L4_29.pptx - Topics for Today Thermal expansion(18-3...

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Topics for Today Thermal expansion (18-3) Absorption of Heat (18-4) First law of thermodynamics (18-5) Heat transfer mechanism (18-6) Someone left an article of clothing at the last exam, please see me if it is yours.
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Most materials expand as the temperature increases. A temperature change of Δ T produces a change in length of Δ L = L α Δ T, where α is the “coefficient of linear expansion”. The change in length is proportional to the origin length – why? Typically α ≈ 10 -5 for metals. The change in length will occur for all dimensions of an object. Since Volume = Length 3 , the change in volume is Δ V = V β Δ T , where β = “coefficient of volume expansion” = 3α. Do demo 4A30.20 - Expansion & Contraction Demo - Ball and Ring. Thermal Expansion
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An engineering application of thermal expansion is the bimetal strip thermostat. If you bond together two different materials with different thermal expansion coefficients, then they will bend as the temperature changes. Many thermostats use a bimetal strip, or more usually a bimetal coil, as the temperature sensing elements. A mercury switch turns on/off according to the ambient temperature.
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