The correct option is **C. Liquid-in-glass thermometer**. ### Detailed Explanation 1. **Understanding the Liquid-in-Glass Thermometer**: - A liquid-in-glass thermometer consists of a glass tube that contains a liquid, usually mercury or colored alcohol. - As the temperature increases, the liquid expands and rises in the tube. Conversely, when the temperature decreases, the liquid contracts and falls back down the tube. - The scale on the thermometer is calibrated to correspond to specific temperature values, allowing for accurate readings. 2. **Why Option C is Correct**: - The defining characteristic of a liquid-in-glass thermometer is its reliance on the thermal expansion of a liquid. This principle is based on the fact that most liquids expand when heated and contract when cooled. - Mercury is often used because it has a high thermal conductivity, a wide range of liquid state (it remains liquid at a wide range of temperatures), and does not wet the glass, allowing for clear readings. - Alcohol is used in some thermometers because it is less toxic than mercury and can measure lower temperatures effectively. 3. **Why the Other Options are Incorrect**: - **A. Bimetallic thermometer**: - A bimetallic thermometer uses two different metals that expand at different rates when heated. These metals are bonded together, and as they expand, they bend, moving a pointer on a dial to indicate temperature. This method does not involve liquid expansion, making it different from the liquid-in-glass thermometer. - **B. Infrared thermometer**: - An infrared thermometer measures temperature from a distance by detecting the infrared radiation emitted by an object. It does not use any liquid and operates on a completely different principle, focusing on thermal radiation rather than thermal expansion. - **D. Thermocouple**: - A thermocouple consists of two different metals joined at one end. When the junction is heated, it produces a voltage that can be correlated to temperature. Like the infrared thermometer, it does not involve the expansion of a liquid, making it distinct from the liquid-in-glass thermometer. ### Formulas and Concepts - **Thermal Expansion**: The principle behind the liquid-in-glass thermometer can be described by the formula for linear expansion: \[ \Delta L = L_0 \alpha \Delta T \] Where: - \(\Delta L\) = change in length (or volume for liquids) - \(L_0\) = original length (or volume) - \(\alpha\) = coefficient of linear expansion (or volumetric expansion for liquids) - \(\Delta T\) = change in temperature ### Common Pitfalls - **Confusing Types of Thermometers**: Students often confuse different types of thermometers. It’s important to remember the specific mechanism each type uses to measure temperature. - **Assuming All Thermometers Use Liquid**: Not all thermometers use liquid; understanding the principles behind each type helps clarify their applications and limitations. ### Revision Summary - The **liquid-in-glass thermometer** measures temperature through the expansion of a liquid (mercury or alcohol). - **Bimetallic thermometers** use metal expansion, **infrared thermometers** measure radiation, and **thermocouples** rely on voltage generation. - Understanding the principles of thermal expansion is crucial for grasping how liquid-in-glass thermometers function. - Always differentiate between the mechanisms of various thermometer types to avoid confusion.