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16.2 Heat and Thermodynamics. When you use an oven to bake something, it is important to wear an oven mitt (metal pans are good conductors, so are good to cook your food, but also can burn your hands). Why don’t oven mitts cover your whole arm? Air is a poor conductor. Conduction.
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16.2 Heat and Thermodynamics • When you use an oven to bake something, it is important to wear an oven mitt (metal pans are good conductors, so are good to cook your food, but also can burn your hands). • Why don’t oven mitts cover your whole arm? • Air is a poor conductor.
Conduction • The transfer of thermal energy with no overall transfer of matter is conduction. • Materials must touch for conduction to occur (ex. Newton’s cradle). • Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often. • Particles vibrate. • Metals conduct better due to free electrons.
Thermal Conductors • A material that conducts thermal energy well is a thermal conductor. • Metals conduct heat quickly, which is why they are used for cooking, but are dangerous. • Thermal conductors do not need to be hot. • Tile feels cold compared to our room’s floor because it is a better conductor and steals your heat.
Thermal Insulators • A poor thermal conductor is called a thermal insulator. • Examples are air, Ar, wool, plastic, and wood. • Insulators slow conduction to reduce heat loss. • They can even trap air to keep things warm (wool sweaters and Styrofoam cups).
Convection • The transfer of thermal energy when particles of a fluid move from one place to another is convection. • Why hot air rises: air near a heat source warms up, expands, becomes less dense, and floats. • The coolest air in an oven is on the top rack of an oven. • Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior.
Radiation • The transfer of energy by waves moving through space is radiation. • Charcoal grills, heat lamps, and the sun use radiation. • All objects radiate energy. As an object’s temperature increases, the rate at which it radiates energy increases. • The closer an object is to a heat source, the hotter it is.
Thermodynamics • The study of conversions between thermal energy and other forms of energy is thermodynamics. • James Joule made a pulley system with a weight and a paddle where a known mass of water was turned. The water would heat due to friction, and the temperature was measured.
First Law of Thermodynamics • The first law of thermodynamics states that energy is conserved. • Energy cannot be created or destroyed. • Energy could be changed. • All energy must be accounted for. • Sometimes the work is useful (compressed air).
Second Law of Thermodynamics • The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. • Heat spontaneously (chaotically) flows from hot to cold. • A heat engine converts heat into work, but efficiency is less than 100%. • Thermal energy not converted to work is waste heat.
Third Law of Thermodynamics • The third law of thermodynamics states that absolute zero cannot be reached. • Absolute zero is 0 Kelvin. • If we were to reach 0 Kelvin and not go inside out, efficiency would equal 100%. • Scientists have gotten close, 0.000003 degrees Kelvin.
