HEAT FLOW & THE FIRST LAW

Heat can also occur during phase changes because changes in molecular form either release or absorb energy. This occurs during melting or freezing or boiling or condensation or during sublimation or deposition. During these processes, the temperature does not change so they are called Latent heats.

HEAT FLOW & THE FIRST LAW

The Latent Heats are measured in three different units kJ/mol, kJ/kg, cal/g

Your text has a table of kJ/mole (18-2) Latent heats depend on p and T. Latent heat fusion heat

vaporization (H2O) 79.7 cal/g 540 cal/g (1 atm)

First Law of Thermodynamics Applied to the Human Body

Food enters the body containing chemical energy Some is converted to stored chemical energy and some

to thermal energy Then chemical energy is converted to work (mechanical

energy) and some heat is released Sometimes the body has to do work to replace heat lost

(shivering) If the internal energy of the body is constant then food

energy must equal heat loss and work done What happens if not?

FoodChemical Energy Work

HeatThermal Energy

Results of U

Changes in the internal energy result in changes in the measurable macroscopic variables of the system Pressure Temperature Volume

For the human body it is usually temperature or volume (isobaric)

Metabolic Rates (Cal/m2-hr)

Sleeping 35Lying awake 40Sitting 50Standing 60Walking 140Running 600Shivering 250

Your surface area can beapproximated using theformula SA = .202m.425 x h.725

where m is in kg and h is inmeters. Calculateyour surface area

The metabolic rate at rest is the basal metabolic rate. The surface area of a 70 kg man of height 1.55m is about 1.70 m2. His metabolic rate is therefore 40 x 1.70 = 68 Cal/hr while lying awake.

Heat and Life We need energy to function (blood circulation,

cell repair, etc.) Even at rest a 70 kg person consumes about 70

Cal/hr The energy needed depends on a persons weight

and build However, it has been found that human energy

consumption (usage) divided by a person’s surface area is approximately the same for most people

It is given a unit of Cal/m2-hr and called metabolic rate

Measuring Metabolic Rate

The metabolic rate is related to oxygen consumption by

About 80 W is the basal metabolic rate, just to maintain and run different body organs

24.8 oVUt t

Various Metabolic Rates

Aerobic Fitness One way to measure a person’s

physical fitness is their maximum capacity to use or consume oxygen

Energy Output vs. Food Intake Food requirements depend on activity levels Consider this schedule:Activity Energy (Cal/m2) For a person of SA 1.7m2

this is 2320(1.70)8 hr of sleep 280 or 3940 Cal per day.8 hr of moderate activity 1200 This could be met by a 4 hr of reading 240 diet of:1 hr of heavy exercise 300 400g of carbs 1600 Cal3 hr of dressing, eating 300 200g of Protein 800 CalTotal 2320 171g of fat 1540 Cal

3940 Cal

Weight Gain vs. Weight Loss They are stored as tissue (fat or muscle) Lack of caloric intake results in the body getting

energy from stored fat first (9 Cal/g) and then proteins (4 Cal/g)

The average person can go 50 days without food Angus Barbieri of Scotland consumed only tea, coffee

and water from June 1965 to July 1966 reducing his body weight from 472 lbs to 178 lbs

Pregnant women need an extra 136 Cal/day which can come from an increased appetite or a decrease in physical activity

The body cannot eliminate excess calories

Efficiency of the Human Body

Efficiency is the ratio of the mechanical power supplied to the metabolic rate or total power input

Example Hiking Howard’s Knob

Suppose one starts out from King Street and climbs Howard’s Knob. How much

energy is needed? Δh=1400 ft=427 m Assume m=80 kg PE= mgh = 80 kg 9.8 (m/s2) 427 m = 335 kJ/4.186kj/kcal = 80 kcal ε=.2ΔPE(ME)= εΔPEfood ΔPEfood =400 kcal