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Electron Emission refers to the phenomenon of freeing electrons from the surface, stimulated by temperature elevation, radiation, or by strong electric field. There are electrons which are loosely bound to the nucleus (free electrons) on the metal surface, and are present in the outermost valence. When a certain amount of energy is provided to such electrons, they tend to emit the metal surface. This least amount of energy is known as Work Function which is represented by (Φ0).
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Key Terms: Electron Emission, Thermionic Emission, Electron, Work Function, Electric Field, Valence, Electrons, Protons, Neutrons, Handron
What are Electrons?
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Electrons are found in atoms. Simply, every atom consists of three elementary particles namely electrons, protons, and neutrons. The protons and neutrons are present inside the nucleus of an atom but the electrons are present at the outer region from the nucleus and bounded by the attraction force.
Electrons Diagram
This force of attraction acts more on near electrons but with the increase in distance from the nucleus, the force of attraction becomes weaker. The electrons present in the outer shell of the atom experience a small attraction force and are termed free electrons. Some important characteristics related to electron is mentioned below :
| Factors | Values |
|---|---|
| Charge of Electrons | < – 1.602 × 10-19 C |
| Mass of Electrons | 9.1 × 10-31 kg |
| Symbol of Electrons | e or β |
The property of electrical conductivity and magnetism is due to the presence of free electrons in metal atoms. The electrical conductivity of a metal depends on the number of free electrons present in it.
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What is Electron Emission?
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Electron emission is a phenomenon in which the free electrons of a metal atom eject out from its surface. The electrons are bound by the attraction force of the nucleus and are not able to escape the metallic surface at normal temperature.
It requires some amount of energy to cross the potential barrier of an atom and by acquiring this amount of energy from another metal atom or any source it will be able to eject out from a metallic surface.
Electron Emission
This minimum amount of energy is required to eject out from a metallic surface known as the work function for that particular metal atom. When the work function is provided to the metal atom the emission of electrons starts at the same moment. The value of the work function of a metal atom is different for different metals.
The work function of a metal depends on the following factors as mentioned below :
- Material of metals and their homogeneity.
- Purity of metals
- Characteristics of metals
Here are the work function of some important metals :
| Metals | Symbol | Work Function (in eV) |
|---|---|---|
| Sodium | Na | 2.46 |
| Aluminum | Al | 4.08 |
| Copper | Cu | 4.70 |
| Cesium | Cs | 2.14 |
| Zinc | Zn | 4.31 |
| Iron | Fe | 4.50 |
| Platinum | Pt | 6.35 |
Types of Electron Emission
Depending upon the mode of energy supplied to the metal atom to eject out the free electrons, electron emissions are of the following four types :
- Thermionic emission
- Electric field emission
- Photoelectric emission
- Secondary emission
- Thermionic emission: The word “thermionic” is a combination of two words therm and ionic. The term means 'heat ' or temperature and ionic means 'ions'. In thermionic emission thermal energy is given to metal atoms due to which the temperature of metal atoms increases and the kinetic energy of the atom also increases and it becomes able to cross the potential barrier and eject out from the metallic surface. The emitted or ejected electrons are called thermal electrons or thermions.
Thermionic emission
- Electric Field Emission: In electric field emission strong electric field is applied to the metal surface and due to this electrons are ejected out. On the application of a strong electric field of nearly 108 V/m, the electrons get emitted out. The electrons escape out very rapidly as the electric field is applied to them. Due to this, electric field emission is also termed quick electron emission.
- Photoelectric emission: When the light of a certain frequency is made to fall on the metal surface then ejection of electrons occurs. This is photoelectric emission. In this emission, the energy is given to free electrons in the form of packets of energy or photons. The emitted electrons are called photoelectrons. The number of emitting photoelectrons depends on the intensity of incident light.
Photoelectric Effect
The photoelectric effect is the first discovered by Albert Einstein, who received a Nobel Prize for the discovery of this effect.
Frequently Asked Questions Related to Electron EmissionQues. What is Photoelectric Effect? (1 mark) Ans. Photoelectric effect is the phenomenon wherein the electrically charged particles are seen to be released from or within a material after absorbing the electromagnetic radiation. Ques. Define Thermions. (1 mark) Ans. The free electrons that escape the metal surface under the influence of heat energy are known as thermions. |
Laws of Photoelectric Effect
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The laws of photoelectric effect are as follows :
- The number of emitted electrons or photoelectrons is directly proportional to the intensity of incident light or radiation.
- The electrons get emitted out very fast in photoelectric emission. The time lag between the electron's emission is nearly 10-9 sec.
- For photoelectric emission, the minimum frequency of light or radiation required for electron emission is termed the threshold frequency.
- The maximum threshold frequency at which the kinetic energy increases, depends on the frequency or wavelength of the incident light.
- In secondary emission, the primary incident particles of certain energy are incident on the metallic surface and due to its effect the secondary particles get emitted out. A large number of electrons emit in secondary emissions.
Things to Remember
- Electron Emission refers to the phenomenon of the liberation of electrons from the surface that is stimulated by temperature elevation, radiation, or a strong electric field.
- This least amount of energy is known as Work Function which is represented by (Φ0).
- Atom consists of three elementary particles namely electrons, protons, and neutrons.
- The protons and neutrons are present inside the nucleus of an atom but the electrons are present in the outer region of the nucleus and bounded by the attraction force.
- The property of electrical conductivity and magnetism is due to the presence of free electrons in metal atoms.
- Depending upon the mode of energy supplied to the metal atoms to eject out the free electrons, electron emissions are of the following four types: Thermionic emission, Electric field emission, Photoelectric emission, Secondary emission.
- The number of emitted electrons or photoelectrons is directly proportional to the intensity of incident light or radiation.
- The electrons get emitted out very fast in photoelectric emission. The time lag between the electron's emission is nearly 10 -9 sec.
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Previous Year Questions
- Which one among the following shows particle nature of light? [ NEET 2001]
- The work function of the metal is…. [NEET 2014]
- The threshold wavelength for the metallic surface is… [NEET 2016]
- The wavelength\( λe\) of an electron and \(λP\) of a photon of same energy E are…. [NEET 2013]
- The de Broglie wavelength of the emitted electron is… [NEET 2015]
- What will be the photoelectric current if the frequency is halved and intensity is doubled? [NEET 2020]
- The velocity of electron beam will be… [NEET 2001]
- Ultraviolet light wavelength … [BHU UET 2009]
- A charged dust particle of radius … [BHU UET 2009]
- An oxide-coated filament is useful in vacuum tubes … [BHU UET 2010]
- When a metal surface is illuminated by light of wavelengths … [BITSAT 2017]
- We wish to see inside an atom. Assuming the atom … [MU OET 2004]
- If work function of a metal is … [JIPMER 2005]
- Radiation, with wavelength … [MET 2020]
- λ1 and λ2 are used to illuminate the slits … [AMUEEE 2013]
- A metal surface of work function 3 eV is illuminated … [UPSEE 2019]
- Quantum nature can prove … [JCECE 2006]
- Huygens wave theory cant explain … [JCECE 2006]
- The work function of a metal … [KCET 2021]
- A proton moving with a momentum … [KCET 2021]
Sample Questions
Ques. Photons of energies 1 eV and 2 eV are successively incident on a metallic surface of work function 0.5 eV. The ratio of kinetic energy of most energetic photoelectrons in both the cases will be
1:2
1:3
1:1
1:4 (CBSE 2020)
Ans. Kinetic energy = energy - work function
For 1 eV energy
- E = 1 - 0.5
= 0.5
For 2 eV energy
- E = 2 - 0.5
= 1.5
Ratio = 1/3
Therefore, (b) 1:3
Ques. The variation of the stopping potential denoted by (V0) with the frequency (v) of the light incident on two distinct photosensitive surfaces M1 and M2 is given below. Identify the surface which possesses greater value of the work function. (CBSE 2020)
Ans. The threshold frequency of the metal plate M1 is lesser than that of M2.
Since λ ∝ 1/v, the threshold wavelength of the metal plate M1 is greater than that of M2.
The work function of a metal is given as W0 = hv0
Since the threshold frequency of metal M2 is greater, therefore the work function is also greater than that of M1.
Ques. What is “threshold frequency” in the context of photoelectric emission? (CBSE 2019)
Ans. In photoelectric effect, when light with suitable frequency falls on the metal surface, the electrons are ejected from the surface of the metals. The emitted electrons are known as photo electrons and similarly, the current produced is the photoelectric current. For all the metals, there is a minimum frequency of incident radiation below which no emission of photoelectrons occurs and this frequency is called threshold frequency.
Ques. (a) State two main observations of the photoelectric effect experiment which could only be explained by Einstein’s photoelectric equation.
(b) Draw a graph that shows variation of photocurrent with the anode potential of a photocell. (CBSE 2020)
Ans. (a) The two main observations are as follows:
(i) The maximum kinetic energy of an emitted photoelectron is independent of incident light.
(ii) For each photoelectron, there should be a threshold frequency of incident light below which no emission occurs.
(b)
Ques. An α particle and the proton of the same kinetic energy are allowed to pass through a magnetic field 
Ans. As we know, mass of proton,
mp = ¼ x mass of alpha particle, mα
Charge on proton, qp = ½ x charge on alpha particle, qα
Using the formula,
\(r = \frac{mv}{qB}\)We know, mv = √(2Km), here K = kinetic energy
Hence r = √(2Km)/ qB
Where, K, v and B are constant terms.
So,
\(r \alpha \frac{\sqrt{m}}{q}\)Thus the relation will be,
=
\(\frac{r_p}{r_\alpha} = \sqrt{\frac{m_p}{m_\alpha}\frac{q_\alpha}{q_p}}\)=
\(\sqrt{\frac{m_p}{4m_p}\frac{2q_p}{q_p}}\)= 1/1
Therefore, the ratio of the radii of the circular paths described by the particles is 1:1.
Ques. Show how Einstein’s photoelectric equation can be established by using photon picture of light. Write two features of photoelectric effect that can not be explained by wave theory. (CBSE 2017)
Ans. a) Light comprises photons and when a photon interacts with an electron, it lends its entire energy to the electron and then exists no longer. The energy used by electrons to eject is work function (hv) and the rest of the energy given to electrons is kE.
b) The two features of photoelectric effect that can not be explained by the wave theory are,
- Greater intensity has no effect on kE of an electron.
- Secondly, the wave theory is unable to explain the existence of threshold energy.
Ques. (i) What are the important features of Einstein's photoelectric equation.
(ii) Radiation of frequency 1015 Hz is incident on two photosensitive surfaces P and Q where there is no photoemission from the surface P but occurs from the surface Q. but photoelectrons have zero kinetic energy. Explain these observations and find the value of work function for the surface Q. (Delhi 2017)
Ans. i) Important features are,
- In the interaction of radiation with matter, the radiation behaves as if it is made up of photons.
- Each photon possesses energy E = hv and momentum p = hv/c and speed c, the speed of light.
- The intensity of radiation is understood by the number of photons falling per second on the surface.
- Photoelectric effect is understood as a result of the one to one collision between an electron and a photon.
- Photons are electrically neutral which are not deflected by electric and magnetic fields.
ii) As no photoelectric emission occurs from the surface P, therefore the frequency of incident radiation is 1015 Hz is less than its threshold frequency (v0)p.
On the other hand, photo emission takes place from Q but the kinetic energy of photoelectrons is zero. This shows that frequency of incident radiation is equal to the threshold frequency of Q.
Ques. What are the characteristic features in the photoelectric effect that can not be explained based on the wave theory of light, but can be explained only by using Einstein’s equation. (Delhi 2016)
Ans. The characteristics features in the photoelectric effect are,
i) The maximum kinetic energy of the emitted electron must be directly proportional to the intensity of the incident radiations which is not observed experimentally. Again, the maximum kinetic energy of the emitted electrons must not be dependent on the incident frequency according to wave theory, but it is not so.
ii) Threshold frequency should not exist based on the wave theory. Light of all frequencies must emit electrons where intensity of light is sufficient for the electrons to emit.
iii) The photoelectric effect should not be instantaneous according to the wave theory. Wave energy can not be transferred to a particular electron but will be distributed to each electron present in the illuminated portion. Therefore, there must be a time lag between emission of electrons and incidence of radiation.
Ques. (a) State the important properties of photons that are used to establish Einstein’s photoelectric equation.
(b) Use this equation to explain the concept of
(i) threshold frequency and
(ii) stopping potential. (All India 2014)
Ans: (a) The important properties of photons are,
- In the interaction of radiation with matter, the radiation behaves as if it is made up of photons.
- Each photon possesses energy E = hv and momentum p = hv/c and speed c, the speed of light.
- All photons of light of a particular frequency v or wavelength λ have the same energy, E = hv = hc/λ and momentum, p = hv/c = h/λ whatever the intensity of radiation may be. With increasing the intensity of light of a given wavelength, there will be an increase in the number of photons per second with each one having the same energy. Hence, photon energy is independent of intensity of radiation.
- Photons are electrically neutral which are not deflected by electric and magnetic fields.
- The total energy and momentum are conserved in a photon- particle collision. However, the number of photons may not be conserved in a collision, the photon may be absorbed or a new one may be created.
(b) Einstein’s photoelectric equation is,
\(K_{max} = hv - \phi_0\)i) Threshold frequency: since Kmax must be non-negative, the equation (i) suggest that the photoelectric emission is only possible if,
\(hv > \phi_0 or, v> v_0, \)\(where, v_0 = \frac{\phi_0}{h}\)
The above equation shows that the greater the work function, higher the threshold frequency required to emit photoelectrons.
Hence, there exists a threshold frequency below which no photoelectric emission is possible whatever be the intensity of the incident radiation or how long it falls on the surface.
ii) Stopping potential: The minimum value of the negative potential v0. That should be applied to anode in a photocell so that the photoelectric current is zero. This is called stopping potential.
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