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Heat flow, equilibrium and conduction

الأسبوع 5 · الدرس الرابع «طرق انتقال الحرارة» (١): انتقال الحرارة والاتزان الحراري والتوصيل والموصلات والعوازل الحرارية · 3 دروس
2

Conduction: passing energy along a solid

Science Prep 2 Term 1, Unit 1: pp. 30–34
GoalExplain conduction with particles and say that the particles stay in place while energy passes along a solid.
New words
conduction · heat passing through a material as particles bump into their neighboursthermal conductivity · how well a material lets heat pass throughvibrate · to move back and forth in a small space
Explanation

Heat can travel in three ways: conduction, convection and radiation. Conduction is the transfer of heat through a solid, mainly. Imagine a long line of children holding hands. If the first child is pushed hard, the shaking passes along the line, but the children stay where they are. In the same way, when one end of a metal rod is warmed, its particles vibrate harder and bump into their neighbours, which then vibrate harder too, and the energy travels along the rod. The particles do not travel; they only pass the energy on. Materials differ in thermal conductivity. Metals are good conductors: in a metal, free electrons also help carry energy. Wood, plastic, wool and still air are poor conductors, which we call insulators. A metal spoon from a shelf feels colder than a wooden one, although both are at room temperature. The metal is not colder: it is a better conductor, so it takes heat from your hand faster.

Worked examples
metal spoonwarm water
A metal teaspoon is left in a cup of warm tap water. After a few minutes the handle, which is out of the water, feels warm. The particles at the bottom vibrate harder and pass the energy up the spoon.
An ice cube on a metal tray melts faster than one on a wooden board in the same room. Describe what you see before you explain: the metal conducts heat from the room into the ice quicker.
Why does a coat keep you warm? It does not give you warmth. It traps still air, a poor conductor, so your body heat leaves slowly.
Class activity

Spoon feel test, with a grown-up. Take a metal spoon and a wooden spoon that have been on the same shelf for an hour. Touch both to your cheek or the inside of your wrist and say which feels colder. Read both with a digital thermometer if you have one and compare the two readings. Explain why they feel different. Do not use boiling water or flames.

Practice
1
In conduction, do the particles travel along the solid?
2
Name two good conductors and two insulators.
3
Why does a metal spoon feel colder than a wooden one at room temperature?
4
A student says a wool jumper “gives” warmth. Say what really happens.