Question 1

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

Accepted Answer

When electrons move through a gas, they collide with the gaseous particles and lose energy. This increases the resistance and, hence, reduces the current. But at low pressure, as the gas particle are widely spread, there are fewer collisions. So, the electrons can pass easily and with less collisions.If the pressure is reduced to nearly zero, the current through the gas will decrease. This is because the mean free-path of the electrons (distance that the electrons travel between collisions) is longer and they can, therefore, be accelerated to higher speeds before collision with an atom and they have more chance of causing ionisation.

Question 2

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

Accepted Answer

If the diode and resistor are in series, for the positive half cycle of AC, the current through the resistor will be DC. But for the next half cycle the current through the resistor will be zero. As a diode is a device that converts AC into DC, current through the resistor will be DC.

Question 3

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

Accepted Answer

If the filament current is increased, it will increase the temperature of the cathode (metal) and the cathode will emit more electrons. This will lead to an increase in the number of thermions emitted per unit time. As a result, the thermionic current will increase.

Question 4

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

Accepted Answer

We will prefer a material with high melting point to be used as the cathode in a diode. The material of the cathode of a diode should be resistant to high temperature, have high melting point and be electrically conductive because thermionic emission occurs at high temperature.

Question 5

Would you prefer a material with a high work-function or a low work-function to be used as a cathode in a diode?

Accepted Answer

We will prefer a material with low work-function to be used as a cathode in a diode, so that electron emission can occur using a small amount of energy.

Question 6

An isolated metal sphere is heated to a high temperature. Will it become positively charged due to thermionic emission?

Accepted Answer

Yes, it will become positively charged due to thermionic emission. When the metal sphere is heated, average kinetic energy of the electrons will increases, due to which, the free electrons of the metal sphere will be able to escape. As a result, they will leave a positive charge on the isolated metal sphere.

Question 7

A diode value 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 become more and more positively charged?

Accepted Answer

No, the cathode does not become more and more positively charged. In the diode circuit,  the cathode of the diode valve is always connected to the negative terminal of the battery. So, the battery will supply the electrons to the cathode and the cathode will continuously emit electrons without becoming more and more positively charged.

Question 8

Why does thermionic emission not take place in non-conductors?

Accepted Answer

For thermionic emission, material should have low work function and large number of free electrons. But nonconductor does not have free electrons and they have higher work function. So, thermionic emission does not takes place in non-conductors.

Question 9

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?

Accepted Answer

As the rate of emission of thermions is directly proportional to the surface area of the surface emitting it, the plate current doubles if the cathode of a diode valve is replaced by another cathode of double the surface area.Hence plate current increases.

Question 10

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

Accepted Answer

When the operating point lies on the linear portion of the characteristics curve, change in voltage across the load resistance follows the pattern of the input signal, but the amplitude is much larger. This can be done by choosing the linear portion of triode characteristic.

Question 11

Cathode rays constitute a stream of:

Accepted Answer

(a) electronsCathode rays consist of a stream of fast moving electrons.

Question 12

Cathode rays are passing through a discharge tube. In the tube, there is:

Accepted Answer

(c) an electric as well as a magnetic fieldCathode 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.

Question 13

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​)?

Accepted Answer

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

Question 14

When the diode shows saturated current, dynamic place resistance is?

Accepted Answer

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

Question 15

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

Accepted Answer

No appreciable current will pass through the diode. 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.

Question 16

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 __________ .

Accepted Answer

(b)50 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.

Question 17

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

Accepted Answer

(c) remain almost the sameDynamic resistance (rp​)  is given by,P​=Δip​ΔVp​​ The triode is operating in the linear region. Therefore, Change in the value of voltage = Change in the value of current So,  ((P​) will remain almost the same.

Question 18

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

Accepted Answer

positive 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.

Question 19

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

Accepted Answer

the separations of the grid from the cathode and the anode 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.

Question 20

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

Accepted Answer

(a) positive ions(b) negative ions,(c) electronsSome 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 jons and electrons.

Chapter 19 : Electric Current through Gases