Advertisements
Advertisements
प्रश्न
In an experiment on photoelectric effect, the emitter and the collector plates are placed at a separation of 10 cm and are connected through an ammeter without any cell. A magnetic field B exists parallel to the plates. The work function of the emitter is 2.39 eV and the light incident on it has wavelengths between 400 nm and 600 nm. Find the minimum value of B for which the current registered by the ammeter is zero. Neglect any effect of space charge.
Advertisements
उत्तर
Given:
Separation between the collector and emitter, d = 10 cm
Work function, ϕ = 2.39 eV
Wavelength range, λ1 = 400 nm to λ2 = 600 nm
Magnetic field B will be minimum if energy is maximum.
For maximum energy, wavelength λ should be minimum.
Einstein's photoelectric equation :
`E = (hc)/lambda - phi`
Here , h = Planck's constant
`lambda` = Wavelength of light
c = speed of light
`therefore E = 1242/400 - 2.39`
= `3.105 - 2.39 = 0.715 "eV"`
The beam of ejected electrons will be bent by the magnetic field. If the electrons do not reach the other plates, there will be no current.
When a charged particle is sent perpendicular to a magnetic field, it moves along a circle of radius,
`r = (mv)/(qB)`,
where m = mass of charge particle
B = magnetic field
v = velocity of particle
q = charge on the particle
Radius of the circle should be equal to r = d, so that no current flows in the Circuit .
⇒ `r = sqrt((2mE)/(qB))` `(∵ mv = sqrt(2mE))`
⇒ `0.1 = sqrt((2 xx 9.1 xx 10^-31 xx 1.6 xx 10^-19 xx 0.715)/(1.6 xx 10^-19 xx B))`
⇒ `B = sqrt((2 xx 9.1 xx 1.6 xx 0.715 xx 10^-50)/(1.6 xx 10^-20))`
⇒ `B = sqrt((2 xx 9.1 xx 1.6 xx 0.715)/(1.6)) xx 10^-5`
⇒ `B = 2.85 xx 10^-5 "T"`
APPEARS IN
संबंधित प्रश्न
Light of frequency 7.21 × 1014 Hz is incident on a metal surface. Electrons with a maximum speed of 6.0 × 105 m/s are ejected from the surface. What is the threshold frequency for photoemission of electrons?
(a) A monoenergetic electron beam with electron speed of 5.20 × 106 m s−1 is subject to a magnetic field of 1.30 × 10−4 T normal to the beam velocity. What is the a radius of the circle traced by the beam, given e/m for electron equals 1.76 × 1011 C kg−1?
(b) Is the formula you employ in (a) valid for calculating the radius of the path of a 20 MeV electron beam? If not, in what way is it modified?
An electron gun with its collector at a potential of 100 V fires out electrons in a spherical bulb containing hydrogen gas at low pressure (∼10−2 mm of Hg). A magnetic field of 2.83 × 10−4 T curves the path of the electrons in a circular orbit of radius 12.0 cm. (The path can be viewed because the gas ions in the path focus the beam by attracting electrons, and emitting light by electron capture; this method is known as the ‘fine beam tube’ method. Determine e/m from the data.
If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?
| Metal | Work Function (eV) |
| Na | 1.92 |
| K | 2.15 |
| Ca | 3.20 |
| Mo | 4.17 |
Define the term "cut off frequency" in photoelectric emission. The threshod frequency of a metal is f. When the light of frequency 2f is incident on the metal plate, the maximum velocity of photo-electrons is v1. When the frequency of the incident radiation is increased to 5f, the maximum velocity of phto-electrons is v2. Find the ratio v1 : v2.
Visible light has wavelengths in the range of 400 nm to 780 nm. Calculate the range of energy of the photons of visible light.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
Two neutral particles are kept 1 m apart. Suppose by some mechanism some charge is transferred from one particle to the other and the electric potential energy lost is completely converted into a photon. Calculate the longest and the next smaller wavelength of the photon possible.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
In an experiment on photoelectric effect, light of wavelength 400 nm is incident on a cesium plate at the rate of 5.0 W. The potential of the collector plate is made sufficiently positive with respect to the emitter, so that the current reaches its saturation value. Assuming that on average, one out of every 106 photons is able to eject a photoelectron, find the photocurrent in the circuit.
Work function of aluminium is 4.2 eV. If two photons each of energy 2.5 eV are incident on its surface, will the emission of electrons take place? Justify your answer.
The stopping potential in an experiment on photoelectric effect is 1.5V. What is the maximum kinetic energy of the photoelectrons emitted? Calculate in Joules.
For a given frequency of light and a positive plate potential in the set up below, If the intensity of light is increased then ______.
In various experiments on photo electricity, the stopping potential for a given frequency of the incident radiation is ______.
The electromagnetic theory of light failed to explain ______.
Cathode rays can be deflected by
In photoelectric effect, the photoelectric current
An increase in the intensity of the radiation causing photo-electric emission from a surface does not affect the maximum K.E. of the photoelectrons. Explain.
