2A4_Lecture02_2008

# Even though your hand is not in the fire it will

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Unformatted text preview: re and the other Place in your hand. Even though your hand is not in the fire, it will quickly become hot as a result of conduction through the bar conduction T1 T2 q T1>T2 Fourier’s Law q′′ = −k∇T Convection Convection • Convection – A hot metal plate is exposed to ambient air. Cooling will hot be by convection. be – What happens to the rate of cooling if we move the air in What some way i.e. using a fan? some – Why is this the case? We are not changing the Why temperature of the air in any way. temperature T∞ q Ts Ts > T∞ Newton’s Law of Cooling q′′ = h( Ts − T∞ ) Radiation Radiation • Radiation – Even though there is no medium through which Even the heat can flow, the sun heats the earth. The mechanism for this heating is radiation mechanism – Every solid emits radiant energy. Amount and rate depend on solid temperature Tabs rate Stefan – Boltzmann Law q′′ = σ Tabs 4 Advice It is extremely important that you understand the physical mechanisms that are occurring in physical heat transfer problems in order to be able to identify and use the proper equations to mathematically describe what is happening mathematically Example Example Think about a coffee cup Think Heat is transferred by: Heat – Conduction and p...
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