41: Electric Current Through Gases : Cbse-xi-physics

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Qstn #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?
Ans : 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.
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Qstn #2
An AC source is connected to a diode and a resistor in series. Is the current thorough the resistor AC or DC?
Ans : 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.
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Qstn #3
How will the thermionic current vary if the filament current is increased?
Ans : 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.
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Qstn #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?
Ans : 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.
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Qstn #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?
Ans : 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.
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Qstn #6
An isolated metal sphere is heated to a high temperature. Will it become positively charged due to thermionic emission?
Ans : 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.
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Qstn #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?
Ans : 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.
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Qstn #8
Why does thermionic emission not take place in non-conductors?
Ans : 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.
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Qstn #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?
Ans : 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.
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Qstn #10
Why is the linear portion of the triode characteristic chosen to operate the triode as an amplifier?
Ans : 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.
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