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Chapter 20 The First Law of Thermodynamics EXAMPLES

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Chapter 20. Heat and the First Law of Thermodynamics: Examples

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Example 20.1 A very Hot Frying Pan Accidentally an empty iron frying pan gets very hot on the stove (~300 o C). What happen when you dunk it into a few inches of cool water in the bottom of the sink? Assume: M water ≈ M pan Water warms up (~10 o C to 20 o C) Water does not come close to boiling. Since: M water ≈ M pan & c water ~ 10c iron from heat Q leaving the pan enters the water, and the iron’s pan ΔT will be 10 times greater than ΔT of the water. If you let a few drops of water fall onto the hot pan, that very small mass of water will sizzle and boil away since: M pan ~ 100M water

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Example 20.2 Cooling a Hot Ingot A 0.05-kg ingot of metal is heated to 200 o C and then dropped into a beaker containing 0.4-kg of water initially at 20 o C. If the final equilibrium temperature of the mixed system is 22.4 o C, find specific heat (c) of the metal.

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Calorimetry Problem-Solving Strategy Units of measure must be consistent For example, if your value of c is in J/kg.o C, then your mass must be in kg, the temperatures in o C and energies in J Transfers of energy are given by Q =mc ΔT only when no phase change occurs If there is a phase change, use Q = mL Be sure to select the correct sign for all energy transfers Remember to use Q cold = –Q hot The ΔT is always T f - T i

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Example 20.3 Cooling the Steam What mass of steam initially at 130 o C is needed to warm 200-g of water in a 100-g glass container from 20 o C to 50.0 o C Steam loses energy in 3 stages: 1st stage: Steam 100 o C, DT = –30 o C 2nd stage: Steam water 3rd stage: Water 50 o C, DT = –50 o C

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Example 20.3 Cooling the Steam, final Adding the energy transfer in the 3 stages: Water and Glass increasing temperature: DT = 30 o C Final: Q cold = –Q hot

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Example 20.4 An Isothermal Expansion A 1.0 mol of an ideal gas is kept at 0 o C during an expansion from 3.0 L to 10.0 L. (A). Find the work done on the gas during the expansion. Using equation (B). How much energy transfer by heat occurs with the surroundings in this process? From the first law: ΔE int = Q + W

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Example 20.4 An Isothermal Expansion, final (C). Find the work done on the gas during the expansion. Work done in an isobaric process is: W = P (V f – V i ) where V f =10.0L and V i =3.0L (reverse of part A)

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Example 20.5 Quick Quiz 20.5 Two rods from different materials with the same length and diameter are connecting two regions of different temperatures. In which case is the rate of energy transfer by heat (P ) larger Series Parallel Therefore P P = 4 P S : In parallel, the rods present a larger area A and a smaller length L through which the energy can transfer.

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Material from the book to Study!!! Objective Questions: 6-12 Conceptual Questions: 3-5 Problems: Material for the 2 nd Midterm

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