Chapter 9 Thermal Energy

Thermal Energy

“the total energy of all its atoms and molecules as they wiggle & jiggle, twist & turn, vibrate or race back and forth”

Temperature is the measure of the average kinetic energy in a substance

Measure of the total energy (potential & kinetic) in a substanceVaries with the number

of molecules

50 ml of warm water and 100 ml of warm water at the same temperature. Compare the thermal energies.

Thermometer

Measures temperature by expansion or contraction of a liquid, usually colored alcohol (transfer of thermal energy)

Temperature has no upper limit Temperature has a lower limit

Absolute zero: -273ºC

Heat

The thermal energy transferred from one substance to another due to a temperature difference between the two substances

Direction of thermal energy flow is always from a warmer substance to a cooler one

Matter doesn’t contain heat, it contains thermal energy

Measured in joules

Touch the inside of a 220°C hot oven and burn yourself. But when the 1800°C white hot sparks form a 4th-of-July-type sparkler hit your skin, you're okay. Why?

Cold

An object is cold because it lacks thermal energy; cold is the result of lowered thermal energy

Calorie

Another unit used to measure heat Amount of heat needed to change 1 gram

of water by 1ºC 1 cal = 4.2 J

Laws of Thermodynamics

1st law – whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred

2nd law – heat never flows from a cold substance to a hot substance

3rd law – no system can reach absolute zero

Specific heat capacity The amount of heat required to change the

temperature of a unit mass of the substance by 1 degree

What a substance does with the thermal energy that it acquires (stores the energy as potential or uses it as kinetic energy – raises the temperature)

Examples: apple pie filling vs crust; soup vs toast; boiling water vs milk

Thermal Expansion Most substances expand when heated (because

molecules in substance “jiggle faster” and move farther apart) and contract when cooled

Different substances expand at different rates Bimetallic strips

Liquids expand more than solids with increases in temperature

When you can’t loosen a metal lid on a glass jar, how can you use the concept of thermal expansion to rescue the situation?

A Concorde supersonic airplane is 20 cm longer when in flight then when parked on the ground. Explain.

When the temperature of a metal ring increases, does the hole become larger, smaller, or stay the same?

When the temperature of the piece of metal is increased and the metal expands, will the gap between the ends become narrower, or wider, or remain unchanged?

Expansion of water - heating Water expands when heated except in the temperature range

between 0ºC and 4ºC Figure 9.19 Ice has a crystalline structure with open

structured crystals which takes up a greater volume than molecules in the liquid phase

0ºC - 4ºC collapsing ice crystals causes a decrease in volume as the molecules begin moving faster and filling in the empty spaces

After 4ºC expansion overrides contraction because most ice crystals have collapsed

Expansion of water - cooling Figure 9.22 as water cools, it sinks until the

entire pond is at 4ºC. Then, as water at the surface is cooled further, it floats on top and can freeze. Once ice is formed, temperatures lower than 4ºC can extend down into the pond

Very deep bodies of water are not ice-covered even in the coldest of winters

Because of water’s high specific heat and poor ability to conduct heat, the bottom of deep bodies of water in cold regions remain at a constant 4ºC year round