Electric Current Through Gases
Questions for Short Answer
1. 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?
ANSWER: When the pressure is low, the density of a gas is also low. So the free-conducting electrons move longer before colliding with another molecule. Thus they face less resistance to moving. So the conduction is easier at low pressure.
The conduction does not improve if the pressure is made as low as zero because there are very few molecules to get ionized. Also, the electrons emitted from the cathode have a mean free path greater than the length of the tube and they strike the walls of the tube, and the conduction stops.
2. An AC source is connected to a diode and a resistor in series. Is the current through the resistor AC or DC?
ANSWER: Since a diode allows current to flow only in one direction through it, the current through the resistor will be DC. But for each AC cycle, the DC current in the resistor will be only half the cycle, and the other half cycle will have zero current (when the current in AC is in the reverse direction).
3. How will the thermionic current vary if the filament current is increased?
ANSWER: If the filament current is increased, the temperature of the cathode will increase resulting in more emission of electrons. This will result in an increase in thermionic current.
4. Would you prefer a material having a high melting point or a low melting point to be used as a cathode in a diode?
ANSWER: Since the thermionic emission from a diode occurs at high temperatures, a high melting point material with good conductance will be preferred.
5. Would you prefer a material having a high work function or a low work function to be used as a cathode in a diode?
ANSWER: Since work function is the amount of energy used to detach an electron from an atom, a material with a low work function will be preferred so that the electrons needed for the current in a diode are available at the low expense of energy.
6. An isolated metal sphere is heated to a high temperature. Will it become positively charged due to thermionic emission?
ANSWER: Since the metal sphere is isolated, due to the thermionic emission of electrons the metal sphere will have a net positive charge remaining. So it will become positively charged.
7. 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?
ANSWER: No. Since a constant current flows in the circuit, there is a continuous supply of electrons from the negative terminal of the battery/source to the cathode. So it does not get more and more positively charged.
8. Why does thermionic emission not take place in nonconductors?
ANSWER: For thermionic emission, a material should have a low work function so that free electrons are easily available. Nonconductors have very high work functions and electrons are not easily detached from the atoms. So the thermionic emission does not take place.
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 plate current decrease, increase, or remain the same?
ANSWER: According to the Richardson-Dushman equation, the number of thermionic electrons ejected by a metal is proportional to its surface area. Since the cathode of a diode valve emits thermions, doubling its surface area keeping the temperature and other conditions the same, double the thermions will be ejected. So the plate current will increase.
10. Why is the linear portion of the triode characteristic chosen to operate the triode as an amplifier?
ANSWER: In the linear portion of the triode characteristic, the change in the voltage across the load resistance is proportional to the input signal but with a much higher amplitude. This quite amplifies the input signal. That is why the linear portion of the triode characteristic is chosen for the operation of the amplifier.
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Links to the Chapters
Links to the Chapters
CHAPTER- 40- Electromagnetic WavesCHAPTER- 39- Alternating CurrentCHAPTER- 38- Electromagnetic Induction
CHAPTER- 37- Magnetic Properties of MatterCHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 40- Electromagnetic Waves
CHAPTER- 39- Alternating Current
CHAPTER- 38- Electromagnetic Induction
CHAPTER- 37- Magnetic Properties of Matter
CHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 32- Electric Current in ConductorsCHAPTER- 31- CapacitorsCHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
OBJECTIVE -I
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-11 to Q-18
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 32- Electric Current in Conductors
CHAPTER- 31- Capacitors
CHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Exercises (11-20)
CHAPTER- 7 - Circular Motion
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → Questions for Short Answer
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
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CHAPTER- 5 - Newton's Laws of Motion
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
Click here for "The Forces" - OBJECTIVE-I
Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
Click here for "Questions for short Answers"
Click here for "OBJECTIVE-I"
Click here for EXERCISES (Question number 1 to 10)
Click here for EXERCISES (Question number 11 to 20)
Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Physics and Mathematics"
CHAPTER- 2 - "Physics and Mathematics"
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