Download Force in the Thermal System

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
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How does heat flow?
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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
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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
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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
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How do we measure temperature?
• There are three different temperature scales
• Fahrenheit (F)
• Celsius (C)
• Kelvin (K)
Force in Thermal Systems
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Thermocouple