Thermo Lecture 5

# Thermo Lecture 5 - Heat is energy in transit Internal...

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Heat is energy in transit! Internal energy relates to the energetic content of an object or a system. Whereas Heat is energy in transit! More specifically, the heat, Q , relates to the energy transferred due to temperature difference alone . While, W , incorporates all other sorts of energy transfer, most commonly mechanical work . The amount of heat transferred to a system, Q , is a characteristic of the specific process in which it is transferred. Consequence: heat capacity and molar specific heat of ideal gas are characteristics of the process in which heat is transferred. T Q C Δ = / ) /( T n Q C molar Δ =

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Constant volume (isochoric) process – V = const. 0 = W U Q Δ = By definition of the molar specific heat at constant volume, C v , Therefore: T U n C v Δ Δ = 1 Measuring C v we learn about internal energy of the gas as a function of temperature! W Q U = Δ T nC Q v Δ = n – number of moles of the gas. T nC U v Δ = Δ
V P From the 1 st law and equation for C v : T R C n V P T nC W U Q v v Δ + = = Δ + Δ = = + Δ = ) ( Molar specific heat at constant pressure (definition): T nC Q p Δ = R C C v p + = Why is specific heat at constant pressure higher than at constant volume? Heat capacity is defined as, C=Q / Δ T . It depends on heat transferred to the system, NOT on the change in the internal energy. Therefore, heat capacity is a function of the process and is different for different processes. More specifically, if the volume is kept constant, the entire heat is used to increase the system’s internal energy. If volume increases, the heat is used in two ways: to increase the internal energy and to perform work, Q = Δ U + W . Therefore, more heat is required per 1° temperature increase.

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Thermodynamic processesE W Q U = Δ nRT PV = = PdV W
What is the heat transferred to the gas in the process? = PdV W W U Q + Δ = T nC U v Δ = Δ R PV nT = R V P V P T n 1 1 2 2 = Δ + = PdV R V P V P C Q v 1 1 2 2

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Adiabatic processes – no heat transfer (thermal insulation). W U = Δ nRT PV = 0 = Q = PdV W
no heat transfer (thermal insulation). W U = Δ 0 = Q Positive work, W , is done by the expanding gas at expense of reduction of its internal energyE Since there is no heat supplied from the outside to replenish the gas energy, the temperature decreases. BTW, what is the molar specific heat of a gas in an adiabatic process?

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## This note was uploaded on 02/05/2011 for the course BILD 2 taught by Professor Schroeder during the Spring '08 term at UCSD.

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Thermo Lecture 5 - Heat is energy in transit Internal...

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