Modern physics NEB important question

 A beam of electrons moving with a velocity of 1 * 10 ^ 7 * m * s ^ - 1 enters mid-way between two horizontal parallel plates P. Q in a direction parallel to the plates in figure P and Q are 5 cm long, 2 cm apart and have a p.d.V applied between them. Calculate V, if the beam is deflected so that it just grazes the edge of the lower plate Q.

Electric intensity between plates

E = V/d = V/(2 * 10 ^ - 2)

Downward acceleration on electron,

a = (force)/(mass) = (Ee)/me

So vertical distance, 

y = 1/2 * a * t ^ 2 = 1/2 * [(Ee)/me] * t ^ 2

But y = 1cm = 10 ^ - 2 m and 

t = (5 * 10 ^ - 2)/(1 * 10 ^ 7)

= 5 * 10 ^ - 9 s

Since the horizontal velocity is not affected by the vertical electric field and remains constant. 

So, 10 ^ - 2 = 1/2 * V/(2 * 10 ^ - 2) * 1.8 * 10 ^ 11 * (5 * 10 ^ - 9) ^ 2

V = 89V

1.Why is clock oil used in Millikan's of the clock oil?experiment? Can we use water in place

Ans: In Millikan's oil drops experiment, the size of the droplet should be small and should not evaporate. Due to the small droplet, it acquires small terminal velocity. In water we do not find such types of condition because water may evaporate due to its high vapour pressure and the droplets of water are quite large. The above requirement will be fulfilled in the case of clock oil and hence it is used in Millikan's experiment.

2.Why does electric discharge take place at low pressure difference? and high potential

Ans: At low pressure and high potential, ionized particles can move towards electrodes which acquire sufficient kinetic energy and in their way can ionize other gas atoms. More ions and electrons are produced in the gas due to collisions. The electrons move towards the anode and the positive ions move towards the cathode. Hence electric discharge takes place at low pressure and high potential through the gas.

3.What is the importance of Millikan's oil drop experiment? 

Ans: Following are the importance of Millikan's oil drop experiment:

 (I) It shows that electronic charge is the              smallest possible charge on a charged             particle or ion

(ii) It provides the evidence of quantization        of charge i.e. a body can carry an                    integral multiple of minimum charge i.e. Q = (+-)ne, whre n = 1, 2, 3,...

(ii) By combining the result of Millikan's               experiment with the result of                           Thomson's experiment the mass of                 electron can be determined.

4.The value of e/m is constant for cathode rays but not for positive rays. Why? 

Ans: Cathode rays are the stream of electrons. The mass and charge of electrons are constant and hence, the specific charge () of cathode rays is constant. But positive rays are stream of positive ions. Masses of positive ions are different for different substances. Hence, the value of is not constant for positive rays.

5.gases are insulator at ordinary pressure and start 

Ans:- At low pressures, ions have a chance to reach their respective electrodes and constitute a current. At ordinary pressures, ions have no chance to do so because of collisions with gas molecules and recombination.

2.state Planck constant?

Ans:-the quantum nature of energy and relates the energy of a photon to its frequency. In the International System of Units (SI), the constant value is 6.62607015×10−34 joule-hertz−1 (or joule-seconds).

                          Photons 

*. What is threshold frequency? Does it depend upon intensity of light?

Ans:- The minimum frequency required to a remove electron from the surface of a metal.

Threshold frequency is independent of intensity of light in photoelectric effect.

# stopping potential define ?

Ans :- The minimum negative potential applied to anode for which photocurrent become zero.

#define photoelectric effect ?

The phenomenon of emissions of electrons from a metal surfaces when light radiation of suitable frequency fall on it...

#effect of potential on photocurrent 

- when intensity of light increase

- saturation current increase 

-stopping potential remaining same

#effect  of frequency of incident light stopping potential 

- when frequency light increase

- saturation current remaining same

- stopping potential increasing 

#Einsteins photoelectric equation