Phys141 Principles of Physical Science

Chapter 5 Temperature and Heat

Instructor: Li Ma

Office: NBC 126Phone: (713) 313-7028Email: malx@tsu.edu

Webpage: http://itscience.tsu.edu/ma

Department of Computer Science & PhysicsTexas Southern University, Houston

Sept. 27, 2004

Topics To Be Discussed

TemperatureHeatSpecific Heat and Latent HeatHeat TransferPhases of Matter

Heat &Temperature

Commonly used termsWell-known phenomena related to

them: cold when holding a piece of ice, hot near to the fire, warm when rubbing your hands or standing in sun shine, how is the weather for everyday, etc.

Very important concepts in our daily lifeThey have different & distinct meanings

Molecules of Substance

Most substances are made of very small particles called molecules, which are chemical combinations of atoms

Molecules are the smallest unit for substance

Temperature

It is a relative measure or indication of hotness or coldness

It is a measure of the average kinetic energy of the molecules of a substance

The temperature perception in our sense of touch is not reliable

Quantitatively measure the temperature: thermometer

Temperature (cont)

Thermometer: a instrument that utilizes the physical properties of materials for the purpose of accurately determining temperature

Thermal expansion: temperature-dependent property used to measure temperature

Liquid-in-glass thermometer

Temperature (cont)

Thermometers are calibrated so that numerical values can be assigned to different temperature

Two reference (fixed) points:– Freezing (ice) point & boiling (steam) point

A choice of unit:– Fahrenheit scale: 32°F/212°F/180 equal

units– Celsius scale: 0°C/100°C/100 equal units

Temperature (cont)

Absolute zero: the lower limit of temperature– -273°C, -460°F

Another temperature scale: Kelvin scale– Unit is kelvin (K)– 0 K is at the absolute limit– No negative temperature– The kelvin and the degree Celsius are

equal intervals

Temperature (cont)

Conversion between different temperature scales:– TK = TC + 273

– TF = 1.8TC + 32 or TC = (TF – 32)/1.8

Examples:– TF = 98.6°F (normal human body

temperature), what is it on Celsius scale?

– TC = (TF – 32)/1.8

= (98.6 – 32)/1.8 = 37°C

Heat

Heat is a form of energyMolecules of a substance have energy

– Vibrate, rotate, move – kinetic– The molecular bond, vibration and/or

rotation of atoms within molecules – potential

Internal energy:– Total energy (kinetic + potential) within an

object

Heat (cont)

Heat is net energy transferred from one object to another because of a temperature difference

Unit in SI: joule (J)Common and traditional unit: calorie

– The amount of heat necessary to raise one gram of pure water by one Celsius degree at normal atmospheric pressure

– 1 cal = 4.186 J (≈ 4.2 J), 1 kcal = 1000 cal

Heat (cont)

One effect of heating a material is expansion– Nearly all matter expands when heated

and contracted when cooledException of this rule

– When water is frozen, it expands– Ice at 0°C occupies a larger volume than

the same mass of water at 0°C

Heat (cont)

Heat expansion characteristics plays a major role in many aspects in our life:– Cracks in highway: the concrete will not

buckle in summer– Expansion joint designed into bridges– The first steam motor

Specific Heat

Heat is added to material, temperature may change

Same amount of heat is added to equal mass of different materials, the temperature increased may be different:– Same amount of heat is added, same amount of

internal energy is gained.– The internal forces of materials are different– The more internal energy goes into internal kinetic

energy, the higher temperature will increase

Specific Heat (cont)

The specific heat of a substance is the amount of heat necessary to raise the temperature of one kilogram of the substance one Celsius degree

Unit for specific heat is J/kg·°C or kcal/kg·°C

Water has a specific heat of 1.00 kcal/kg·°C (c = 1.00 kcal/kg·°C)– Table 5.1 on page 95

Specific Heat (cont)

Water has one of the highest specific heats and so can store more heat energy for a given temperature change– Use water to store solar energy collected

during the day time

amount ofheat to changetemperature

= mass xspecific

heatx

temperaturechange

H = mcΔTor

Latent Heat

Three phases of matter: solid, liquid and gas– The molecules of a gas are relatively father

apart than the molecules in a liquid or solidHeat is added to material, temperature

may not change, but phase changesDuring a phase change, the heat energy

goes into the work of separating the molecules, instead of increasing the molecular kinetic energy

Latent Heat (cont)

The latent heat is the heat associated with phase

The latent heat of fusion of the substance is the amount of heat necessary to change one kilogram of a solid into a liquid at the same temperature – This temperature is melting point (solid =>

liquid) or freezing point (liquid => solid)

Latent Heat (cont)

The latent heat of vaporization of the substance is the amount of heat necessary to change one kilogram of a liquid into a gas at the same temperature – This temperature is boiling point (liquid =>

gas)Sublimation & deposition

Latent Heat (cont)

amount ofheat to melta substance

= mass xlatent heatof fusion

H = mLfor

amount ofheat to boila substance

= mass xlatent heat

of vaporization

H = mLvor

Latent Heat (cont)

For water – Lf = 80 kcal/kg = 3.35 x 105 J/kg

– Lv = 540 kcal/kg = 2.26 x 106 J/kg

Example:– Calculate the amount of heat necessary to change

0.20 kg of ice at 0°C into water at 10°C

– H = Hmelt ice + Hincrease temperature

– H = mLf + mcΔT

= 0.20 kg x (80 kcal/kg + 1.00 kcal/kg·°C x 10 °C)

= 18.0 kcal

Heat transferThree methods of heat transfer:

– Conduction: transfer of heat by molecular collision

thermal conductor, thermal insulator

– Convection: transfer of heat by the movement of a substance from one position to another, heated gas or water

– Radiation: transferring energy by means of electromagnetic waves, heat got from Sun, no need of material medium

Phases of MatterThree common phases of matter: solid,

liquid & gas– Figure 5.5 on page 99– A solid has definite shape and volume– A liquid has a definite volume but no

definite shape– A gas has no definite volume or shape

Sometimes they are also referred to as states of matter