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# Physics 114 – Lecture 40 - PowerPoint PPT Presentation

Physics 114 – Lecture 40. Chapter 14 Heat Heat Flow: Spontaneously occurs from the hotter to the colder body. Thermal equilibrium §14.1 Heat as Energy Transfer Flow of heat – 18 th century view, flow of caloric from the hotter to the colder body

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• Chapter 14Heat

• Heat Flow: Spontaneously occurs from the hotter to the colder body. Thermal equilibrium

• §14.1 Heat as Energy Transfer

• Flow of heat – 18th century view, flow of caloric from the hotter to the colder body

• 19th century – heat was viewed as being similar to work, which was equivalent to viewing heat as a form of energy

• Kinetic Theory for gases: KEave = ½ mv2ave = (3/2) kT

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• Definition of calorie (cal):

• Amount of heat needed to raise 1 gram of water through a rise in temperature of 10C – more specifically from 14.5 0C to 15.5 0C

• 1 kcal = 1000 cal is the amount of heat required to raise 1 kg of water through a temperature rise of 10 C

• 1 kcal ≡ 1 Cal =1000 cal, the Cal being the unit of energy used by nutritionists

• British system of units: 1 British thermal unit (Btu) is the amount of heat required to raise 1 lb of water through a rise in temperature of 1 0F

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• Mechanical Equivalent of Heat

• Count Rumford – boring cannons

• Joule (~ 1850) showed that

4.186 J = 1 cal

which is equivalent to

4.186 kJ = 1 kcal

• Thus heat flow is a

transfer of energy from one

body to another

• Study example 14.1

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• §14.2 Internal Energy

• The internal energy of a system is the sum of all the energy of each molecule or atom in that system

• Internal energy is sometimes referred to as thermal energy

• Temperature, Heat and Internal Energy

• Temperature in K: a measure of KEave of the atoms and molecules in the system

• Heat: transfer of energy from one body to another because of a temperature difference between those bodies

• Internal Energy: sum of energy of each molecule or atom in the system

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• Internal Energy of an Ideal Gas

• For a system of N atoms, the internal energy, U, is:

• U = N X KEave = N(½ mv2ave) = (3/2) N kT

• With N k = n R this becomes:

• U = (3/2) n RT

• Note that the temperature, T, must be expressed in K

• When this expression is used

for molecules, the rotational

KE must be taken into

consideration

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• §14.3 Specific Heat

• Consider an amount of heat, Q, flowing into a body of mass, m, which produces a rise in temperature, ΔT. Experimentally it is observed that Q is directly proportional to m and ΔT, but that Q does depend on the composition of the body.

• The specific heat of a material is defined as:

• Q = m c ΔT units of c cal/(g.0C)

• Note that, from the definition of the calorie, that the above definition also defines c for water to be

cwater = 1 cal/(g.0C)

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• Study examples 14.2 and 14.3

• §14.4 Calorimetry – Solving Problems

• We need to exercise care in describing various types of system, which is the collection of bodies under consideration. Systems are of three main types

• Closed System: No mass enters or leaves but heat may be exchanged with the environment

• Open System: Mass and energy may enter or leave

• Isolated System: Neither mass nor energy may enter or leave

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• Calorimetry problems are solved by applying the principle of conservation of energy – heat energy to be specific →

• heat lost by one body or bodies = heat gained by the other body or bodies

• which is the same statement as

• energy lost by one body or bodies = energyt gained by the other body or bodies

• Heat lost by one body = m c (Ti – Tf)

• Heat gained by one body = m c (Tf – Ti)

• Study examples 14.4, 14.5 and 14.6

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