Temperature is a measure of the kinetic energy due to the movement of molecules in a substance.
As a substance gets warmer, its molecules travel faster and further.
Temperature is (roughly speaking) a measure of the average speed of the molecules in a substance.
Temperature is independent of mass.
· Temperature indicates tells how warm [or cold] a substance is.
· Warmth results from the average kinetic energy (motion) of moving atoms or molecules
How the conventional glass type works
· Put it in thermal contact with the substance to be measured.
· Soon it reaches thermal equilibrium with the substance (i.e., it becomes the temperature of the substance)
· Read the thermometer. In other words, look at the scale printed on the tube to measure the amount of mercury (or other liquid) has expanded from the bulb into the tube.
Thermometers have low heat capacity
· A good thermometer does not noticeably change the temperature of the substance to be measured.
· The temperature of the thermometer should reach the temperature of the substance with minimal exchange of heat energy.
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In other words, the thermometer should have low heat capacity
(little thermal inertia)
A thermometer tube expands very little when heated.
· When you read a thermometer, you really view the difference between the expansion of the liquid and the expansion of the tube.
· With narrow exceptions, substances expand when heated.
· So, as temperature increases, both the mercury and the tube expand.
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The tube of a good thermometer is made of a material that
expands much less than mercury when heated.
Thermostat
· A type of thermometer
· Made of a bimetallic strip
· Curves when temperature changes because top metal and bottom metal expand at different rates when heated
· Used to open and close electrical circuits, e.g., for air conditioner.
Thermometers are calibrated to show one or more Temperature Scales
· Celsius: 0 to 100 are freezing and boiling points of water.
· Kelvin (the SI unit): each Kelvin is the same size as a Celsius degree, but kelvins start at –273 C. This value is “absolute zero,” the temperature at which molecular movement ceases.
· Fahrenheit: 32 to 212 are freezing and boiling. Essentiall arbitrary.
Thermometers are based on the principle that thermal expansion is directly
proportional to temperature.
Consider two substances at the same temperature. Their the average molecular speed is identical. But the two may contain different amounts of internal energy due to different masses or heat capacities.
· Five grams of 90 degree water has more molecules, and thus more kinetic energy, than 1 gram of 90 degree water.
· One gram of 90 degree water (high heat capacity) has more internal energy than 1 gram of lead (low heat capacity).
Heat is the energy that transfers from one object to another because of a temperature differential.
Heat is energy in transit from a warmer thing and to a cooler thing. Similarly, water flowing through a hydroelectric dam does work, and the water behind the dam has more potential energy than that below it, but none of the water contains work.
Heat flows when two substances:
· are in thermal contact with one another, and
· have different temperatures.
Heat flow stops when the temperatures are the same (thermal equilibrium).
Heat flows in one direction: from higher temperature object to lower temperature object. Analogy: water only flows downstream.
An amount of heat transferred to a substance is measured by the magnitude of the resulting temperature change, the mass of the substance, and the heat capacity of the substance.
Calorie is commonly used unit for heat energy, even though the Joule is the SI unit for all types of energy.
One 1 calorie is the quantity of heat required to raise the temperature of one gram of water by one degree C (or Kelvin).
In food, kilocalories are called Calories
Specific Heat Capacity (or “specific heat”, or “heat capacity”) is the quantity of heat required to raise the temperature of a unit mass of a substance by one 1degree Celsius.
Indicates “thermal intertia” and stored energy
Some internal energy is kinetic (molecular movement), and some is potential.
As heat energy is added to a substance with low heat capacity, much of it increases molecular motion (i.e., temperature), while less of it is stored as potential energy.
As heat is added to a substance with a high heat capacity, little of it increases molecular motion, and more of it is stored as potential energy.
Only the kinetic energy increases temperature
In water, much of the energy is potential, so lots of heat has to be added to increase temperature (molecular movement)
This is slightly oversimplified explanation, but it’s good enough for our purposes. The speed and distance of molecular
Formula for the relationship between heat capacity, change in temperature, heat energy, and temperature change:
Lower rates in solids than liquids than gasses
With narrow exceptions (e.g., water), substances expand when heated.
Adding heat energy to a substance makes its molecules move faster
When molecules move faster, they also move further back and forth
So each molecule takes up more space when the substance is heated.
Cross references: see thermometer section, above.
The hole formed by a ring gets larger when the ring is heated.
· Consider the circular layer of molecules that line the inside of the ring or jar.
· The space occupied by each of these molecules gets larger when heated.
· The increased space increases the circumference of the inner circle.
· The same is true regardless of the thickness of the ring.
· Example: the inner circle of the lid of a jar expands when you put it under hot water.
Test 21 will probably have one problem (about 2 points) on ratio/proportion
applications of thermal expansion, like the yellow box on p. 318, and like
problems 46 and 47 (the third- and second-to-last Think and Solves). 47 is worked out below.
1/100,000 ∙ 20º ∙ 1,500,000 m = .3 meters (30 cm)
General rule: substances are more dense as a solid than as a liquid, and they are more dense as a liquid than a gas.
But water is an exception. Remember that ice floats. So the solid phase of water is less dense than the liquid phase. Why is water different?
· Ice has a crystalline structure, and liquid water does not.
· Ice crystals have a structure that occupies more empty space. (See Fig. 21.14.)
· As ice melts, the crystals collapse.
· Each gram water occupies less space as a liquid than as a solid, and thus water is more dense than ice.
Substances generally expand when heated.
· Molecular motion increases
· The space occupied by each molecule increases.
Substances generally become less dense when heated.
· When the space occupied by each molecule increases, so does the volume of every gram.
· So the mass/volume ratio decreases.
· Thus the density of the substance decreases.
Exception for water.
Space per molecule increases with heat.
But very cold water has some crystals, and these melt as temperature increases.
Between 0 and 4 degrees, the effect of the collapsing crystals is greater than the effect of the increased molecular volume.
So as water is heated from 0 to 4 degrees, its volume decreases, and it becomes more dense.
Water has high specific heat
So it has lots of potential energy and thermal inertia.
Examples
In Lab 50, the temperature of the lead weight changed more than the temperature of the water.
Hot water bottle stays warm longer than a brick.
. It was easier to change the temperature of the nails than the water.
Consider a hot water bottle. It stays hot for a long time. Similarly, if you heat a bowl of soup (mostly water) in a microwave, and you cover the bowl with a paper towel (low specific heat capacity) to prevent splattering, the soup will stay hot much longer than the paper towel.
Water and climate
· Generally, regions near large bodies of water have better climates.
· Water moderates climate
· Air flowing off water across land in effect a region more thermal inertia. (See figure 21.8.)
· Temperatures in such regions generally change slowly and do not reach extreme highs or lows.
Generally, substances are more dense as a solid than as a liquid, and they are more dense as a liquid than a gas. Water is an exception. Solid water is less dense than liquid water.
· Ice has a crystalline structure.
· The crystals have empty space in them.
· The crystals collapse when ice melts. (Liquid water is not crystalline.)
· Each gram water occupies less space as a liquid than as a solid.
Generally, substances become less dense as they are heated. Water does so above 4 degrees, but it becomes more dense when heated from 0 to 4 degrees.
· As with any material, the space occupied by each molecule of water increases when heated.
· So the volume of every gram of the structure increases.
· Thus the density of the substance decreases.
· This phenomenon holds true for water, even in the 0 to 4 range.
· But the increased occurs In the temperature range between
Very cold water contains microscopic (and crystalline) slush. The crystals tend to decrease density.
This reduces the density causes water to become less dense as it is heated. So it is less dense than , it is less dense than
Density of water
· Solid phase is less dense than liquid phase (unusual) due to from crystalline structure of ice
· Ice floats due to its lower density
· Between 0 and 4 degrees, water contains microscopic slush (some crystals). As the slush melts, water becomes more dense
· At temperatures above 4 degrees, water volume increases when heated.
Cold Lakes
· Ice cannot form unless all water in the lake is 4 degrees or below
· Because as water temperature at the surfaces gets close to 4 degrees, it becomes more dense than the rest of the water, and thus sinks to the bottom.
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