13.2
Gęstość
Gęstość to ważna cecha substancji, kluczowa dla określenia, czy obiekt tonie, czy pływa w płynie. Jego jednostką SI jest kg/m3, a jednostką cgs jest g…
Jeśli drewniana kłoda i kamień o tej samej masie zostaną wrzucone do rzeki, kamień tonie, podczas gdy kłoda unosi się na wodzie. Wynika to z fizycznej właściwości materiału znanej jako gęstość.
Gęstość jest definiowana jako masa obiektu na jednostkę objętości i może pomóc w określeniu, czy obiekt będzie unosił się na wodzie, czy tonie w płynie.
W powyższym przykładzie gęstość kamienia jest większa niż gęstość wody, a więc tonie. Natomiast gęstość drewna jest mniejsza, a więc unosi się na wodzie.
Obiekty z tego samego materiału o różnych masach mają tę samą gęstość.
Jeśli obiekt ma różną gęstość w swojej objętości lub dla niejednorodnej mieszaniny dwóch lub więcej różnych materiałów, definiujemy gęstość średnią.
Średnia gęstość tych obiektów jest równa całkowitej masie podzielonej przez jego całkowitą objętość.
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Q1: What is density and how is it calculated?
Density is a physical property defined as the mass of an object per unit volume. It is calculated by dividing an object's total mass by its total volume. The SI unit for density is kg/m³, while the CGS unit is g/cm³. Density helps determine whether an object will float or sink in a fluid, connecting to density and archimedes principle.
Q2: Why does a stone sink while a wooden log floats in water?
Objects sink or float based on their density relative to the fluid. The stone sinks because its density is greater than water's density, causing it to be denser and heavier per unit volume. The wooden log floats because its density is lower than water's density. This principle applies regardless of the objects' masses.
Q3: What is the difference between homogeneous and heterogeneous substances?
A homogeneous substance has constant density throughout its volume, such as a solid iron bar where any sample has the same density as the average. A heterogeneous substance has varying density, like Swiss cheese containing both solid material and gas-filled voids. For heterogeneous materials, local density describes the density at specific locations.
Q4: How does temperature affect the density of liquids and gases?
Gases have densities that vary considerably with temperature because they expand and contract freely. Liquids, however, have densities that vary little with temperature, so their densities are often treated as constant and equal to their average density. This difference reflects the structural differences between gases and liquids.
Q5: Why do gases have much lower densities than liquids and solids?
Gases have much lower densities than liquids and solids because their atoms are separated by large amounts of empty space. In contrast, the atoms in liquids and solids are in close contact, making their densities roughly comparable. This structural difference explains the significant density variation between phases of matter.
Q6: What is specific gravity and why is it useful?
Specific gravity is a dimensionless quantity defined as the ratio of a material's density to water's density at 4.0°C and one atmosphere of pressure, which is 1000 kg/m³. It provides a convenient, unit-free way to compare densities of different substances without worrying about unit conversions, making material comparisons straightforward.
Q7: Can objects with the same mass have different densities?
No, objects of the same material with different masses have the same density because density is a material property independent of mass. However, objects made of different materials can have the same mass but different densities. For example, a small stone and a large piece of wood might weigh the same but have different densities.