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