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Download L 17 - Thermodynamics [2] Thermal Expansion Coefficients of linear
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Transcript
Thermocouples L 17 - Thermodynamics [2] Science dealing with the relationship between heat (random molecular motion) and work (organized motion) and the conversion of one into the other 350 C two different metals Today’s topics AMPS – Practical thermal effects – Devices for temperature measurement – Mechanisms of heat transfer Thermocouples- measure Temp. • The temperature sensor you stick into your turkey is a thermocouple • it is composed of 2 wires composed of different metals welded together - when it gets hot it makes electrical current flow 350.0 °F Thermocouples used as safety devices • a thermocouple is used in gas heaters and dryers to protect against explosions • a thermocouple is placed in the pilot light • as long as the pilot light is on, the thermocouple is hot and current flows • a circuit senses the current and allows the main gas valve to open • if the pilot light is out, the circuit prevents the main gas valve from opening Thermocouple Pilot light current gas Thermal Expansion • Most substances expand when heated • We use a parameter, called the coefficient of thermal expansion to quantify this effect • The length of a metal bar increases from L0 to L (L = L – L0), when it is heated from T1 to T2 (T = T2 –T1) • Change in length = L = L0 T • For copper, = 17×10-6 per deg. C • Change L = (17×10-6 per C)(1 m)(100 C-22 C) = 0.0013 m = 1.3 mm Coefficients of linear expansion SUBSTANCE aluminum brass glass rubber Ice lead steel concrete (per deg C) 23 × 106 19 × 106 9 × 106 80 × 106 51 × 106 29 × 106 11 × 106 10 × 106 1 Thermal Expansion winter/summer expansion gaps L1 Metal bar at T1 Metal bar at T2 > T1 L2 • Expansion occurs in all dimensions • Expansion must be taken into account when designing roads and bridges in climates that vary significantly from winter to summer – all materials expand, steel, concrete, asphalt . . . expansion gaps on bridges Thermal Expansion problems No leave room for expansion result buckling Areas and volumes expand too! Does the whole get bigger or smaller when heated? cold cold hot cold hot hot Both the inner and outer diameters increase when the ring is heated 2 Hot water causes the lid to expand, making it easier to unscrew it. Fire sprinklers are triggered by thermal expansion of a liquid Bi-Metal strips Heating a Bi-metal strip • thermal expansion of metals is put to good use in a bi-metallic strip. • this is two strips of different metals bonded together • when heat is applied to the bi-metallic strip, both metals expand, but by different amounts! • result: The metal with the higher thermal expansion coefficient is on top metal A metal B The red side expanded more than the blue side, so the strip bends toward the blue side. Heat Flow Bi-Metal strip thermal switch HEAT HOT ON OFF when a preset temperature is reached, the bi-metallic strip opens up, turning the power off COLD • Heat is the energy that flows from one body to another because of their temperature difference • There are 3 ways that heat can be transferred: – convection – conduction – radiation 3 Heat transfer by Convection • heat is transferred from one location to another by the bulk movement and mixing of liquids or gases (fluids), but NOT in solids. • when water is boiled, hot water at the bottom rises and mixes with cooler water at the top • Hot air rises: • want heated air into lower level of house in the winter • cooled air into upper levels in the summer heat conduction Heat transfer by conduction • heat is transferred directly through a material, with no bulk movement of material • only energy moves (molecules in a solid are not free to move, but can vibrate) iron is a particularly poor conductor of heat Thermal Conductivity: a parameter that Cross sectional area A quantifies the ability of a material to conducting heat. Material metals wood glass L HOT COLD Heat Flow wool Goose down Styrofoam Thermal conductivity 14 - 400 0.15 0.8 Metal Silver Copper Aluminum Thermal Conductivity 406 385 205 0.04 0.025 0.01 Brass Iron Steel /SS 109 80 50/14 Heat Flow rate depends on A / L and a property unique to the material, called thermal conductivity Heat transfer by Radiation • The warmth you feel from the sun is the sun’s thermal radiation • It travels through the vacuum of space to reach earth, no material is necessary (takes 8 minutes) • you can feel its effects even though you cannot see the radiation. • you can feel the thermal radiation from a fireplace • Objects not in contact with liquids gases or other solids loose heat by radiation 4