Heat Flow and Thermal Equilibrium
Aligned to HS-PS3-4 — Next Generation Science Standards.
What this lesson teaches
When two objects at different temperatures touch, thermal energy always flows from hotter to colder until both reach the same temperature. This one-way spreading toward a more uniform energy distribution is the second law of thermodynamics; energy never spontaneously piles back into the hot object.
Worked example
A 90°C metal block dropped into 20°C water does not stay hot. Energy leaves the block and enters the water until both settle at one in-between temperature, perhaps 35°C. The total thermal energy is conserved, but it is now shared more evenly.
Practice questions
- State the direction thermal energy flows between a hot and a cold object in contact.
- Predict the final temperature when equal masses of 80°C and 20°C water (same substance) are mixed, and explain your reasoning.
- Explain why a warm room never spontaneously makes one corner freezing while another turns hot, using the second law.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Flujo de Calor y Equilibrio Térmico
Cuando dos objetos a diferentes temperaturas se tocan, la energía térmica siempre fluye del más caliente al más frío hasta que ambos alcanzan la misma temperatura. Esta dispersión en un solo sentido hacia una distribución de energía más uniforme es la segunda ley de la termodinámica; la energía nunca se acumula de vuelta espontáneamente en el objeto caliente.
Ejemplo: Un bloque de metal a 90°C dejado caer en agua a 20°C no se queda caliente. La energía sale del bloque y entra al agua hasta que ambos se estabilizan en una temperatura intermedia, quizá 35°C. La energía térmica total se conserva, pero ahora se reparte de forma más pareja.
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