Questions OBJECTIVE - II and Answer Thermal and Chemical Effects of Electric Current HC Verma Part II

Q#1.  

Two resistors having equal resistances are joined in series and a current is passed through the combination. Neglect any variation in resistance as the temperature changes. In a given time interval,

(a) equal amounts of thermal energy must be produced in the resistors

(b) unequal amounts of thermal energy may be produced

(c) the temperature must rise equally in the resistors 

(d) the temperature may rise equally in the resistors.          

Answer: (a), (d).       

Since both resistors are connected in series, the same current i passes through them. Suppose the resistance of each of them = R, then in time t, the thermal energy produced in each of them, H = i²Rt. Hence option (a) is true.  

The rise of temperature in resistors

∆T = H/ms, where m =mass of a resistor and s =specific heat of the material.  

The product 'ms' may or may not be the same for the resistors. Hence option (d) is true.        

Q# 2.  

A copper strip AB and an iron strip AC are joined at A. Junction A is maintained at 0°C and the free ends B and C are maintained at 100°C. There is a potential difference between

(a) the two ends of the copper strip

(b) the copper end and the iron end at the junction

(c) the two ends of the iron strip

(d) the free ends B and C.    

Answer: All.  

The two ends of the copper strip are at different temperatures, hence there will be emf between the two ends (Thomson effect). So there is a potential difference between the two ends of the copper strip. Option (a) is true. 

Though the temperature of the ends of copper and iron strips at the junction is the same, there will be a potential difference between these two ends of the metals due to the Peltier emf developed on account of the different natures of the metals. Option (b) is true. 

Option (c) is correct exactly due to the reason mentioned in explanation (a). 

The nature of the materials of strips AB and AC are different. Also, the lengths may be different. Hence the potential difference between A and B, (say α) and the potential difference between A and C (say ß) will be different. So there will be a potential difference between free ends B and C equal to α-ß. Option (d) is also correct.  

So, all options are correct.         

Q#3.  The constants a and b for the pair silver-lead are 2.50 µV/°C and 0.012 µV/(°C)² respectively. For a silver-lead thermocouple with a colder junction at 0°C,

(a) there will be no neutral temperature

(b) there will be no inversion temperature 

(c) there will not be any thermo-emf even if the junctions are kept at different temperatures

(d) there will be no current in the thermocouple even if the junctions are kept at different temperatures.  

Answer: (a), (b).  

Since the signs of the constants a and b are the same, there will be no neutral temperature or inversion temperature above 0°C for the silver-lead thermocouple. Options (a) and (b) are correct. 

The two junctions if kept at different temperatures will have different Peltier EMFs. Thus there will be a net thermo-emf and hence a current between the two junctions. So, options (c) and (d) are not correct.  

Q#4.  

An electrolysis experiment is stopped and the battery terminals are reversed.

(a) The electrolysis will stop

(b) The rate of liberation of material at the electrodes will increase

(c) The rate of liberation of material will remain the same.

(d) Heat will be produced at a greater rate.        

Answer: (c)  

With the reversal of the terminal, only the polarity of the terminals will change. The potential difference across the electrodes and the resistance of the electrolyte will remain the same. Since the current in the electrolyte is due to the movement of ions in the electrolyte, the movement of the ions will now be reversed. The rate of liberation of material will remain the same. Option (c) is correct.     

Q#5.  

The electrochemical equivalent of a material depends on  

(a) the nature of the material

(b) the current through the electrolyte containing the material

(c) the amount of charge passed through the electrolyte

(d) the amount of material present in the electrolyte.       

Answer: (a)  

The electrochemical equivalent of a material is the ratio of the relative atomic mass of the substance to its valency. Since the atomic mass and the valency of a substance is the nature of the substance, option (a) is correct.