HC Verma Questions and Solutions: Chapter 41: Electric Current through Gases- 1 | HC Verma Solutions - JEE PDF Download

Cathode rays consist of a stream of fast moving electrons.

Cathode rays consist of beams of electrons that constitute current and, hence, magnetic field. We know electric field is produced by a charge, whether it is stationary or moving, So, electric field will also be present inside the tube.

Since the thermionic current is directly proportional to the square of the temperature of the surface emitting thermions, the graph is parabolic.

Saturated current is the maximum current beyond which, there is no effect of plate voltage (V_P) on the plate current (I_P). At this stage, dynamic resistance is infinite.

If the anode is given a negative potential relative to the cathode, the electrons are pushed back to the cathode. Hence, no current will flow through the diode.

The number of current pulses is equal to the frequency of the AC source because one current pulse passes through the diode for one oscillation of the AC source.

Dynamic resistance (r_P) is given by,

The triode is operating in the linear region. Therefore,
Change in the value of voltage = Change in the value of current
So, (r_P) will remain almost the same.

If the grid voltage is made positive, it will help the electrons move towards the anode, which will help in increasing the current. Thus, the plate current in the triode value is maximum when the potential of the grid is positive.

When the triode operating in the linear region, value of plate voltage, grid voltage and plate current is already specified (or fixed). If the grid is near the cathode, then we assume that grid lie in the region where the space charge density is more. There is a chance that grid will pick up electrons from the space charge region of cathode and effect the plate current and hence amplification factor (as we know that amplification factor will depend on the plate current). If the grid is far from the cathode, it will reduce the grid voltage and hence the amplification factor also get affected.

Some ions are always present in gases due to cosmic rays and other factors. When we apply potential difference  across a discharge tube, the ions get accelerated due to the electric field. If the potential difference is large, then the ions get enough energy to ionise the molecules on collision.
Thus, a large number of ions are produced and conduction starts. Generally, an electron gets detached from a molecule to make the molecule a positive ion. At low pressure, this electron moves through a large distance and gets attached to another molecule and forms a negative ion.
Thus, electric conduction takes place in a discharge tube due to the movement of positive ions, negative ions and electrons.

A cathode ray travels in a straight line. When cathode rays strike a solid metal, X-rays are emitted from the metal. When a cathode ray tube is brought in a magnetic field, the path of cathode rays gets deflected. Cathode rays are the matter waves that consists of electrons thus they are not electromagnetic waves.

A diode valve consists of a negatively-charged region between the cathode and anode, called the space charge region. This negatively-charged region repels the electrons coming from the cathode and, thus, it reduces the plate current. On the other hand, this region has no effect on plate voltage, rate of emission of thermions and the saturation current.

Since the triode value consists of a space charge region near the cathode, this space charge region is affected by the grid voltage. If the grid voltage is made negative, it repels the electrons coming from the cathode and reduces current. If this voltage is made positive, it helps the electrons move towards the anode. Hence, this voltage is more effective in changing the plate current. Since, the saturated current in the triode is the maximum value of plate current, the saturated current in the triode valve is effectively controlled by the grid voltage. More negative is the grid voltage, less is the value of saturated current.

A diode valve and a triode valve allow current to flow only in one direction. Since a rectifier is a device that converts alternating current (bi-directional) into direct current (uni-directional), a diode valve and a triode valve can be used as rectifiers.
A triode valve can control its output in proportion to the input signal. That is, it can act as an amplifier, whereas a diode valve cannot control its output in proportion to the input signal. So, it cannot be use as an amplifier.

The plate current varies directly with the plate voltage. Therefore, if the plate voltage is zero, the plate current is also zero. Due to the same reason, if the plate voltage is negative, the plate current will be zero. Now, if the plate is slightly positive, then it may be the reason that the plate voltage is not able to reduce the effect of space charge. Hence, the current may be zero. Now, as the temperature of the filament is low, it will not be able to emit electrons and the resulting plate current will be zero. Hence, all the options are possible.

If the temperature of the filament is high, then it will emit electrons. Other conditions for the  plate current in the diode are:
(1) Positive grid voltage, V_g
(2) Positive plate voltage, V_P
If any of the above conditions is not satisfied, then plate current in a triode will be zero.