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A discharge tube contains helium at a low pressure. A large potential difference is applied across the tube. Consider a helium atom that has just been ionized due to the detachment of an atomic electron. Find the ratio of the distance travelled by the free electron to that by the positive ion in a short time dt after the ionization.
A molecule of a gas, filled in a discharge tube, gets ionized when an electron is detached from it. An electric field of 5.0 kV m–1 exists in the vicinity of the event.
(a) Find the distance travelled by the free electron in 1 μs assuming no collision.
(b) If the mean free path of the electron is 1.0 mm, estimate the time of transit of the free electron between successive collisions
The mean free path of electrons in the gas in a discharge tube is inversely proportional to the pressure inside it. The Crookes dark space occupies half the length of the discharge tube when the pressure is 0.02 mm of mercury. Estimate the pressure at which the dark space will fill the whole tube.
Two discharge tubes have identical material structure and the same gas is filled in them. The length of one tube is 10 cm and that of the other tube is 20 cm. Sparking starts in both the tubes when the potential difference between the cathode and the anode is 100 V. If the pressure in the shorter tube is 1.0 mm of mercury, what is the pressure in the longer tube?
The saturation current from a thoriated-tungsten cathode at 2000 K is 100 mA. What will be the saturation current for a pure-tungsten cathode of the same surface area operating at the same temperature? The constant A in the Richardson-Dushman equation is 60 × 104 A m-2K-2 pure tungsten and 3.0 × 104 A m-2K-2 for thoriated tungsten. The work function of pure tungsten is 4.5 eV and that of thoriated tungsten is 2.6 eV.
The constant A in the Richardson-Dushman equation of tungsten is 60 × 104 A m-2 K-2. The work function surface area 2.0 × 10–5 m2 is heated by a 24 W electric 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 constant =6 × 10-8 W m-2 K-4. Assume that the thermions take only a small fraction of the heat supplied.
The plate resistance of a triode is 8 kΩ and the transconductance is 2.5 millimho.
(a) If the plate voltage in increased by 48 V, and the grid voltage is kept constant, what will be the increase in the plate current?
(b) With plate voltage kept constant at this increased value, how much should the grid voltage be decreased in order to bring the plate current back to its initial value?
The plate resistance and the amplification factor of a triode are 10 kΩ and 20. The tube is operated at plate voltage 250 V and grid voltage –7.5 V. The plate current is 10 mA
(a) To what value should the grid voltage be changed so as to increase the plate current to 15 mA?
(b) To what value should the plate voltage be changed to take the plate current back to 10 mA?
The plate current, plate voltage and grid voltage of a 6F6 triode tube are related as
Ip = 41(Vp + 7 Vg) 1.41, Where Vp and Vg are in volts and ip in microamperes. The tube is operated at Vp = 250 V, Vg = –20 V. Calculate
(a) the tube current,
(b) the plate resistance,
(c) the mutual conductance and
(d) the amplification factor.
Figure shows two identical triode tubes connected in parallel. The anodes are connected together, the grids are connected together and the cathodes are connected together. Show that the equivalent plate resistance is half to the individual plate resistance. The equivalent mutual conductance is double the individual mutual conductance and the equivalent amplification factor is the same as the individual amplification factor