Final answer:
Good thermal contact with liquid nitrogen maintains objects at 77 K due to the process where the nitrogen evaporates as it absorbs heat, cooling the object. This effective cooling method is largely because the phase change from liquid to gas requires significant energy, thereby extracting heat from the object. Therefore, the correct option is c) Nitrogen evaporates, cooling the object.
Step-by-step explanation:
Understanding Thermal Contact with Liquid Nitrogen
When objects are in good thermal contact with liquid nitrogen, they can be maintained at a steady temperature of 77 K, which is the boiling point of liquid nitrogen at atmospheric pressure. This process of temperature maintenance involves several phenomena:
• Heat absorption: As the object in contact with the liquid nitrogen is likely at a higher temperature, the nitrogen absorbs heat from the object.
• Constant temperature source: Liquid nitrogen acts as a constant temperature source by maintaining its temperature at 77 K while it remains in its liquid state.
• Evaporation cooling: The liquid nitrogen evaporates as it absorbs heat, which cools the object. This phenomenon is based on the endothermic process of phase change from liquid to gas.
• Heat conduction: Nitrogen, though not the best conductor, can transfer heat away from the object to some extent.
However, the most critical process that allows liquid nitrogen to keep objects at a constant temperature is the evaporation cooling effect. As the nitrogen absorbs heat, it transitions from liquid to gas, a process that necessarily consumes energy - thereby lowering the temperature of the object in contact with it.
The mention of liquid nitrogen as a coolant in various contexts highlights its importance. For example, in the field of superconductivity, materials can become superconducting at temperatures close to 90 K, which are achievable using liquid nitrogen. This provides an inexpensive cooling solution compared to liquid helium. Concerning the question, the correct option that explains how good thermal contact with liquid nitrogen can keep objects at 77 K is: (c) Nitrogen evaporates, cooling the object.