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6.2 Thermal Energy & Temperature

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1 6.2 Thermal Energy & Temperature
Recall that the total amount of kinetic and potential energy possessed by the atoms or molecules of a substance is called thermal energy (ETH). Thermal energy is responsible for an object’s warmth or coldness; it affects the temperature of an object. When an object or substance absorbs thermal energy, it warms up; when it releases thermal energy, it cools down. As a substance absorbs thermal energy from its surroundings, part of this energy increases the kinetic energy of the substance’s particles; increasing the substance’s temperature. Part if the absorbed energy also increases the potential energy of the particles. The increase in potential energy does not increase the temperature of the substance.

2 6.2 Heat Heat is a term used in physics to describe the transfer of thermal energy from a warmer substance to a colder one; the transfer of thermal energy from a substance of higher temperature to a substance of lower temperature. It is preferable to use the word ‘heat’ as a verb as opposed to a noun. Always use the word ‘heat’ as a transfer of thermal energy, not the thermal energy itself. For example, A glass of hot water has more thermal energy, not heat, than a glass of cold water. The stove is used to heat the water up. Thermal energy can be transferred in three different ways: conduction, convection, and radiation.

3 6.2 Conduction Thermal energy can transfer from a warmer object to a colder object through the process of thermal conduction. This occurs when the two objects are in direct physical contact with each other. Thermal conduction occurs when the fast moving particles of a warmer substance collide with the slow moving particles of the colder substance, causing the slower particles to speed up, and the faster particles to slow down. As a result, the warmer object’s temperature decreases; the colder object’s temperature increases.

4 6.2 Convection In fluids (liquids & gases), thermal energy can transferred by convection. This occurs when colder, denser fluid falls and pushes up warmer, less dense fluid. When a fluid is continuously heated, the warmer fluid moves upward away from the heat source, it cools down, increases in density, and sinks into the warmer, less dense fluid below. This process repeats itself, resulting in a continuous convection current.

5 6.2 Radiation Thermal energy can also be transferred by radiation. Radiation is a thermal transfer that involves electromagnetic waves. These waves are emitted from various sources such as lamps, flames, and the Sun. The Sun is our largest source of radiant energy. All particles that have kinetic energy emit some radiant energy. These waves travel through materials such as air, glass, and even empty space.

6 6.2 Thermal Energy Transfer

7 6.2 Conductors Why does a metal faucet feel much colder to touch than a plastic countertop, even though both are at room temperature? Metals are known to be thermal conductors because they allow thermal energy to pass through them quickly and easily. So when your hand touches a metal faucet, thermal energy passes quickly and easily from your hand to the faucet. Since metals conduct thermal energy so well, we commonly make pots and pans out of metals so that thermal energy is passed quickly from the stove into raw foods when cooking.

8 6.2 Insulators Materials such as plastic, tend to be poor conductors of thermal energy. They are known as thermal insulators. Gases such as air also tend to be poor conductors because their molecules are spread far apart. Animals make good use of this form of thermal insulation by trapping air within hair, fur, and feathers to help keep warm. This makes it difficult for the thermal energy to move from the animal’s body to the external environment. The best thermal insulator of all is a vacuum because it contains very few or no particles at all.

9 6.2 Homework Questions # 1-5 p.280

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