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

Home Learn Fast select Course

Change Unit Change Subject Last Menu

CBSE-XI-Physics

41: Electric Current through Gases

with Solutions - page 2

Note: Please signup/signin free to get personalized experience.

10 minutes can boost your percentage by 10%

Note: Please signup/signin free to get personalized experience.

Qstn #3
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)?
Ans : (d)
Since the thermionic current is directly proportional to the square of the temperature of the surface emitting thermions, the graph is parabolic.
Page No 352:

Qstn #4
When the diode shows saturated current, dynamic place resistance is
(a) zero
(b) infinity
(c) indeterminate
(d) different for different diodes
digAnsr: b
Ans : (b) infinity
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.
Page No 352:

Qstn #5
The anode of a thermionic diode is connected to the negative terminal of a battery and the cathode to its positive terminal.
(a) No appreciable current will pass through the diode.
(b) A large current will pass through the diode from the anode to the cathode.
(c) A large current will pass through the diode from the cathode to the anode.
(d) The diode will be damaged.
digAnsr: a
Ans : (a) 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.
Page No 352:

Qstn #6
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) 25
(b) 50
(c) 100
(d) 200.
digAnsr: b
Ans : (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.
Page No 352:

Qstn #7
A triode is operated in the linear region of its characteristics. If the plate voltage is slightly increased, the dynamic plate resistance will
(a) increase
(b) decrease
(c) remain almost the same
(d) become zero
digAnsr: c
Ans : (c) remain almost the same
Dynamic resistance (r_P) is given by,
r_P = `` \frac{∆{V}_{P}}{∆{i}_{P}}``
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.
Page No 352:

Qstn #8
The plate current in a triode value is maximum when the potential of the grid is
(a) positive
(b) zero
(c) negative
(d) non-positive
digAnsr: a
Ans : (a) 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.
Page No 352:

Qstn #9
The amplification factor of a triode operating in the linear region depends strongly on
(a) the temperature of the cathode
(b) the plate potential
(c) the grid potential
(d) the separations of the grid from the cathode and the anode
digAnsr: d
Ans : (d) 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.
Page No 352:

#
Section : iii

Qstn #1
Electric conduction takes place in a discharge tube due to the movement of
(a) positive ions
(b) negative ions
(c) electrons
(d) protons
digAnsr: a,b,c
Ans : (a) positive ions
(b) negative ions
(c) electrons
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.
Page No 352:

Qstn #2
Which of the following is true for a cathode ray?
(a) It travels in a straight line.
(b) It emits an X-ray when it strikes a metal.
(c) It is an electromagnetic wave.
(d) It is not deflected by a magnetic field.
digAnsr: a,b
Ans : (a) It travels in a straight line.
(b) It emits an X-ray when it strikes a metal.
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.
Page No 352:

Qstn #3
Due to the space charge in a diode valve,
(a) the plate current decreases
(b) the plate voltage increases
(c) the rate of emission of thermions increases
(d) the saturation current increases
digAnsr: a
Ans : (a) the plate current decreases
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.
Page No 352:

Qstn #4
The saturation current in a triode valve can be changed by changing
(a) the grid voltage
(b) the plate voltage
(c) the separation between the grid and the cathode
(d) the temperature of the cathode
digAnsr: a
Ans : (a) the grid voltage
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.
Page No 352:

Qstn #5
Mark the correct options.
(a) A diode valve can be used as a rectifier.
(b) A triode valve can be used as a rectifier.
(c) A diode valve can be used as an amplifier.
(d) A triode valve can be used as an amplifier.
digAnsr: a,b,d
Ans : (a) A diode valve can be used as a rectifier.
(b) A triode valve can be used as a rectifier.
(d) A triode valve can be used as an amplifier.
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.
Page No 352:

Qstn #6
The plate current in a diode is zero. It is possible that
(a) the plate voltage is zero
(b) the plate voltage is slightly negative
(c) the plate voltage is slightly positive
(d) the temperature of the filament is low
digAnsr: a,b,c,d
Ans : (a) the plate voltage is zero
(b) the plate voltage is slightly negative
(c) the plate voltage is slightly positive
(d) the temperature of the filament is low
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.
Page No 352:

Qstn #7
The plate current in a triode valve is zero. The temperature of the filament is high. It is possible that
(a) V_g > 0, V_p > 0
(b) V_g > 0, V_p < 0
(c) V_g < 0, V_p > 0
(d) V_g < 0, V_p < 0
digAnsr: b,c,d
Ans : (b) V_g > 0, V_p < 0
(c) V_g < 0, V_p > 0
(d) V_g < 0, V_p < 0
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.
Page No 352: