Thermodynamics - Lecture 6

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First Law of Thermodynamics

-Energy Conservation

Q is positive when heat transferred IN

W POSITIVE when work done by system on SOMETHING ELSE

First Law for Body

-For the body along with the first law of thermodynamics, we also need to include the ENERGY INPUT, due to the metabolism of FOOD

Food Energy, E

-Calorific Value in Cal, or KiloCal BUT we use JOULES OR KILOJOULES j or kJ

-1 cal = 4.19kJ

-measured by the heat produced with food oxidised (by combustion) then adjusted for how we metabolise food

-recommended intake 8MJ females, 10MJ males

Work Efficiency

W- mechanical work done by body

(E - change in U) - NET metabolic energy required to carryout work

-If body does not add/use stored energy then (change in u) = 0 . Thus n= W/E ( then Q= -E)

If (change in U) is NEGATIVE = WEIGHT LOSS

-In body usually quite low ( 2-10%), 20-25%during exercise

-When n -heat loss rate is usually about same at metabolic

Body Thermoregulation

Temp sensitive receptors are

-hypothalmus; w & c, integrates info from both skin and central receptors, provides signals (control rate heat loss/heat production)

-skin - w & cold

-abdominal visera (only warmth)

These DICTATE thermal CONTROL options

-perspiration

-vasoconstriction

-vasodilation

-shivering

Perspiration

-max moisture loss 2000w/p hour

-if RH low, skin dry. Even when lots of sweat

-RH increases

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