Understanding Sensible Heat

Sensible heat is a fundamental concept in thermodynamics and heat transfer. It refers to the heat energy that, when added or removed from a substance, causes a change in its temperature without changing its phase (like solid to liquid or liquid to gas). The term "sensible" comes from the fact that this type of heat transfer results in a noticeable, or "sensible," change in temperature.

Analyzing the Options for Sensible Heat

Let's examine each statement provided in the options:

• Option 1: "It is the amount of energy given to a system to increase its enthalpy."

  Enthalpy is a measure of the total energy of a thermodynamic system. While adding sensible heat does increase the internal energy part of enthalpy (related to temperature), increasing enthalpy can also involve latent heat (during phase change) or work. This statement is too broad and not a specific definition of sensible heat.

• Option 2: "It is the molecular energy of a system stored in it because of random motion of the molecules of the system."

  This describes the internal energy of a system related to its temperature. Sensible heat is the energy transfer that changes this internal energy, and thus temperature, but it is not the stored energy itself. This is a description of a state property (internal energy), not the energy transfer process (sensible heat).

• Option 3: "It is the amount of energy needed to increase or decrease the temperature of refrigerant."

  This statement accurately defines sensible heat, specifically applied to a refrigerant. Sensible heat is precisely the heat added or removed to change the temperature of a substance (like a refrigerant) while it remains in the same phase. Increasing temperature requires adding sensible heat, and decreasing temperature requires removing sensible heat.

• Option 4: "It is the amount of energy required to convert liquid refrigerant into vapour refrigerant."

  This statement describes latent heat, specifically the latent heat of vaporization. Latent heat is the energy absorbed or released during a phase transition at a constant temperature. It is distinct from sensible heat, which causes temperature change.

Based on the definitions, the statement that correctly describes sensible heat, especially in the context of refrigerants often discussed in refrigeration cycles, is the one referring to the energy needed to change temperature.

Correct Statement on Sensible Heat

The correct statement about sensible heat is that it is the amount of energy needed to increase or decrease the temperature of a substance, such as a refrigerant, without causing a phase change. This is directly captured by option 3.