18.3
Thermometers and Temperature Scales
Any physical property that depends consistently and reproducibly on temperature can be used as the basis of a thermometer. For example, volume increas…
A thermometer is a device used to measure the temperature of a body or object.
Some of the most important types of thermometers are liquid-in-glass, bimetallic, digital, infrared, and gas thermometers.
Thermometers measure temperature on three main scales: Celsius, Fahrenheit, and Kelvin. These scales were designed based on the freezing and boiling points of water.
On the Celsius temperature scale, the freezing point of water is labeled as zero, whereas the boiling point of water is labeled as one hundred. There are one hundred degree intervals between these two points, which is why it is also known as the centigrade scale.
Here, the unit of temperature is degree Celsius.
On the Fahrenheit temperature scale, the freezing point of water is indicated at 32, while the boiling point is 212. The unit of temperature on this scale is degree Fahrenheit.
Temperature on the Fahrenheit scale can easily be converted to Celsius by employing the conversion equation.
The Kelvin temperature scale is an absolute temperature scale. Here, zero kelvin indicates absolute zero, which is the lowest possible temperature.
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Q1: What are the main types of thermometers used to measure temperature?
The most important thermometer types include liquid-in-glass, bimetallic, digital, infrared, and gas thermometers. Liquid-in-glass thermometers use alcohol or mercury that expands with temperature. Infrared thermometers measure radiation emission and are commonly used for body temperature measurement in the ear canal, offering greater accuracy than traditional methods.
Q2: How do alcohol thermometers work to measure temperature changes?
Alcohol thermometers rely on thermal expansion, where the liquid expands more rapidly than the glass encasing it. When temperature increases, alcohol from the bulb is forced into a narrow tube, producing a large visible change in column length for small temperature changes. This design amplifies the temperature signal for easy reading.
Q3: What is the difference between Celsius and Fahrenheit temperature scales?
On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees, creating 100 equal intervals also called centigrade. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees. Temperature can be converted between these scales using a mathematical conversion equation.
Q4: What does absolute zero represent on the Kelvin temperature scale?
The Kelvin scale is an absolute temperature scale where zero kelvin represents absolute zero, the lowest possible temperature. Unlike Celsius and Fahrenheit, which use arbitrary reference points based on water's freezing and boiling points, the Kelvin scale is grounded in fundamental physical principles and is essential for scientific calculations.
Q5: How are temperature scales created and standardized?
Temperature scales are created by identifying two reproducible reference temperatures, typically the freezing and boiling points of water at standard atmospheric pressure. These fixed points establish the scale's range and intervals. All three common scales—Fahrenheit, Celsius, and Kelvin—use this principle to ensure consistent, reliable temperature measurement across different thermometer types.
Q6: What physical properties can be used as the basis for thermometer design?
Any physical property that depends consistently and reproducibly on temperature can form a thermometer's basis. Common properties include volume expansion in liquids, electrical resistance, color changes, and infrared radiation emission. This variety of measurable properties explains why so many different thermometer types exist for specialized applications.
Q7: Why are infrared thermometers more accurate for measuring body temperature?
Infrared thermometers measure radiation emission, which varies predictably with temperature, providing rapid and accurate readings. When placed in the ear canal, they detect infrared radiation directly from the tympanum, avoiding contact errors. This method is more accurate than traditional alcohol thermometers placed under the tongue or armpit, which depend on thermal contact and can be affected by placement variations.