Heat, Temperature, and Expansion

Chapter 21

All matter is composed of atoms which are constantly in motion. Previously we learned that kinetic energy or “energy of motion” can be expressed with the equation: http://phet.colorado.edu/en/simulation/states-of-matter-basics

KE = ½ mv2

Therefore, it makes sense that the faster atoms or molecules move, the more kinetic energy they contain. We can increase this motion many different ways. Here are some examples:

• stirring a liquid• passing an electric current through a wire• increasing the pressure of a gas• hitting a penny with a hammer• friction between two surfaces

The temperature of the atoms and molecules in all of these examples is represented as their average kinetic energy.

Temperature Scales:

Fahrenheit scale - named after German physicist Daniel Gabriel Fahrenheit, this scale was proposed in 1724 and is still used in the United states but has been replaced by the Celsius scale in most countries. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees.

Celsius scale – named after the Swedish astronomer Anders Celsius, this scaled is used by most countries. Water freezes at 0 degrees on this scale and boils at 100 degrees.

Kelvin Scale – absolute temperature scale named after Lord Kelvin who introduced this scale in 1848. Water freezes at 273.15 degrees Kelvin and boils at 373.15 degrees. Zero degrees Kelvin on this scale corresponds to a state in which atoms of a substance have zero kinetic energy. This state is called absolute zero.

Temperature Scales:

It is incorrect to say that matter contains heat. All atoms and molecules are in constant motion, and contain energy by virtue of their motion (kinetic energy).

calorie = amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius

1 Calorie = 1000 calories = 1 kilocalorie

4.184 joules = 1 calorie

Specific heat = amount of energy needed to raise the temperature of 1 gram of a substance by 1 degree Celsius

Heat capacity = amount of energy needed to raise the temperature of a substance by 1 degree Celsius. ***NOTE HEAT CAPACITY DEPENDS ON THE MASS OF THE SUBSTANCE*** 1 gallon of water has more heat capacity than 1 cup of water.

Specific Heat vs. Heat Capacity

Compare the cup of water to the ocean on the right:

Which has the greater specific heat? Answer: Neither. The specific heat in both places is 4.18 J/g C ̊

Which has the greater heat capacity? Answer: The lake. There is more water in the lake which can absorb more total heat energy.

Cup of Water Lake

Equation for specific heat:

Q = mC∆T

Q = heat in joulesm = mass of substance C = specific heat of substance∆T = change in temperature (Tf – Ti)

(Tfinal – Tinitial)

Thermal Expansion

Different materials expand at different rates. In a bimetallic strip, two sides of different materials, say one of brass and the other of iron are welded together. When the strip is heated, the difference in the amounts of expansion of brass and iron shows up easily.

A thermostat is a practical application of a bimetallic strip.

Bimetallic strip used in a thermostat:

Above picture: Expansion Joint on a bridge

Expansion of Water

• Solids, liquids, and gases expand while heated and contract when cooled.

• Water is an exception to this rule.

• Water is less dense as solid than it is as a liquid.

*** Note water molecules are closer together in liquid form ***