Advertisements
Advertisements
Question
Why should gases be insulators at ordinary pressures and start conducting at very low pressures?
Advertisements
Solution
At atmospheric pressure, the ions of gases have no chance of reaching their respective electrons because of collision and recombination with other gas molecules. Hence, gases are insulators at atmospheric pressure. At low pressures, ions have a chance of reaching their respective electrodes and constitute a current. Hence, they conduct electricity at these pressures.
RELATED QUESTIONS
Find the maximum frequency of X-rays produced by 30 kV electrons.
Quarks inside protons and neutrons are thought to carry fractional charges [(+2/3)e; (–1/3)e]. Why do they not show up in Millikan’s oil-drop experiment?
Would you prefer a material with a high work-function or a low work-function to be used as a cathode in a diode?
An isolated metal sphere is heated to a high temperature. Will it become positively charged due to thermionic emission?
Why does thermionic emission not take place in non-conductors?
The cathode of a diode valve is replaced by another cathode of double the surface area. Keeping the voltage and temperature conditions the same, will the place current decrease, increase or remain the same?
The anode of a thermionic diode is connected to the negative terminal of a battery and the cathode to its positive terminal.
The constant A in the Richardson−Dushman equation for tungsten is 60 × 104 A m−2K−2. The work function of tungsten is 4.5 eV. A tungsten cathode with a surface area 2.0 × 10−5 m2 is heated by a 24 W electric heater. In steady state, the heat radiated by the heater and the cathode equals the energy input by the heater and the temperature becomes constant. Assuming that the cathode radiates like a blackbody, calculate the saturation current due to thermions. Take Stefan's Constant = 6 × 10−8 W m−2 K−1. Assume that the thermions take only a small fraction of the heat supplied.
The work function of aluminum 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.
If a light of wavelength 330 nm is incident on a metal with work function 3.55 eV, the electrons are emitted. Then the wavelength of the wave associated with the emitted electron is (Take h = 6.6 × 10–34 Js)
Emission of electrons by the absorption of heat energy is called ____________ emission.
Why do metals have a large number of free electrons?
A 150 W lamp emits light of the mean wavelength of 5500 Å. If the efficiency is 12%, find out the number of photons emitted by the lamp in one second.
In which case is electron emission from a metal not known?
Photoelectric emission is observed from a metallic surface for frequencies ν1 and ν2 of the incident light (ν1 > ν2). If the maximum value of kinetic energy of the photoelectrons emitted in the two cases are in the ration 1 : n then the threshold frequency of the metallic surface is ______.
Consider Figure for photoemission.
How would you reconcile with momentum conservation? Note light (photons) have momentum in a different direction than the emitted electrons.
Name the factors on which photoelectric emission from a surface depends.
