L 17 - Thermodynamics [2] Thermal Energy and Work Energy Science dealing with the relationship between thermal energy (random molecular motion) and work (organized • The random motion of motion), and the conversion of one into the atoms is thermal the other energy Today’s topics • The upward motion of – Practical thermal effects the piston is work – Devices for temperature measurement energy – Mechanisms of heat transfer
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Thermocouples Thermocouples- measure Temp.
• Is composed of 2 wires of different metals 180 F welded together • when the tip gets hot an electrical current is produced • The current is proportional to the temperature • It can be used over a wide range of temperatures
two different metals AMPS 500 °F
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Thermocouples used as safety devices Thermal Expansion
L • a thermocouple is used in gas Thermocouple 1 heaters, dryers, and fireplaces Pilot light Metal bar at T1 to protect against explosions • a thermocouple is placed in the pilot light Metal bar at T2 > T1 • as long as the pilot light is on, L2 the thermocouple is hot and current flows • The length of a bar of metal increases when it is heated • a circuit detects the current and allows the main gas valve • Actually, all dimensions expand by the same percentage to open • Expansion must be taken into account when designing • if the pilot light is out, the roads and bridges in climates that vary significantly from circuit prevents the main gas winter to summer – all materials expand, steel, concrete, valve from opening asphalt . . . current gas 5 6
1 Thermal Expansion Coefficients of linear expansion
• Most substances expand when heated SUBSTANCE (per deg C) • We use a parameter, called the coefficient of aluminum 23 × 106 thermal expansion to quantify this effect brass 19 × 106 • The length of a metal bar increases from L to L 0 glass 9 × 106 (L = L – L0), when it is heated from T1 to T2 rubber 80 × 106 (T = T2 –T1) Ice 51 × 106 • Change in length = L = L0 T 6 • For copper, = 17×10-6 per deg. C lead 29 × 10 • Change L = (17×10-6 per C)(1 m)(100 C-22 C) steel 11 × 106 = 0.0013 m = 1.3 mm concrete 10 × 106
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expansion gaps on bridges winter/summer expansion gaps
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Thermal expansion problems Areas and volumes expand too!
cold hot cold hot No room for thermal expansion result buckling 11 12
2 Does the whole get bigger or Hot water causes the lid to smaller when heated? expand, making it easier to unscrew it.
cold hot
Both the inner and outer diameters increase when the ring is heated 13 14
Fire sprinklers are triggered by Bi-Metal strips thermal expansion of a liquid
• thermal expansion of metals is put to good use in a bi-metallic strip. • it is two strips of different metals bonded together
metal A
metal B
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Heating a Bi-metal strip Bi-Metal strip thermal switch used to turn power off when a preset • when heat is applied to the bi-metallic temperature is reached strip, both metals expand, but by different amounts!
• result: The metal with the higher thermal expansion coefficient is on top ON OFF
The red side expanded more Used in coffee makers and hair dryers than the blue side, so the strip bends toward the blue side. 17 18
3 Heat Flow Heat transfer by Convection • heat is transferred from one location to another by the bulk movement and subsequent mixing of HEAT liquids or gases (fluids), but NOT in solids. HOT COLD • when water is boiled, hot water at the bottom rises and mixes with cooler water at the top • Hot air rises: • Heat is the energy that flows from one body to – in winter, want hot air in at lower level another because of their temperature difference – in summer, cold air in at upper level • There are 3 ways that heat can be transferred: – convection convection – conduction currents – radiation 19 20
Heat transfer by conduction heat conduction Cross sectional • heat is transferred directly through a material, area A with no bulk movement of material • only energy moves (molecules in a solid are not free to move, but can vibrate)
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iron is a poor HOT COLD conductor Heat Flow of heat Heat Flow rate depends on A / L and a property
21 unique to the material, called thermal conductivity22
Thermal Conductivity: a parameter that Heat transfer by Radiation quantifies the ability of a material to conducting heat. • The warmth you feel from the sun is the sun’s thermal radiation Material Thermal Metal Thermal conductivity Conductivity • It travels through the vacuum of metals 14 - 400 Silver 406 space to reach earth, no material is necessary (takes 8 minutes) wood 0.15 Copper 385 • you can feel its effects even though glass 0.8 Aluminum 205 you cannot see the radiation. wool 0.04 Brass 109 • you can feel the thermal radiation Goose down 0.025 Iron 80 from a fireplace Styrofoam 0.01 Steel /SS 50/14 • Objects not in contact with liquids gases or other solids loose heat by radiation
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4 Thermal Radiation
• The amount of thermal radiation emitted by an object is proportional to its temperature raised to the fourth power ~ T4 • Doubling the temperature will increase the amount of thermal radiation by 24 = 2 x 2 x 2 x 2 = 16
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