الأسبوع 5 · الدرس الرابع «طرق انتقال الحرارة» (١): انتقال الحرارة والاتزان الحراري والتوصيل والموصلات والعوازل الحرارية · 3 دروس
1
Heat flows from hot to cold until equilibrium
Science Prep 2 Term 1, Unit 1: pp. 30–34
GoalExplain that heat flows from hot to cold until thermal equilibrium, and predict the final temperature when equal masses of water are mixed.
New words
thermal equilibrium · the state when two objects in contact reach the same temperatureheat transfer · the movement of heat from a hotter place to a colder placetemperature difference · how much hotter one thing is than another
Explanation
Put a hot drink in a cooler room. The drink cools down and the air around it warms a tiny bit. Heat always flows from the hotter object to the colder one by itself, never the other way, and it goes on until both have the same temperature. That state is thermal equilibrium. A temperature difference is the cause; contact is what lets it happen. Cold does not flow: a cold object does not send “cold” out, it simply takes heat in. When we mix equal masses of water at different temperatures, the final temperature is about the average, ignoring losses: equal masses of water at 50 °C and 10 °C end near 30 °C. If the hot water is larger, the final temperature ends nearer to its temperature: 200 g at 60 °C with 100 g at 15 °C end near 45 °C, because there is twice as much hot water. The average rule works only for equal masses.
Worked examples
The graph shows hot water (50 °C) and cold water (10 °C) mixed in equal amounts: both temperatures move towards each other and meet at 30 °C. After that the temperature stays the same: thermal equilibrium.
A cup of tea at 70 °C stands in a room at 25 °C. Heat flows from the tea to the room until the tea reaches about 25 °C. It will not go below the room temperature by itself.
Mix 300 g of water at 20 °C with 300 g at 40 °C. The masses are equal, so the final temperature is the average: (20 + 40) ÷ 2 = 30 °C.
Class activity
Mixing test, with a grown-up. Use warm tap water, not hot, and cold tap water. Put equal cups of each in a big jug and read both temperatures first. Mix, wait a minute, and read again. Compare the result with the average of the two starting temperatures. Describe what you see. Then say why a lot of the heat in a small cup does not stay in the cup.
Practice
1
In which direction does heat flow by itself?
From the hotter object to the colder object.
2
What is thermal equilibrium?
When two objects in contact have reached the same temperature and no net heat flows.
3
250 g of water at 80 °C is mixed with 250 g of water at 20 °C. What is the final temperature, ignoring losses?
(80 + 20) ÷ 2 = 50 °C.
4
A student says an ice cube sends cold into your hand. Correct this.
Cold does not flow: heat flows from your warmer hand into the ice.
