Thomson (1897)
Mass
9.10938291(40)×10−31 kg
Electric charge
−1.602176565(35)×10−19 C
Magnetic moment
−1.00115965218076(27) μB[7]
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Spin
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⁄2

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Photon
a photon is a discrete bundle (or quantum) of
electromagnetic (or light) energy.
E  h
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Photo electric effect
Electrons are present on the surface of conductor.
These electrons are loosely bounded with the
nucleus. In few metals these electrons are released
from its surface when the metal is expose to light.
This effect is known as photo electric effect.
“Phenomenon of emission of electrons by metal
surface under the action of light is called as photoelectric effect. The emitted electrons in this process
are called as photo-electrons.” Alkali metals like
sodium, potassium shows this effect.
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1. Frequency of incident radiation.
1. Initially P.D. & intensity of radiation are kept
constant.
It is observed that, at low frequency, photoelectric current cannot flow.
This certain minimum frequency at and above
which photoelectric current starts is called as
threshold frequency ( ).
0
Threshold frequency is differs for different
metals. Wavelength of light corresponding to
threshold frequency is called as threshold
wavelength (λ0)
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2. Now,
frequency
(greater
than
threshold
frequency) & potential difference are kept
constant.
It
is
observed
that
photoelectric
current
increases as intensity of incident radiation
increases.
Photoelectric current is directly proportional to
intensity of incident radiations.
3. Now
frequency
(greater
than
threshold
frequency) & intensity incident radiation are
kept constant
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It is observed the photo-electric current stops at
certain -ve potential. This value of potential is
called as stopping potential (Vs).
It is also observed that value of stopping
potential increases as the frequency of incident
radiation increases.
The relation between kinetic energy (K.E.) and
stopping potential (Vs) is K.E.=Vs.e
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4. As soon as light radiation incident on photo
sensitive material, electron emission starts and
current flows. That means photoelectric effect is
an instantaneous process.
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Characteristics of photo electric effect.
1. Photo-electric effect occurs only when the
frequency of incidence radiation is greater than
certain minimum frequency called as threshold
frequency (v0).
2. The number of electrons emitted per second is
directly proportional to intensity of incident
radiation.
3. The maximum K.E. of photo-electrons depends
upon frequency of incident radiation K.E.
increases with increase in frequency.
4. Photo-electric
effect
process.
10
is
as
instantaneous

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Einstein’s photo-electric equation.
E  Work Function  K.E.
1
h  W0  mv 2max
2
Definition:
“Minimum amount of light energy required to
release an electron from metal surface is called as
photo electric work function.”
If frequency of incident radiation is decreased upto
threshold frequency then, kinetic energy becomes
zero. i.e. when ν = ν0, K.E. = 0
∴ hν0 = W0
Substitute this value in equation
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1
2
mv max
2
1
2
h  h  mv max
2
1
2
 h     0   mv max
2
h  h 0 
1. From this relation it is clear that for emission of
electrons frequent n of incident radiation n must
be greater than threshold frequency.
2. From above equation it is also clear that, as
frequency of incident radiations increases K.E.
of emitted electrons increases.
3. The intense beam of light contains maximum
number of photons. Due to this, number of
collisions are more & number of electrons
emitted per second are more. That is emission
of electrons is directly proportional to intensity
of incident radiations.
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4. Photoelectric
effect
process.
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is
an
instantaneous

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Photoelectric cell: Cathode is a concave shape electrode & coated
with photo sensitive material like sodium cesium
etc. Anode is a platinum rod. There are two pins
from t electrodes to connect the cell into external
circuit. There is a non conducting base to the
photocell.
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Applications of photo cell
Photocell is used in the following applications:
1. Exposure meter:
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2. Lux meter :
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3. Burglar alarm:
4. Production of sound recorded on cine film:
Sound is recorded on the part of cine film.
When light from this part incident on photocell
from this part, due to change in intensity,
magnitude of current changes. Using this
current, a sound is produced.
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