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Force in the Thermal System Overview Force in Thermal Systems What is the “prime mover” in the thermal system? • temperature difference • ΔT – “delta” T, or • change in temperature • T2 – T1 Force in Thermal Systems How does heat flow? During the winter, how does heat flow between your house and outside? • from high temperatures to low temperatures • the hot air from inside flows through the wall to the outside Force in Thermal Systems During the summer, how does heat flow between your house and outside? • the hot air from outside flows through the walls to the inside How do you prevent this heat flow? • insulators – materials that prevent heat from • flowing R-factors (R-13, R-30…) Force in Thermal Systems What is the difference between heat and temperature? • heat – thermal energy that flows from high to • • low – includes both the kinetic energy and potential energy temperature – average kinetic energy (energy of motion) of the molecules of a substance Note: temperature is a scalar quantity Force in Thermal Systems How do we measure temperature? • There are three different temperature scales • Fahrenheit (F) • Celsius (C) • Kelvin (K) Force in Thermal Systems Facts about the Fahrenheit scale • used commonly in US • 32º is freezing point of water • 212º is the boiling point of water Force in Thermal Systems Facts about the Celsius scale • Metric scale for temperature • 0º is freezing point of water • 100º is boiling point of water Fahrenheit vs Celsius Force in Thermal Systems Facts about the Kelvin scale • most commonly used in scientific • • • • measurement based on absolute zero 0 K is the lowest possible temperature 273 K is freezing point of water 373 K is boiling point of water Force in Thermal Systems How do you convert from Celsius to Fahrenheit? • ºF = (9/5) ºC + 32º How do you convert from Fahrenheit to Celsius? • ºC = (5/9)(ºF – 32º) Force in Thermal Systems Example – Convert 50º F to ºC • ºC = (5/9)(ºF – 32º) • ºC = (5/9)(50º – 32º) • ºC = (5/9)(18º) • ºC = 10ºC Force in Thermal Systems Example – Convert 20º C to ºF • ºF = (9/5) ºC + 32º • ºF = (9/5) 20º + 32º • ºF = 36º + 32º • ºF = 68ºF Force in Thermal Systems Writing temperatures and temperature differences • (absolute) temperatures are expressed as °F • Ex. The room was at 70°F • Differences (or ranges) are expressed as F°, or C° • Ex. There was a 30F° difference between the outside and inside the house in summer • As 100C° = 180F°, then • 1C° = 1.8F°, or 1F° = 1/1.8C° or = 0.55C° Force in Thermal Systems What is a simple instrument for measuring temperature? • Thermometer (analog) • a liquid inside (mercury or alcohol) expands as the temperature increases • as the liquid expands, it rises up the capillary tube which has been marked with a temperature scale • Thermometer (digital) • electronic Force in Thermal Systems What is a thermocouple? • a temperature measuring device made of two • dissimilar metals joined at two places called junctions a voltage is produced that is proportional to the difference in temperatures applied to the two junctions Force in Thermal Systems Thermocouple