11_-_Convection

# 11_-_Convection - energy Therefore the thermal gradient is...

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11 - Convection 2 The Plan • Why is there convection? – At low temperatures radiation is not able to conduct sufficient heat, it goes as T 3 T. – Convection depends on the circulation of physical material. • When does it work? 3 Convection I Outside Inside the bubble the bubble (1) Since the bubble rises quickly it's adianatic, so 1 (2) if + note 0 P T d dP dT P T P d dP P d d and d γ ρ ρ δρ δρ = - = < + < < Ideal gas law The condition for convection 4 Convection II Outside Inside the bubble the bubble This is the condition for convection to start. Using (1) and (2) we get 1 1 Divide by 1 1 P dP dT P P T dT dP T P dr dT T dP dr P dr dT T dP dr P dr δ < - - < - < - >

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5 • Convection starts when: • Convection is extremely efficient at conducting
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Unformatted text preview: energy. Therefore, the thermal gradient is held close to the value where convection first starts. Convection III 1 ( ) ( ) dT T r dP dr P r dr γ-= 6 The outer 28% of the Sun Convection is present in about ~28% of the Sun’s outer radius. 7 Summary • At lower temperatures convection is likely to be present. • The actual onset of convection is determined by the absolute value of the radial temperature gradient, |dT/dr|. 1 ( ) ( ) dT T r dP dr P r dr-=...
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11_-_Convection - energy Therefore the thermal gradient is...

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