EGR 1301

Temperature, Heat,& Combustion

EGR 1301: Introduction to Engineering

EGR 1301

Models

• “A system of postulates, data, and inferences presented as a mathematical description of an entity or state of affairs”

• Quantitative approximation of reality Mathematical equations Computer simulations Physical scale models

• Why do we use them? Reality is too complex!!!

Source: Merriam-Webster .com, 2010

EGR 1301

Energy Conversion Tables

Source: Foundations of Engineering, Holtzapple & Reece, 2003

“For those who want some proof that physicists are human, the proof is in the idiocy of all the different units which they use for measuring energy.” Richard Feynman

EGR 1301

Utility of Energy for Analysis

• Incandescent bulb Resistance heating

in filament Light• When filament

reaches sufficiently high temperature Light is radiated

60 Watts of electricity• 800 lumens of light• ~10 cals of heat

• Fluorescent bulb Stream of electrons

collide with Hg electronsLight• Generates very little

heat

23 Watts of electricity• 800 lumens of light• ~1 cal of heat

Source: http://www.fullspectrumsolutions.com/26w_powercompact_65_prd1.htm

EGR 1301

Temperature – What Is It?

• NOT the same as HEAT• A quantitative measure of “hotness”• More accurately described on an atomic

scale Measures vibrational kinetic energy

EGR 1301

Temperature vs. Heat

• Temperature A measure of the intensity of internal energy

in a system (gas, liquid, or solid)• Heat

A measure of the total quantity of thermal energy flow into or out of a system

EGR 1301

Temperature vs. Heat

• Example: A cup of water at 60°C has much less energy

than a hot water heater full of water at 60°C.

BUT, the intensity of heat is the same.

EGR 1301

Heat Capacity

• Energy required to raise temperature of matter by one degree (at constant pressure or constant volume)

Q = energy in calories m = mass in grams ΔT = temperature change in degrees (C or K)

TmQC

EGR 1301

Constant Pressure Heat Capacities

Source: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

Converting Work into Heat:Joule’s Experiment

Source: Foundations of Engineering, Holtzapple & Reece, 2003

TCmQ

xFW maF

QU

outinoutinoutinpk MMQQWWUEE

QW

EGR 1301

Heat CapacityExample Problem

• In Joule’s experiment, Beaker contains 5 kg of water Mass spinning the stirrer is 90 kg

(g=9.81m/s2) The water increases in temperature by 0.1°C How far did the mass travel?

TCmQ xFW

maF QW

EGR 1301

Heat Capacity

calCkggkg

CgcalQ 5001.0

11000500.1

Jsmkg

calJ

smkg

calmaQ

FWx

1

1

1184.4

81.990

500 2

2

2

mx 4.2

TCmQ xFW maF QW

EGR 1301

States of Matter

Source: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

Phase Diagram

Source: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

Phase Change

• Constant temperature process of transition between phases Melting / Solidification Boiling (vaporization) / Condensation

EGR 1301

Phase (or State) Change Energy

• Where m = mass (kg) ΔHvap = latent heat of vaporization (kJ/kg) ΔHfus = latent heat of fusion (kJ/kg)

vapvap HmQ fusfus HmQ

EGR 1301

Phase-Change EnergySource: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

Combustion

• Similar to phase change

• Where Qcomb = energy released (MJ) m = mass (kg) ΔHcomb = specific heat of combustion (MJ/kg)

• Table 22.4

combcomb HmQ

EGR 1301

Example 1:Phase-Change Energy

• When water changes from solid to liquid, it must absorb 333.56 kJ/kg from the surroundings What is the energy absorbed to melt ice in

units of cal/g?

Jcal

kJJ

gkg

kgkJ

12390.01000

1000156.333

gcal7.79

EGR 1301

Example 2a:1st Law of Thermodynamics

• If you have 100 g of water at 22°C and add 20 g of ice at 0°C, what will be the temperature of the 100 g of water once all the ice has melted to form 20 g of water at 0°C?

CTgm

22100

1

1

CTgm

020

2

2

:Given

EGR 1301

Example 2a:1st Law of Thermodynamics

CTgm

22100

1

1

CTgm

020

2

2

:Given :Eqns

HmHfusion

TCmQ

gcalg 7.7920 Tg

Cgcal

10000.1

CT 94.15 CCT

b 06.694.1522

1

EGR 1301

Example 2b:1st Law of Thermodynamics

• What will be the final temperature when the system temperature is uniform (i.e., water from melted ice has warmed and surrounding water has further cooled so that all water is at one temperature)?

CTgmCTgm

b

020

06.6100

2

2

1

1

)06.6(10000.1 CTgCgcal

f

:Given

)0(2000.1 CTgCgcal

f

CTf

05.5

EGR 1301

Example 3:Latent Heat

• If the latent heat of vaporization for water is 2,256.7 kJ/kg, what is the latent heat for water in cal/g?

Jcal

kgkJ

12390.07.2256

gcal4.539

EGR 1301

Example 4a:1st Law of Thermodynamics

• If by sweat and evaporation, you lose 0.031 slugs of water during exercising, how many calories of energy in the form of heat is removed from your body? Weight (mg) of 1 lb is associated with mass of 0.031

slugs Note on p.688 the conversion from slugs to grams

gcal

sluggslug 4.53914594031.0

cal244032

EGR 1301

Example 4b:1st Law of Thermodynamics

• If your body mass is 68,100 g (i.e., 150 lbs), how much would your body temperature rise if you did not sweat and evaporate the sweat in order to cool yourself? Assume the heat capacity for your body is that for

water, because your body is ~ 75% water.

TCmQ TgCg

calcal

)68100(1244032

FCFCT

45.65958.3