Question

various heat sources

Answer 1

Heat is transferred in various ways like through friction, conduction, and convection. Some of the various sources of heat are from the sun to the environment, through electricity like the iron, and by nuclear reaction.

Answer 2

the various heat sources are 
i) the sun  
ii) burning things 
iii) most fissionable material 
iv) electric current through resistive wire
v) any exothermic chemical reaction 
vi) physical work like friction