Consider the following statements about free electrons in a metal and electron emission:
(A) The free electrons cannot normally escape from a metal, because the metal surface becomes positively charged and pulls them back.
(B) The work function of a metal depends on the properties of the metal but not on the nature of its surface.
(C) In thermionic emission, the free electrons get enough energy to come out when the metal is suitably heated.
(D) The negatively charged particles given out by metals in ultraviolet light have a larger e/m than cathode ray particles.
(E) One electron volt is the energy gained by an electron when it is accelerated through a potential difference of 1 volt.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
A free electron needs a minimum energy to leave the metal surface. This energy is called the work function. It can be given to the electron by heating, by a strong electric field or by light.
(A) The free electrons cannot normally escape from a metal, because the metal surface becomes positively charged and pulls them back. — Correct statement. An electron that tries to leave makes the surface positive. The pull of the ions then holds it inside the metal.
(B) The work function of a metal depends on the properties of the metal but not on the nature of its surface. — Incorrect statement. The work function depends on the metal and also on the nature of its surface. So two pieces of one metal with different surfaces can have different work functions.
(C) In thermionic emission, the free electrons get enough energy to come out when the metal is suitably heated. — Correct statement. Heating gives the free electrons enough thermal energy to leave the metal.
(D) The negatively charged particles given out by metals in ultraviolet light have a larger e/m than cathode ray particles. — Incorrect statement. These particles have the same e/m as cathode ray particles. This showed that both are the same particles, the electrons.
(E) One electron volt is the energy gained by an electron when it is accelerated through a potential difference of 1 volt. — Correct statement. This is the meaning of the unit electron volt, and \( 1\ \mathrm{eV}=1.602\times10^{-19}\ \mathrm{J} \).
NCERT (2026-27):
Electron Emission – “… If an electron attempts to come out of the metal, the metal surface acquires a positive charge and pulls the electron back to the metal. …”
Electron Emission – “… The work function (φ0) depends on the properties of the metal and the nature of its surface.”
Electron Emission – “(i) Thermionic emission: By suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal.”
Introduction – “… The value of e/m of these particles was found to be the same as that for cathode ray particles. …”
Electron Emission – “… One electron volt is the energy gained by an electron when it has been accelerated by a potential difference of 1 volt, so that 1 eV = 1.602 ×10–19 J.”