Lec 3

Question 1

What is advantage of complete circuit ????

Answer 1

Electron Cathode se gaye cathode ko aaye , cathode neutral rha toh usko electron nikalne mein takleef nhi hui

Question 2

Cathode agar neutral na hota circuit complete na hoti toh kya hota?

Answer 2

electron ke jaana pr cathode deficiency of electron ke karan positively charged hota aur metal baaki electrons ko iss attraction ke karan metal surface se kjaane mein takleef deta

Question 3

Current kaise aaya circuit mein?
Kya vaccum mein electron travel krte h?

Answer 3

Nhi vaccum mein electron travel nhi krte
We irradiated metal surface with monochromatic light with sufficient high frequency due to which it emits photoelectrons which are responsible for current.

Question 4

Variable potential kya h?

Answer 4

It is variable
Potential of Anode wrt cathode
Va-Vc

Question 5

When ext voltage is 0 , why there is photocurrent?

Answer 5

When V=0, light will irradiate metal cathode plate, and photoelectrons will emit out of metal plate , Some photoelectrons will be fast ,no waste of energy in collision ,they travel with high kinetic energy while other photoelectrons will waste some energy in collision due to which they will travel with less speed and less kinetic energy .Due to presence of space charge (electrons) these photoelectrons with less kinetic energy are repelled by them and they reverse their path toward cathode and do not reach anode and do not contribute in photocurrent.While other photoelectrons with high kinetic energy,reach anode and contribute in photocurrent.
Thus, their is little current even in absence of external voltage

Question 6

Why space charge is present?

Answer 6

Space charge refers tothe collection of electrons which are emitted from the metal surface, after the application of tube current, at a short distance away from the metal surface. These electrons collect and form a cloud of negative charge around the metal surface.2

Question 7

What is accelerating voltage?Accelerating voltage laagane se kya hoga aur kyu?

Answer 7

Accelerating voltage means Va-Vc>0 , when potential of Anode is greater than potential of cathode , we know electric field originate from higher potential and terminate to lower potential .So electric field originate from anode to cathode. And force on electron acts in opposite direction to electric field from cathode to anode , This force on photoelectrons by electric field eE. Greater the potential of Anode wrt cathode , greater will be electric field from anode to cathode ,greater will be force on electron by electric field from cathode to anode , easier will be to compensate repulsion of photoelectrons by space charge,possibly more electrons reach anode and hence there will be more photocurrent

Question 8

What happens if u keep on increasing potential of Anode wrt cathode??

Answer 8

A time shall come, when all the electrons will reach anode and photocurrent will be maximum and it will not increase after that. On increasing potential of Anode wrt cathode,the electric field increases, the force on electron due to electric field increases.

Question 9

What is saturation current?

Answer 9

When all photoelectrons emitted from cathode reach anode .Current is maximum in this case and can’t increase after that.

Question 10

What is regarding voltage?

Answer 10

When potential of Anode is lower with respect to cathode,
Va-Vc<0

Question 11

What happens if you increase retarding potential?

Answer 11

Electric field cathode se anode
Force on photoelectrons anode se cathode and repulsion due to space charge the possibility of photoelectrons reaching anode decreases and so photocurrent also decreases.Not all photoelectrons but some photoelectrons with less kinetic energy=less speed are repelled by space charge and not reach anode but some do reach anode . That’s y photocurrent decreases

Question 12

What is quantum efficiency?

Answer 12

=no of photoelectrons emitted*100/no of photoelectrons incident

Question 13

What is saturation current formula?
Formula of number of photoelectrons incident in 1 sec?
Derive saturation current.

Answer 13

i=quantum efficiencyPwavelengthe/100hc
ni=Pwavelength/hc
Write quantum efficiency
Write ne=no of photoelectrons emitted in 1 sec
ne=ni*quantum efficiency/100
Put value of ni from above in this

I=q/t
q=ne
Aur t time mein electron emitted pr dhyaan Dene
I milega

Question 14

If power is doubled , wavelength is fixed ,what is affected?

Answer 14

Power is directly proportional to
Number of photons
Power double hui toh no of photons bhi double hue

Question 15

Write formula of saturation current again, keeping lambda fix , what is relation between power and i and graph and it’s slope

Answer 15

Saturation current is directly proportional to power
Graph jiska y intercept zero hai aur slope =quantum efficiencylambdae/100hc

Question 16

Red light
100watt pr i current
200 watt pr …… current
50watt pr…….. current

Answer 16

2i
i/2

Question 17

If source is at r distance from cathode photocurrent is i what is photocurrent when it is at distance 2r?

Answer 17

I=P/4pirsq I is intensity
P=IA=PA/4pirsq
Pnew=P/4
Inew=I/4
As P is directly proportional to i saturation

Question 18

Power ka formula baatade?

Answer 18

P=IA=PA/4pirsq
P is directly proportional to 1/rsq
i saturation is directly proportional to 1/rsq

Question 19

Explain stopping potential?
Cathode pr kya aur anode pr kya potential rkhu iske liye?

Answer 19

When potential of cathode is zero and potential of Anode is made negative wrt cathode ,this negative potential of Anode wrt cathode is -Vs(stopping potential) , On increasing this Va-Vc<0, electric field from cathode to anode increases as cathode at 0 potential is higher than anode at negative potential wrt cathode,force on photoelectrons opposite to electric field increases, So electron which has speed in forward direction has its speed decreases due to force by electric field,so possibility of photoelectrons reaching anode also decreases ,also acts space charge repulsion and that minimum value of negative Vs potential for which no photoelctron reach anode and don’t contribute to photocurrent is called stopping potential.
Cathode at zero potential
Anode at lower potential wrt cathode
Sabse zyada kinetic energy Walla photoelctron ki bhi forward direction mein speed ghatti jaarhi due to electric field force in opposite direction and anode pe just pahuchne ke phele uski kinetic energy zero hui aur electric field force usko cathode pr le gyi vapas

Question 20

Relation between kmax and Vs

Answer 20

By total mechanical energy conversation,
kmax+0(-e)=0+-Vs*-e
kmax=eVs

Question 21

Consistency of phoelectric effect with an idea???

Answer 21

Individual photons of EM radiation are absorbed by individual electrons in a material,
With electron gaining photon’s energy.

Question 22

What are parameters affecting stopping potential or Kmax? 4hai

Answer 22

Write Einstein EQ in eVs form
Stopping potential
Depends on energy of individual photon
Does not depend on number of photons
Greater energy of photons,greater will be energy of photoelectrons as individual photons absorbed by individual electrons with electron gaining photon’s energy , greater frequency,lower wavelength and higher kinetic energy, higher will be stopping potential to stop than electron from going anode.
If cathode metal plate has less work function,easier to emit electron, more kinetic energy,more stopping potential