Thermal Contact and Heat Flow Principle

When two objects are placed in thermal contact, they can exchange internal energy in the form of heat. This energy transfer, or heat flow, occurs spontaneously between them.

Condition for Heat Flow

• Heat naturally flows from a region of higher thermal energy to a region of lower thermal energy.
• This difference in thermal energy is directly related to the temperature of the objects.
• Therefore, heat flow between two bodies in thermal contact happens if and only if they have different temperatures.

Why Other Options Are Incorrect

• Mass: While mass affects how much heat is needed to change temperature (specific heat), it doesn't directly cause heat flow itself.
• Density: Density is mass per unit volume and is not the primary driver for heat transfer between objects in contact.
• Specific Heat: Specific heat capacity relates to how much energy is required to raise the temperature of a substance. It influences the rate of temperature change but doesn't initiate heat flow.

Conclusion

The fundamental principle governing heat transfer between objects in thermal contact is the difference in their thermal energy levels, which is measured by temperature. Heat will flow until thermal equilibrium is reached (i.e., when their temperatures are equal).