How does a hot cup of tea lose heat ?

Thermodynamics is the field that studies energy transformations that occur in matter.

A cup of hot tea is actually an open system in which energy can be transferred from the hot tea to the surroundings. The molecules in the hot tea are moving faster than those in the surrounding environment and there will be a transfer of heat to the surroundings due to convection. If the tea is very hot, the air above it gains heat energy and the molecules move apart from each other causing them to become less dense and they rise. Eventually, as heat dissipates to the surroundings, the molecules of the atmospheric gases move closer together, become more dense and will sink. The process of convection occurs when matter is in the liquid or gaseous states.

Conduction will occur if someone places their hand on the hot tea cup. The higher temperature of the cup of hot tea will transfer heat by conduction to the cooler object--the person's hand when the faster moving molecules transfer energy to the slower moving molecules.

Therefore, both convection and conduction cause the hot tea to lose heat energy to the surrounding environment.