UNIT 5: HEAT

• What is heat?

• What is temperature?

• How are these related to each other?

KINETIC MOLECULAR THEORY

• All matter consists of tiny particles

• These particles are in constant motion

• The amount of motion is proportional to temperature. Increased temperature means increased motion

• ex water- solid when below zero, liquid when 0-100 degrees and steam above 100

• Solids, liquids and gases differ in the amount that their particles move and how much they interact

KINETIC MOLECULAR THEORY

HEAT

• Is the thermal energy transferred from one object to another due to differences in temperatures. Measured in Joules (J)

Qheat= Ethermal

where Qheat = amount of heat;

E= change in thermal energy

TEMPERATURE

• Is a measure of the average kinetic energy of the molecules of a substance. Can be measured with a thermometer

• Measurements of a thermometer:

• Celsius (oC)- freezing point is 0oC and boiling point is 100 oC

• Kelvin Scale (K)- sets absolute zero (temperature where no motion occurs) as 0 K (-273.15oC).

K= oC +273 or oC= K-273

TEMPERATURE CONVERSIONS• Convert these temperature from Celsuis to Kelvin

• 27oC = 300 K

• 560 oC = ? K

• -184 oC = ? K

• -273 oC = ? K

• Convert these temperatures from Kelvin to Celsius

• 110K = -163 oC

• 22 K = ? oC

• 402 K= ? oC

• 323 K = ? oC

THERMAL EXPANSION

• When matter is heated, the particles move faster and move apart from each other. This causes the volume to increase. We call this expansion.

• When matter is cooled, the opposite happens and we get contraction.

EVERYDAY EXAMPLES

• Air pressure

• Tires have less pressure in the cold temperatures

• Experiment: two balloons about the same volume- what will happen when one balloon is put outside in the cold

EVERYDAY EXAMPLES

• Bridges and sidewalks are built in segments. They have spaces called expansion joints between them. The expansion joints allow the concrete and steel to expand without buckling and cracking.

• The thumping sound you hear when you drive over a bridge in a car or bus is the sound of the tires going over the expansion joints.

EVERYDAY EXAMPLES

• Thermometers- liquid in a thermometer will expand with increased temperatures making them rise

• Global warming is causing ocean levels to rise (due to the expansion of the volume of water) threatening cities at sea level

THERMAL EXPANSION

• Substances vary in the amount of thermal expansion

• Demo in small groups- ring and ball demo

- compound bar

LINEAR EXPANSION- FOR A SOLID

• Depends on the initial length, temperature change, and the type of materials it is made from

• Change in length (Δ L) is directly proportional to the change of temperature (Δ T) and the original length (Lo)

ΔL= α Lo * Δ T

Where: L= length in meters (m); α = coefficient of linear expansion; Lo= original length (m); Δ T= change in temperature

EXAMPLE

• A lead rod is 2.5 m long at 20 oC. What is the change in length if it is heated to 100 oC and α = 27 x 10 -6 oC-1.

SOLUTION

• L = Lo T

= 27 x10-6 oC-1 x 2.5 m x 80.0 oC

= 0.0054 m

VOLUME EXPANSION- FOR GASES

• ΔV= βVo ΔT

Where: ΔV- change in volume (m3 or L)

β – coefficient of volume expansion

Vo – original length (m3 or L)

ΔT- change in temperature

Note: β is different for different materials

EXAMPLE

• 1000 L of gasoline is in a tank at 20oC. What is the change in volume when the temperature drops to -20oC?( β = 950 x 10-6 oC)

SOLUTION

• V= V0T

= 950x10-6oC-1 X 1000L X -40oC

= -38 L

ASSIGNMENT

• do expansion practice problems- check answers as you go

• Expansion problems- do #1-8