Why is conduction easier in gases if the pressure is low? Will the conduction continue to improve if the pressure is made as low as nearly zero?

An AC source is connected to a diode and a resistor in series. Is the current through the resistor AC or DC?

How will the thermionic current vary if the filament current is increased?

Would you prefer a material having a high melting point or a low melting point to be used as a cathode in a diode?

Would you prefer a material having 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?

A diode valve is connected to a battery and a load resistance. The filament is heated so that a constant current is obtained in the circuit. As the cathode continuously emits electrons, does it get more and more positively charged?

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 plate current decrease, increase or remain the same?

Why is the linear portion of the triode characteristic chosen to operate the triode as an amplifier?

Cathode rays constitute a stream of

Cathode rays are passing through a discharge tube, in tube, there is

Let i0 be the thermionic current from a metal surface when the absolute temperature of the surface is T0. The temperature is slowly increased and the thermionic current is measured as a function of temperature. Which of the following plots may represent the variation in (i/i0) against (T/T0)?

When the diode shows saturated current, dynamic plate resistance is

The anode of a thermionic diode is connected to the negative terminal of a battery and the cathode to its positive terminal.

A diode, a resistor and a 50 Hz AC source are connected in series. The number of current pulses per second through the resistor is

A triode is operated in the linear region of its characteristics. If the plate voltage is slightly increased, the dynamic plate resistance will

The plate current in a triode valve is maximum when the potential of the grid is

The amplification factor of a triode operating in the linear region depends strongly on

Electric conduction takes place in a discharge tube due to the movement of

Which of the following are true for cathode ray?

Because of the space charge in a diode valve,

The saturation current in a triode valve can be changed by changing

Mark the correct options.

The plate current in a diode is zero. It is possible that

The plate current in a triode valve is zero. The temperature of the filament is high. It is possible that

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?

Calculate n(T)/n (1000 K) for tungsten emitter at T = 300 K, 2000 K and 3000 K where n(T) represents the number of thermions emitted per second by the surface at temperature T. Work function of tungsten is 4.52 eV.

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.

A tungsten cathode and a thoriated-tungsten cathode have the same geometrical dimensions and are operated at the same temperature. The thoriated-tungsten cathode gives 5000 times more current than the other one. Find the operating temperature. Take relevant data from the previous problem.

If the temperature of a tungsten filament is raised from 2000 K to 2010 K, by what factor does the emission current change? Work function of tungsten is 4.5 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 current in a diode is 20 mA when the plate voltage is 50 V or 60 V. What will be the current if the plate voltage is 70V?

The power delivered in the plate circuit of a diode is 1.0 W when the plate voltage is 36 V. Find the power delivered if the plate voltage is increased to 49 V. Assume Langmuir – Child equation to hold.

A triode valve operates at V_p = 225 V and V_g = –0.5 V. The plate current remains unchanged if the plate voltage is increased to 250 V and the grid voltage is decreased to –2.5 V. Calculate the amplification factor.

Calculate the amplification factor of a triode valve which has plate resistance of 2 kΩ and transconductance of 2 millimho.

The dynamic plate resistance of a triode valve is 10 kΩ. Find the change in the plate current if the plate voltage is changed from 200 V to 220 V.

Find the values of r_p, μ and g_m of a triode operating at plate voltage 200 V and grid voltage –6 V. The plate characteristics are shown in figure.

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

I_p = 41(V_p + 7 V_g) ^1.41, Where V_p and V_g are in volts and ip in microamperes. The tube is operated at V_p = 250 V, V_g = –20 V. Calculate

(a) the tube current,

(b) the plate resistance,

(c) the mutual conductance and

(d) the amplification factor.

The plate current in a triode can be written as

Show that the mutual conductance is proportional to the cube root of the plate current.

A triode has mutual conductance = 2.0 millimho and plate resistance = 20 kΩ. It is desired to amplify a signal by a factor of 30. What load resistance should be added in the circuit?

The gain factor of an amplifier is increased from 10 to 12 as the load resistance is changed from 4 kΩ to 8 KΩ. Calculate (a) the amplification factor and (b) the plate resistance.

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