Download - Chapter 14--Heat
Chapter 14--HeatSections 1-9
14.1 Heat as Energy Transfer
Two objects at different temperatures transfer heat; hotcold
heat flow spontaneously from hot to cold
14.1 Heat as Energy Transfer
calorie(c or cal)--amount of heat necessary to raise the temperature of 1-g of water 1C
Calorie (C) or kilocalorie (kcal)--1000 calories; dietary calorie
BTU (British thermal unit)--heat needed to raise 1 lb. of water 1 F
0.252 kcal = 1 BTU = 1055 J
14.1 Heat as Energy Transfer
Mechanical equivalent of heat James Joule conducted experiments to show
that work is done by heat transfer (& vise versa)
4.186 J = 1 cal joule =SI energy unit
14.1 Heat as Energy Transfer
Heat energy that is transferred from one body to
another due to a difference in temperature
14.2 Distinction Between Temperature, Heat and Internal Energy
Thermal energy (internal energy) sum of all the energy of all the molecules in
an object units = joules total energy of all the molecules in an
object
14.2 Distinction Between Temperature, Heat and Internal Energy
Temperature measure of the average kinetic energy of
individual molecules units = kelvin
14.2 Distinction Between Temperature, Heat and Internal Energy
Heat transfer of energy (such as thermal energy)
from one object to another due to a temperature difference
14.2 Distinction Between Temperature, Heat and Internal Energy If 50-g of water at 30C is mixed with 200-g
of water at 25C heat will flow from water at 30C to water at 25C even though there is more internal energy in 200-g of water (25C ) than in 50-g water (30C)
14-3 Internal Energy of an Ideal GasInternal energy of an ideal monatomic gas U = N(1/2mv2) = 3/2NkT = 3/2 nRT where:
• U = internal energy (J)• N = # of atoms• m = mass of atom• v = average speed• n = number of moles• T = temperature (K)• k = 1.38 x10-23 J/K (Boltzmann constant)• R = 8.315 J/molK or 0.0821 L atm/mol K
14-3 Internal Energy of an Ideal Gas Internal energy of an ideal gas T and n if the gas is polyatomic then rotational and
vibrational energy (?) of the molecules must be taken into account (U will be greater than it is for monatomic gases but still T)
When Heat is added to a substance it either: Increases its temperature
• Energy goes into increasing Kinetic Energy—causes particles to move faster
Or Changes its phase
• Energy goes into increasing Potential Energy—causes particles to become further apart (overcome intermolecular forces)
14.4 Specific Heat
Q = mcT where: Q = heat lost/gained m = mass of substance (kg) T = change in temperature (T2- T1) c = specific heat ***this heat added causes a change in
temperature***
14.4 Specific Heat
Specific Heat characteristic of material (changes slightly
with temperature) water: 1kcal/kgoC or 4186 J/kg oC (20oC) ice: 2100 J/kg oC (-5oC) steam: 2010 J/kg oC (110oC)
14.4 Specific Heat
When: Q = -; T = -; heat is transferred out of
substance Q = +; T = +; heat is transferred into of
substance
14.5 Calorimetry
In an isolated system energy is conserved If heat is lost by one component of the system it is gained by
another component of the system. -(Q+) = +(Q-) maca(T2-T1) = - mbcb(T1-T2)
14.5 Calorimetry
Calorimetry Technique used to quantitatively measure heat
exchangeCalorimeter Device used to quantitatively measure heat
exchange within a system Used to find specific heat of an unknown substance Well insulated so no heat exchange with
environment
14.5 Calorimetry
If 200 mL of tea at 95oC is poured into a 150 g cup at 25oC what will be the final temperature of the tea?
(cwater = 4186 J/kgoC cglass = 840 J/kgoC )
14.5 Calorimetry
Answer: 85.8oC
14.6 Latent Heat
Latent Heat Heat required to change the phase of a
substance solid liquid, liquid gas Units: kJ/kg, J/kg, J/g, Kcal/kg
14.6 Latent Heat
Latent HeatTwo types: Heat of fusion—heat required to melt 1kg
of a substance• Lf H2O = 333 kJ/kg
Heat of vaporization—heat required to vaporize 1 kg of a substance• Lv H2O =2260 kJ/kg
14.6 Latent Heat
Heating Curve A graph of Temperature vs. Heat added Shows what happens as heat is added to a
sample of substance
14.6 Latent HeatHeating Curve:Water What happens in
each part? What equation do
we use to find heat?
T
Q
14.6 Latent Heat
How much heat is needed to change the temperature of 1,350 mL of water from –17°C to 145°C?
14.6 Latent Heat
Answer: 4.24 x 106 J
There are three ways heat can move from one place to another: Conduction Convection Radiation
14.7 ConductionConduction Heat moves in an object without the net movement
of its particles. Results of molecular collisions Hot molecules collide with cold molecules and ???? Only occurs when there is a temperature difference Rate of flow temperature difference, area in
contact , 1/length(of motion)
14.7 Conduction
Conductors Substances where heat moves quicklyInsulators Substance when heat moves slowly
14.7 Conduction
At room temperature why does a carpet feel warmer than a tile floor?
Why are storm doors so good at insulating if glass is a good conductor of heat?
Why do you layer clothes to keep warm?Building materials: R-Value—rates how
resistant a material is to the flow of heat. R = l/k
14.8 Convection
Convection Heat is transferred by the mass
movement of molecules from one place to another
Forced vs Natural convection Convection Currents
14.8 Convection
Convection In the human body 20% of food energy is
used to do work, 80% goes into heat If this heat was not dissipated body
temperature would increase by 3°C per hour Blood moves heat by convection to beneath
skin and it is conducted to the surface
14.9 Radiation
Radiation Transfer of heat without a medium Most heat from fire comes by way of
radiation Infrared Radiation form the Sun is
responsible for heating the Earth
14.9 Radiation
Stefan-Boltzmann equation Shows the rate at which an object radiates
energy
Q/t = eAT4 where: = Stefan-Boltzmann constant (5.67 x 10-8 W/m2•K4)• e = emissivity; number between 0 &1 that shows how
well an object absorbs (emits) radiation (1 = pure black)
14.9 Radiation
Stefan-Boltzmann equation So rate of emission of radiation
(Q/t) is proportional to Temperature4!
14.9 Radiation
Radiation from the Sun strikes the Earth at 1350 J per second per square meter or 1350 W/m2 (solar constant)
During cloudy conditions about 70% reaches ground when clear 1000W/m2