L 17 - Thermodynamics [2] Thermocouples
Science dealing with the relationship 350 C between heat (random molecular motion) and work (organized motion) and the conversion of one into the other
two different metals Today’s topics AMPS – Practical thermal effects – Devices for temperature measurement – Mechanisms of heat transfer
Thermocouples- measure Temp. Thermocouples used as safety devices
• The temperature sensor you stick into your • a thermocouple is used in gas Thermocouple turkey is a thermocouple heaters and dryers to protect Pilot light • it is composed of 2 wires composed of different against explosions • a thermocouple is placed in metals welded together - when it gets hot it the pilot light makes electrical current flow • as long as the pilot light is on, 350.0 °F 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 current gas
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 – L ), when it is heated from T to T 0 1 2 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 6 • Change L = (17×10-6 per C)(1 m)(100 C-22 C) steel 11 × 10 = 0.0013 m = 1.3 mm concrete 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 hot cold hot cold hot Both the inner and outer diameters increase when the ring is heated
2 Hot water causes the lid to Fire sprinklers are triggered by expand, making it easier thermal expansion of a liquid to unscrew it.
Bi-Metal strips Heating a Bi-metal strip
• thermal expansion of metals is put to good use • when heat is applied to the bi-metallic in a bi-metallic strip. strip, both metals expand, but by different • this is two strips of different metals bonded amounts! together • result: The metal with the metal A higher thermal expansion coefficient is on top
metal B The red side expanded more than the blue side, so the strip bends toward the blue side.
Bi-Metal strip thermal switch Heat Flow
HEAT
HOT COLD
ON OFF • 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: when a preset temperature is reached, – convection the bi-metallic strip opens up, – conduction turning the power off – radiation
3 Heat transfer by Convection Heat transfer by conduction
• heat is transferred from one location to another • heat is transferred directly through a material, by the bulk movement and mixing of liquids or gases (fluids), but NOT in solids. with no bulk movement of material • when water is boiled, hot water • only energy moves (molecules in a solid at the bottom rises and mixes are not free to move, but can vibrate) with cooler water at the top • Hot air rises: iron is a particularly • want heated air into lower poor conductor of heat level of house in the winter • cooled air into upper levels in the summer
heat conduction Thermal Conductivity: a parameter that Cross sectional quantifies the ability of a material to conducting heat. area A Material Thermal Metal Thermal conductivity Conductivity metals 14 - 400 Silver 406 wood 0.15 Copper 385 L glass 0.8 Aluminum 205 wool 0.04 Brass 109 HOT COLD Goose down 0.025 Iron 80 Heat Flow Styrofoam 0.01 Steel /SS 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
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