Answer:
(a) 
(b) 
Explanation:
(a) The surface current density of a conductor is the current flowing per unit length of the conductor. 
                                    
Considering a wire, the current is uniformly distributed over the circumferenece of the wire.
                                    
The radius of the wire = a
                                     
 The surface current density 
(b) The current density is inversely proportional
                                      
    
                                      
           ......(1)
k is the constant of proportionality
                                      
                                      
     ........(2)
substituting (1) into (2)
                                      
                                      
                                      
                                      
                                      
substitute 
                                      
 
        
             
        
        
        
A boiling pot of water (the water travels in a current throughout the pot), a hot air balloon (hot air rises, making the balloon rise) , and cup of a steaming, hot liquid (hot air rises, creating steam) are all situations where convection occurs. 
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I don’t even know I’m so dumb.
        
             
        
        
        
 Wound it be one that dissolves ?
        
             
        
        
        
Electrical energy.................