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Exploring why materials change size when their temperature changes and the practical implications.
Have you ever wondered why bridges have strange metal 'teeth' in the road, or why a tight glass jar lid suddenly pops open after being held under hot water?
According to the Kinetic Molecular Theory, all matter is made of particles in constant motion. When we add heat to a substance, we are increasing its thermal energy. This causes the particles to move faster and vibrate more vigorously. As they collide with more force, they push their neighbors further away. This increase in the average distance between particles results in thermal expansion. When the substance cools, particles lose energy, move less, and settle closer together, leading to thermal contraction.
Quick Check
If the particles in a solid start vibrating with less kinetic energy, what happens to the overall volume of the solid?
Answer
The solid undergoes thermal contraction and its volume decreases.
In solids, we often measure linear expansion, which is the change in length. The amount a solid expands depends on its original length , the change in temperature , and the material's unique coefficient of linear expansion . Different materials expand at different rates because their internal chemical bonds vary in strength. For example, aluminum expands much more than glass when heated by the same amount.
A steel rod is long at . If it is heated to , how much will it grow? (Steel )
1. Identify variables: , . 2. Use the formula: . 3. Calculate: . 4. Result: (or ).
Liquids generally expand more than solids because their particles are less tightly bound. This is the principle behind thermometers: as the liquid (like alcohol or mercury) warms, it expands and is forced up a narrow tube. Gases expand the most of all because their particles have almost no attractive forces holding them together. If you leave a basketball outside on a cold night, it appears 'flat' not because it leaked, but because the air inside contracted.
A thermostat uses a strip made of brass bonded to iron. Brass has a higher than iron.
1. When heated, the brass side wants to grow longer than the iron side. 2. Since they are stuck together, the strip is forced to curve. 3. The strip curves toward the iron side (the material that expands less). 4. This mechanical movement flips a switch to turn off the heater.
Quick Check
Why does the liquid in a thermometer rise when placed in hot water?
Answer
The liquid particles gain kinetic energy and move further apart, increasing the liquid's volume and forcing it up the tube.
Engineers must account for expansion to prevent disasters. Bridges use expansion joints (the 'teeth' mentioned earlier) to allow segments to grow in summer without buckling. Power lines are hung with a 'slack' so they don't snap when they contract during freezing winters. Even railroad tracks need small gaps to prevent the steel from warping under the hot sun.
You have a glass jar with a stuck metal lid. You run hot water over the lid to loosen it. Why does this work?
1. Both the glass and the metal lid receive heat. 2. Metals typically have a much higher coefficient of expansion () than glass. 3. The metal lid expands significantly more than the glass rim of the jar. 4. The gap between the lid and the glass increases, breaking the friction seal.
Which state of matter typically exhibits the greatest amount of expansion for the same increase in temperature?
What does the symbol represent in the expansion formula?
Thermal contraction occurs because particles get smaller when they lose heat.
Review Tomorrow
In 24 hours, try to explain to a friend how the Kinetic Molecular Theory explains why a bridge gets longer in the summer.
Practice Activity
Look at a concrete sidewalk. Can you find the 'expansion cracks' filled with flexible material? Imagine what would happen to the concrete if those gaps were filled with solid rocks instead.