When ice melts, its temperature does not rise and hence its kinetic energy remains constant. The amount of energy taken by the ice is used in deriving the molecules of ice apart and it is stored in the form of potential energy. Thus, heat can be said as a form of potential energy.
The ordinary steam engines do nit satisfy the ideal conditions stated in carnot engine namely
1) Water does not absorb heat at constant temperature.
2) Condensation of steam takes place before adiabatic expansion is completed.
3) The valves of cylinder are not opened at right time.
4) Some heat escapes through conduction.
Thus efficiency of steam engines is low.
A refrigerator is a heat engine which works in opposite to other heat engines like petrol engine and diesel engine. Thus, a refrigerator extracts heat from the freezing chamber, some work is done on it by electric motor and rejects heat into the surrounding air, thus warming the air. Therefore if the door of a running refrigerator is left open, the room will not be cooled down but instead heated slightly.
The elasticity of steel is more than that of copper. Therefore, for the same applied force, the elongation of steel spring is less than that of copper for same initial length. Therefore the steel springs may bear the higher tension before the elastic limit is crossed. That is why spring is made of steel.
Engineering