Understanding Density
The density of a material, usually represented by the Greek letter ρ (rho), measures how much mass is contained in a unit of volume. It is calculated using the simple formula:
Where:
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ρ is the density
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m is the mass
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V is the volume
Calculating density is straightforward, but it’s crucial to pay attention to units. Using inconsistent units or failing to convert them properly can lead to incorrect results. Always write down the values clearly along with their units, and perform dimensional analysis to ensure that your final result is consistent.
Factors Affecting Density
Density can also vary depending on pressure and temperature:
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Solids and liquids: Density changes are generally small.
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Gases: Density is highly sensitive to temperature and pressure. Increasing pressure decreases volume and increases density, while increasing temperature generally increases volume and decreases density.
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Exceptions: For example, water’s density increases between 0°C and 4°C.
Common Units of Density
Density can be expressed in many units. Below are some of the most commonly used:
| Unit | Conversion to kg/m³ |
|---|---|
| kilogram/cubic meter (kg/m³) | SI Unit |
| gram/cubic centimeter (g/cm³) | 1000 |
| kilogram/liter (kg/L) | 1000 |
| gram/liter (g/L) | 1 |
| pound/cubic foot (lb/ft³) | 16.02 |
| pound/cubic inch (lb/in³) | 27,680 |
| ounce/cubic foot (oz/ft³) | 1.001 |
| ounce/cubic inch (oz/in³) | 1,730 |
| pound/gallon (US) | 119.83 |
| pound/gallon (UK) | 99.78 |
| ton (short)/cubic yard | 1,186.6 |
| ton (long)/cubic yard | 1,328.9 |
Density of Common Materials
Here are approximate densities of some well-known materials (in kg/m³):
| Material | Density |
|---|---|
| Earth’s atmosphere at sea level | 1.2 |
| Water (standard temperature & pressure) | 1,000 |
| The Earth | 5,515 |
| Iron | 7,874 |
| Copper | 8,950 |
| Tungsten | 19,250 |
| Gold | 19,300 |
| Platinum | 21,450 |
| Atomic nuclei | 2.3 × 10¹⁷ |
| Black hole | > 1 × 10¹⁸ |