HEAT FLOW & THE FIRST LAW

1. 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.

2. 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)

3. First Law of Thermodynamics Applied to the Human Body Chemical Energy Work • 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? Food Thermal Energy Heat

4. Results of DU • 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)

5. Metabolic Rates (Cal/m2-hr) Sleeping 35 Lying awake 40 Sitting 50 Standing 60 Walking 140 Running 600 Shivering 250 Your surface area can be approximated using the formula SA = .202m.425 x h.725 where m is in kg and h is in meters. Calculate your 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.

6. 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

7. 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

8. Various Metabolic Rates

9. Aerobic Fitness • One way to measure a person’s physical fitness is their maximum capacity to use or consume oxygen

10. 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 Cal 3 hr of dressing, eating300 200g of Protein 800 Cal Total 2320 171g of fat 1540 Cal 3940 Cal

11. 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

12. Efficiency of the Human Body • Efficiency is the ratio of the mechanical power supplied to the metabolic rate or total power input

13. 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=.2 ΔPE(ME)= εΔPEfoodΔPEfood =400 kcal