In an ionically bonded solid the force between two ions is given by where r is the distance between the ions, b and c are constants where b = 1.2x10-28 N·m2 and c = 1.8x10-105 N·m10 . (a) Identify the attractive and repulsive terms in the expression.
(b) Determine the ion separation in Ã… in the solid.
(c) Determine the energy in eV required to separate the ions in the solid to an infinite distance apart.
(d) Sketch E versus r curve for the solid. Label the equilibrium separation and energy minima.
(e) What physical properties would you measure in order to check your answers?

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?