Thermal Energy Spreads Out
Aligned to HS-PS3-4 — Next Generation Science Standards.
What this lesson teaches
When two things at different temperatures touch, energy always flows from the hotter object to the colder one until both reach the same temperature. This one-way spreading toward a more uniform, evenly distributed state is the second law of thermodynamics.
Worked example
Drop a hot metal bolt into cool water and track the temperatures. The bolt's temperature falls and the water's rises until they meet at one shared temperature. Energy left the hot bolt and entered the cool water — never the reverse — and the final mixture is more uniform than the two separate starting temperatures.
Practice questions
- In which direction does thermal energy flow between a hot and a cold object?
- Plan an investigation, listing what you would measure, to show that mixing hot and cold water reaches a uniform final temperature.
- Explain why a cup of coffee cools to room temperature but never spontaneously reheats, using the idea of energy spreading out.
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
La energía térmica se dispersa
Cuando dos cosas a diferentes temperaturas se tocan, la energía siempre fluye del objeto más caliente al más frío hasta que ambos alcanzan la misma temperatura. Esta dispersión en un solo sentido hacia un estado más uniforme y distribuido por igual es la segunda ley de la termodinámica.
Ejemplo: Deja caer un perno de metal caliente en agua fría y sigue las temperaturas. La temperatura del perno baja y la del agua sube hasta que coinciden en una temperatura compartida. La energía salió del perno caliente y entró al agua fría —nunca al revés— y la mezcla final es más uniforme que las dos temperaturas iniciales separadas.
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