Understand that thermal energy is transferred from a region of higher temperature to a region of lower temperature until the two regions reach the same temperature (thermal equilibrium). 🔥❄️
When two objects are in contact, heat flows from the hotter object to the cooler one. Once both objects have the same temperature, no net heat flows – this is thermal equilibrium.
Heat transfer occurs through three mechanisms:
In all cases, energy moves from the higher temperature region to the lower temperature region until equilibrium is reached.
Imagine a cup of hot chocolate in a cool room. The chocolate’s molecules vibrate faster (hot) than the room’s air (cold). Heat flows from the chocolate to the air, cooling the chocolate and warming the air until both reach the same temperature. ☕️➡️🌬️
Heat transferred in a process:
\$Q = mc\Delta T\$
Where \$m\$ = mass, \$c\$ = specific heat capacity, and \$\Delta T\$ = change in temperature.
| Object | Temperature (°C) | Heat Flow |
|---|---|---|
| Object A | 70 | → Object B |
| Object B | 20 | ← Object A |
Tip: When given a question about heat transfer, first identify the direction of heat flow (hot → cold). Then decide whether conduction, convection, or radiation is relevant. Remember that at thermal equilibrium, the temperature difference is zero, so no net heat flows.
Two metal blocks, A and B, are brought into contact. Block A has a mass of 0.5 kg and a temperature of 80 °C. Block B has a mass of 1.0 kg and a temperature of 20 °C. Both have the same specific heat capacity of 900 J kg⁻¹ K⁻¹. What will be the final equilibrium temperature? Show your calculation.
Answer:
Let \$T_f\$ be the final temperature. Energy conservation:
\$mAc(TA - Tf) = mBc(Tf - TB)\$
Substitute values:
\$0.5 \times 900 (80 - Tf) = 1.0 \times 900 (Tf - 20)\$
Simplify:
\$450(80 - Tf) = 900(Tf - 20)\$
\$36000 - 450Tf = 900Tf - 18000\$
\$54000 = 1350T_f\$
\$T_f = 40 °C\$
So the blocks reach 40 °C at equilibrium.
• Heat always flows from hot to cold until temperatures equalise.
• Thermal equilibrium means no net heat transfer.
• Use \$Q = mc\Delta T\$ to calculate heat exchanged.
• Remember the three modes of heat transfer: conduction, convection, radiation.