Temperature, Heat & Thermal Expansion

Temperature

Temperature of an object indicates average internal energy (due to molecular motion) of the object.

Temperature Scales

Physics 1 (Garcia) SJSU

Liquid NitrogenNitrogen gas becomes a

liquid at -320 ºF, which is still over 130 ºF warmer than absolute zero.

Total versus Average

The total amount of money in this room is probably around $1000.

The average amount of money per person is probably around $20.

Temperature of an object depends on the average amount of energy per molecule, not the total.

Bucket of warm water can have more internal energy than cup of hot water.

80 °F

100 °F

Internal EnergyInternal energy of an

object depends on:• Temperature• Mass• Material

1 kg

Temperature Internal

Energy

300 K 120,000 J

200 K 80,000 J

100 K 40,000 J

0 K 0 Joules

Iron

Temperature Internal

Energy

300 K 120 J

200 K 80 J

100 K 40 J

0 K 0 Joules

Iron

1 gram

Temperature Internal

Energy

300 K 1,200,000 J

200 K 800,000 J

100 K 400,000 J

0 K 0 Joules

Water

1 kg

1000 grams

Money and Happiness

Some people need a lot of money to make them happy. Some don’t.

Some materials, such as water, need a lot of energy to raise their temperature.

Some materials, such as iron, need little energy to raise their temperature.

Nicole Richie & Paris Hilton

MAHATMA GANDHI

Increasing Internal Energy

Can increase internal energy (and temperature) by tapping energy sources.

Chemical energy released in fire

Electric energy heats burner

Physics 1 (Garcia) SJSU

Work and Heat

May increase internal energy by exerting a force to do mechanical work.

Rub hands togetherfor warmth

Strike an iron surface with great force and red-hot sparks are created

Demo: Work and Heat

Increase internal energy (and thus temperature) by doing mechanical work on an object.

SHAKE

Bottle of Mercury

Temperature increases by a few degrees

Specific Heat Capacity

Specific heat capacity is the internal energy required to raise one gram of a material by one degree of temperature.

Filling has high specific heat capacityCrust has low specific heat capacity

Filling and crust at same temperature yet mouth burned only by the filling.

Demo: Sparklers

Iron burns red-hot at a temperature of 5000 ºF

Sparks from sparkler don’t burn skin because they have very little energy (small mass and low heat capacity).

Walking on red-hot embers is safe for the same reason.

Demo: Moses Money

Take a dollar bill, soak it in a 50/50 solution of isopropyl alcohol and water; set it on fire.

High specific heat of water prevents the bill from burning. Heat produced by the burning alcohol only enough to warm the water.

Check Yourself

Why does a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a cooler on a hot day?

Why is it that the climate in the desert is so hot during the day yet so cold at night?

Demo: Boil Water in Paper Cup

Because of high heat capacity of water, the large amount of heat added by the flame raises the temperature of the water until boiling.

If the cup is filled with sand instead of water then it burns quickly.

Cup with sand

Cup with water

Thermal Expansion

Due to increased molecular motion, most materials expand as temperature increases.

Sidewalk buckles and cracks due to expansion on a hot summer day

Space allows for expansion

Demo: Slowing Air Molecules

Cool balloon using liquid nitrogen

Air molecules slow down and lose kinetic energy

Balloon slowly warms up,

restoring energy

Balloon returns to its original state

Demo: Expansion of a Ring

Metal ball barely fits past the metal ring.

Not surprising that heated ball won’t pass through cold ring.

Will cold ball pass through heated (expanded) ring?

Demo: Bi-metallic Strip

Different materials have different rates of expansion.

STEEL

Brass

Brass expands more than steel when heated

Thermostat

Bi-metallicSpiral strip

HOT COLD

Demo: Heat, Cool, Break

COOL (quickly)

HEAT

Glass expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass.

GLASS

Cracks form

Demo: Freeze Solid

Materials become brittle when frozen solid.

Organic materials appear solid but cells contain large amounts of liquid water.