Chapter 10 More Heat I. First Law of Thermodynamics

Chapter 10 More Heat
First Law of Thermodynamics
Joule’s Experiment
1) Rubbing your hands together warms them up
a) They are both the same temperature—cold
b) Where does the heat come from?
2) Mechanical Work done on a system warms it up
a) Turning the paddle does work on the water, increasing its temp.
b) Weight loses potential energy, the water gains heat energy
c) 4.19 J of work raises the temp. of 1 g of water 1 oC
Transfer of Heat = transfer of Kinetic Energy (from Work) to the
molecules of the water = increase in internal energy
First Law of Thermodynamics = Increase in internal energy of a system
is equal to the amount of heat added minus the work done by the system
a) DU = Q - W
Work done by the system
-(+W) if work done by system
-(-W) if work done on the system
Restatement of the Conservation of Energy
Heat flow is a transfer of Energy
Internal Energy(U) DU = Q - W
1) Increase in the Kinetic Energy of a molecule = DU KE = ½ mv2
2) Increase in distance between molecules = DU PE = mgh
a) Latent Heats of Fusion and Vaporization
b) Change in phase means change in distance between molecules
3) Internal Energy = KE + PE of molecules
4) U tells us the State of a system
a) Phase, temperature, pressure, etc… known if we know U
b) Heat or Work added or taken away get the system to a particular state
5) System = an organized set, considered separately from its environment
6) Transfer 400 cal of heat to ice water and do work (stir) of 500 J. What is
the increase in U and how much ice melts?
DU  Q  W  (400cal )( 4.19 J / cal )  (500 J )  1680 J  500 J  2180 J
DU  mL f  m 
2180 J
2180 J
 6.5g
(80cal / g )( 4.19 J / cal ) 335 J / g
Food Calories
1) 1 Food Calorie = 1 kilocalorie = 1000 calories = 4,190 J
2) This energy comes from chemical energy released by breaking down food
3) Energy input (Calories) > Energy output(muscular work)
Energy stored (as fats), increase mass
Gas Behavior
Compressing a Gas
1) Work is done on the gas: -W
DU = Q – (-W) = Q + W = increases
2) Wgas = PAd (W = Fd F = PA)
3) Ad = change in volume = DV
4) W = PDV
Work done on/by gas = pressure times change in volume
a) Gas is compressed, work done on gas, DV = (-), -W
b) Gas expands, work done by gas, DV = (+), +W
DU = Q – (+W) = Q - W = decreases
Internal Energy and Temperature
1) Adiabatic Process = no heat flows in or out of the system
2) Ideal Gas = no forces between the molecules (PE)
a) U = KE for ideal gas
b) Absolute temperature only depends on KE of the gas molecules
Adiabatically compress an ideal gas
a) Q = 0
b) DU = 0 –(-W) = +W
c) Temperature goes up
Adiabatically expand an ideal gas
a) Q = 0
b) DU = 0 –(+W) = -W
c) Temperature goes down
d) A refrigerator works this way. It expands a compressed gas to cool.
Gases at Constant Temperature
1) Isothermic Process = no change in temperature
DU = 0 for isothermal process for an ideal gas
If heat is added to the gas, work will be done by the gas
We compress a gas isothermally
a) Normally, if we compress a gas, temperature goes up
b) To keep temperature the same, we must remove heat (-Q)
c) Q = W
-Q = -W
d) We are removing heat that came from the work we did on the gas
Expand a gas isothermally
a) Add Q to expand
b) Work must be done by the gas as it expands (+W)
c) As long as Q = W, the temperature won’t change
Hot air balloon
1) Isobaric process = no change in Pressure
2) Flexible balloon with a certain amount of gas in it
3) Add Q, Temperature increases. To keep pressure same (1 atm), volume
must increase as well
4) Balloon expands, density decreases (d = m/V) and the balloon floats
III. The Flow of Heat
Conduction = Heat flow through a material
1) Rate depends on Thermal Conductivity
a) Different for each compound
b) Metals are very could heat conductors
c) Wood and air are “insulators”
2) Hold a metal block and a wood block
a) Body temp > metal = wood = room temp
b) Heat flows by conduction from hands to blocks
c) Metal feels colder because it removes heat from hand faster
3) Fiberglass insulation: air pockets that really prevent loss of heat
Convection = Transfer of heat due to motion of a fluid
1) Boiling water: hottest water rises and the coolest sinks
2) Flow of air in a heated room transfers heat
3) Warm air/water is less dense than cold air/water
a) Warm fluid rises
b) Cold fluid sinks
Radiation = Heat flow by electromagnetic radiation
1) Electromagnetic radiation is a wavelike form of energy
2) Light, microwave, radio waves are all forms of E. M.
3) Infrared waves (IR) is the form that heat travels in
Shorter than radio and longer than visible
4) Radiation requires no medium, it can travel through a vacuum
a) Thermos bottle won’t keep liquid hot forever
b) Sunlight reaches us through space
i. Greenhouse Effect
ii. Light can get in, IR can’t get out
iii. Increase in Global Temperature
Burning Coal
Or Petroleum