Thermal and Chemical Effects of Electric Current
Questions for Short Answer
1. If a constant potential difference is applied across a bulb, the current slightly decreases as time passes and then becomes constant. Explain.
ANSWER: Initially the bulb filament is at room temperature. As a constant potential difference is applied across the bulb, a part of the electric energy is also converted into heat that raises the temperature of the filament. With the increase in temperature, the resistance of the filament also increases because the resistance is dependent on the temperature. Since the current i = V/R, the current decreases initially.
The heated filament also radiates the heat to the surrounding. After some time the heat radiated and the heat produced is balanced and the resistance of the filament stabilizes and so does the current.
2. Two unequal resistances R₁ and R₂ are connected across two identical batteries of emf ℇ and internal resistance r (figure 33-Q1). Can the thermal energies developed in R₁ and R₂ be equal in a given time. If yes, what will be the condition?
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| The figure for Q-1 |
ANSWER: For the first circuit, the resistance of the circuit =r+R₁,
Current in the circuit, i₁ =Є/(r+R₁)
Similarly the current in the other circuit, i₂ =Є/(r+R₂)
In a time interval t, the thermal energy developed in the first circuit =i₁²R₁t.
And in the other circuit =i₂²R₂t.
For the two thermal energy to be equal,
i₁²R₁t = i₂²R₂t
→Є²R₁/(r+R₁)² =Є²R₂/(r+R₂)²
→R₁r²+R₁R₂²+2rR₁R₂=r²R₂+R₁²R₂+2rR₁R₂
→r²(R₁ -R₂) -R₁R₂(R₁ -R₂) =0
→(R₁ -R₂)(r² -R₁R₂) =0
Since R₁ ≠R₂,
So, r² = R₁R₂,
→r =√(R₁R₂).
At this condition, the thermal energy developed in the two resistors will be equal in a given time interval.
3. When a current passes through a resistor, its temperature increases. Is it an adiabatic process?
ANSWER: In an adiabatic process, there is no exchange of mass and heat between the system and the surrounding. When a current is passed through a resistor, its temperature increases. Due to the temperature difference between the system (resistor) and the surrounding, there is an exchange of heat between them. Thus it is not an adiabatic process.
4. Apply the first law of thermodynamics to a resistor carrying a current i. Identify which of the quantities ΔQ, ΔU and ΔW are zero, which are positive and which are negative.
ANSWER: The first law of thermodynamics states that,
∆U =∆Q -∆W,
where ∆U is the change in the internal energy of the system, ∆Q is the heat added and ∆W is the work done by the system.
Our given system is only the resistor and it is not doing work. The potential difference across it does the work on it by pushing the charges (electrons), hence ∆W is negative. ∆U is positive because with the temperature increase the internal energy of the resistor will increase.
∆Q is the heat added, but no heat is being added here. So ∆Q is zero.
5. Do all the thermocouples have a neutral temperature?
ANSWER: No. There is no neutral temperature above 0°C for the thermocouple the constants a and b have the same sign.
When the cold junction is at 0°C, then the maximum thermo-emf occurs at a temperature θ for the hot junction if θ = -a/b. If the signs of a and b are the same then θ becomes negative which will be less than 0°C and that is not possible because θ is the temperature of the hotter junction.
6. Is inversion temperature always double the neutral temperature? Does the unit of temperature have an effect in deciding this question?
ANSWER: If measured in degree Celcius, the inversion temperature is always double the neutral temperature.
Yes, the above answer is dependent on the unit of temperature. If the unit is other than °C, for eg Kelvin or Fahrenheit then the statement is not true.
7. Is neutral temperature always the arithmetic mean of the inversion temperature and the temperature of the cold junction? Does the unit of temperature have an effect on deciding the question?
ANSWER: Yes, the neutral temperature is always the arithmetic mean of the inversion temperature and the temperature of the cold junction.
The unit of temperature has no effect on deciding the question. Let us see with an example. Suppose A and C are the temperature of the cold junction and the inversion temperature of the thermocouple respectively in °C. Hence the neutral temperature, N =(A+C)/2.
In Kelvin, the temperature is 273+A and 273+C. The neutral temperature now is,
={(273+A)+(273+C)}/2
=273+(A+C)/2
=273+N,
which is the neutral temperature in Kelvin.
Similarly, in Fahrenheit, the temperatures are (9A/5+32) and (9C/5+32) respectively. The neutral temperature now,
={(9A/5+32)+(9C/5+32)}/2
={9(A+C)/5 +2*32}/2
=(9/5)(A+C)/2 +32
=9N/5 +32
which is the neutral temperature in Fahrenheit.
So the neutral temperature is always the mean of the cold junction temperature and the inversion temperature.
8. Do the electrodes in an electrolytic cell have fixed polarity like a battery?
ANSWER: No, the electrodes in an electrolytic cell do not have fixed polarity like a battery. Consider an electrolytic cell made of electrolyte AgNO₃ and Ag electrodes. At the electrode connected to the negative terminal of the battery (Cathode), Ag from the electrolyte is deposited while at the positive connected electrode (Anode) Ag is dissolved in the electrolyte. If we reverse the polarity of the electrodes, the electrolytic cell still works The only difference is that the deposition and dissolution of Ag are reversed.
9. As temperature increases, the viscosity of liquids decreases considerably. Will this decrease the resistance of an electrolyte as the temperature increases?
ANSWER: The charge in an electrolyte is carried by ions and due to the viscosity the ions get resistance in movement. With temperature increase the viscosity decreases and ions move more freely, thus the resistance of the electrolyte decreases.
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Links to the Chapters
Links to the Chapters
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
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Click here for → Exercises (11-20)
CHAPTER- 7 - Circular Motion
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Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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CHAPTER- 6 - Friction
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Click here for → Friction - OBJECTIVE-II
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CHAPTER- 5 - Newton's Laws of Motion
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Click here for → QUESTIONS FOR SHORT ANSWER
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Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
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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"
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Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
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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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