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 Nitrogen Nitrogen 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 Energy Internal energy of an object depends on: Temperature Mass Material 1 kg TemperatureInternal Energy 300 K120,000 J 200 K80,000 J 100 K40,000 J 0 K0 Joules Iron TemperatureInternal Energy 300 K120 J 200 K80 J 100 K40 J 0 K0 Joules Iron 1 gram TemperatureInternal Energy 300 K1,200,000 J 200 K800,000 J 100 K400,000 J 0 K0 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 together for 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 capacity Crust 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-metallic Spiral 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.