How Heat Transfer Reaches Equilibrium

Quick Answer

Heat transfer always moves energy from the hotter object to the colder one until both reach the same temperature, a state called thermal equilibrium. For example, drop an ice cube into a warm drink and heat flows from the drink into the ice until they meet in the middle. In the simulator, two blocks exchange heat by conduction: set each block's temperature, press play, and watch the heat flow shrink as the gap closes. Nothing you do leaves your browser.

Which Way Does Heat Flow?

Heat flows from hot to cold, never the other way on its own. This one-way direction is the second law of thermodynamics in everyday form. Heat does not flow from cold to hot unaided: a fridge can move heat that way, but only by spending energy to do it. In the simulator the arrow of heat flow always points from the warmer block toward the cooler one. Swap which block is hotter and the flow reverses direction to match.

What Is Thermal Equilibrium?

Thermal equilibrium is the state where two objects sit at the same temperature and no net heat flows between them. To reach thermal equilibrium, leave the blocks in contact and let conduction do its work. When the two blocks have equal heat capacity, they settle at the average of their starting temperatures, because energy is conserved: the heat one block loses is exactly the heat the other gains. For example, a block at 80 degrees beside a block at 20 degrees settles near 50 degrees. Set a block temperature by dragging it or typing a value, then watch both readings converge.

Why the Heat Flow Slows Over Time

The rate of heat flow depends on the temperature difference between the blocks. Early on the gap is large, so heat moves fast; as the blocks approach each other the gap shrinks and the flow eases off. That is why the heat flow slows down over time and the temperature curve flattens into exponential cooling. For example, a hot drink cools quickly in its first minutes and then far more slowly as it nears room temperature. The blocks technically only reach equilibrium in the limit, but they get close fast.

High vs Low Thermal Conductance

Thermal conductance sets how quickly heat crosses between the blocks. High conductance is like a metal bridge: heat rushes across and equilibrium arrives sooner. Low conductance is like insulation: the same heat still moves, just slowly. The key point is that conductance changes the speed, not the final temperature. For example, wrapping a drink in a flask slows the cooling but does not change the temperature it eventually reaches. This is also the difference between heat and temperature: heat is the energy in transit, while temperature is how hot each block reads.

Common Mistakes