L4 - Energy in Thermal Processes(5)

# L4 - Energy in Thermal Processes(5) - PH2103 Thermal...

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PH2103 Thermal Physics Lecture 4: Energy in Thermal Processes Massimo Pia Ciamarra [email protected] SPMS-PAP-03-14 Textbook paragraph: 1.4, 1.5, 1.6 (no Enthalpy) Discussion Forum

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1. Specific and latent heat Specific heat Constant volume v.s. constant pressure Latent heat 2. Work in thermodynamic processes PV diagrams Thermal processes Agenda
1. Specific and Latent Heat Specific heat: Heat required to change the temperature of a substance

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Mechanical Equivalent of Heat Joule established the equivalence between mechanical energy and internal energy It takes approximately 4.18 J of mechanical energy to raise 1 g of water by 1 o C James Prescott Joule (1818- 1889) was an English physicist and brewer
Joule’s Experiment Work is done on water . Energy is transferred to water and appears as an increase in temperature We could replace the insulating walls by conducting walls and transfer heat to the system to produce the same increase in temperature The increase in temperature of the system is a consequence of the increase in internal energy Both heat and work can change the internal energy of the system

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Joule’s Experiment Work done: [n = number of drops; h = drop; m = mass] Temperature difference recorded: Water mass: M Mechanical equivalent of heat: Here is an animation . Δ T J = W M ⋅Δ T = 2 m g h n M ⋅Δ T W = 2 m g h n
Question ? What happens if we repeat Joule's experiment, using a different liquid or a gas? More generally, how much energy is required to rise the temperature of a system of mass m by 1 o C ? This quantity is known as the heat capacity C of the system

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Heat capacity & specific heat If we double the size of a system, the amount of energy required to rise its temperature by one degree also doubles (it's intuitive, but control Joule's result) The heat capacity thus depends on the mass of the system: C = m c c is the known as the specific heat or specific heat capacity .
Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1 °C (K)

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Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1 °C (K) The specific heat (c) of a substance is a measure of this amount T m Q c
Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1 °C (K) The specific heat (c) of a substance is a measure of this amount SI units: J/kg.°K T m Q c

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Molar specific Heat Unique amount of energy per unit mole to change the temperature of that substance by 1 °C (K) The molar specific heat (c) of a substance is a measure of this amount SI units: J/mol.°K c = Q n Δ T
Specific Heats of Materials Specific heat measured in standard conditions: P = 1atm T = 25°C

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Specific Heats of Materials Substances with lower specific heat will change temperature more readily
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