L 17 - Thermodynamics  Science dealing with the relationship between thermal energy (random molecular motion) and work (organized motion), and the conversion of one into the other Todays topics Practical thermal effects Devices for temperature measurement Mechanisms of heat transfer 1 Thermal Energy and
Work Energy The random motion of the atoms is thermal energy The upward motion of the piston is work energy 2 Thermocouples 180 F two different metals
AMPS 3 Thermocouples- measure Temp. Is composed of 2 wires of different metals 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 500 F
4 Thermocouples used as safety devices a thermocouple is used in gas heaters, dryers, and fireplaces 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 detects 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 5 Thermal Expansion L1
Metal bar at T1 L2 Metal bar at T2 > T1 The length of a bar of metal increases when it is heated Actually, all dimensions expand by the same percentage 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 . . . 6 Thermal Expansion
Most substances expand when heated We use a parameter, a called the coefficient of thermal expansion to quantify this effect The length of a metal bar increases from L 0 to L (DL = L L0), when it is heated from T1 to T2 (DT = T2 T1) Change in length = DL = a L0 DT For copper, a = 1710-6 per deg. C Change DL = (1710-6 per C)(1 m)(100 C-22 C) = 0.0013 m = 1.3 mm 7 Coefficients of linear expansion SUBSTANCE
a (per deg C) aluminum brass glass rubber Ice lead steel concrete 23 106 19 106
10 Thermal expansion problems No room for thermal expansion result buckling 11 Areas and volumes expand too! cold hot
cold hot 12 Does the whole get bigger or smaller when heated? cold hot
Both the inner and outer diameters increase when the ring is heated 13 Hot water causes the lid to expand, making it easier to unscrew it. 14 Fire sprinklers are triggered by thermal expansion of a liquid 15
Bi-Metal strips 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 16 Heating a Bi-metal strip 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 The red side expanded more than the blue side, so the strip bends toward the blue side. 17 Bi-Metal strip thermal switch used to turn power off when a preset
temperature is reached ON OFF Used in coffee makers and hair dryers 18 Heat Flow HEAT HOT 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 19 Heat transfer by Convection heat is transferred from one location to another by the bulk movement and subsequent 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: in winter, want hot air in at lower level in summer, cold air in at upper level convection currents 20 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 poor conductor of heat 21 heat conduction Cross sectional area A L
HOT COLD Heat Flow Heat Flow rate depends on A / L and a property unique to the material, called thermal conductivity 22 Thermal Conductivity: a parameter that quantifies the ability of a material to conducting heat. Material Thermal
205 109 80 50/14 Goose down Styrofoam 23 Heat transfer by Radiation The warmth you feel from the sun is the suns 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 24 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 25
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