liquid in glass thermometer class 9 | construction and working of liquid in glass thermometer| FBISE

Welcome to my channel ''Lectures of Physics". This channel contains lectures of physics on class 9th, 10th, 11th and 12th in easy way. This video is about Liquid in glass thermometer class 9 physics FBISE NBF. construction and working of liquid in glass thermometer is discussed. Liquid-in-Glass Thermometer: Construction, Principle, and Working A liquid-in-glass thermometer is one of the most common instruments used to measure temperature. It works on the principle that liquids expand when heated and contract when cooled. Because the volume of a liquid changes with temperature, this change can be used to measure temperature accurately. Construction A liquid-in-glass thermometer mainly consists of the following parts: Glass Bulb At one end of the thermometer there is a small bulb made of thin glass. This bulb acts as a container for the thermometric liquid. The thin glass allows heat from the surroundings to transfer quickly to the liquid inside. Thermometric Liquid The bulb is filled with a liquid such as mercury or alcohol. These liquids are chosen because they expand uniformly with temperature and are easily visible in the tube. Mercury is commonly used because it has a clear meniscus, expands uniformly, and does not stick to glass. Alcohol is used when very low temperatures need to be measured because its freezing point is very low. Capillary Tube A very narrow glass tube called the capillary tube is attached to the bulb. The thermometric liquid rises and falls inside this tube when the temperature changes. The narrow diameter of the capillary tube increases the sensitivity of the thermometer because even a small expansion of the liquid causes a noticeable rise in the liquid column. Glass Stem (Outer Glass Tube) The capillary tube is enclosed in a thicker glass stem which protects the capillary and provides support to the instrument. Scale Along the stem, a graduated scale is marked. The scale is usually calibrated in degrees Celsius (°C) or Fahrenheit (°F). The markings indicate the temperature corresponding to the height of the liquid column. Principle of Liquid-in-Glass Thermometer The working of a liquid-in-glass thermometer is based on the thermal expansion of liquids. When temperature increases, the molecules of the liquid gain kinetic energy and move farther apart. As a result, the volume of the liquid increases. Since the bulb and capillary tube are made of glass and have fixed volume, the expanding liquid rises in the narrow capillary tube. When temperature decreases, the liquid molecules lose energy and move closer together. The volume of the liquid decreases, causing the liquid column to fall in the capillary tube. Because the expansion of the liquid is approximately proportional to the temperature change, the height of the liquid column can be used to measure temperature. Working of Liquid-in-Glass Thermometer When the thermometer is placed in contact with a body or environment whose temperature is to be measured, heat transfer occurs between the body and the thermometer bulb. If the surrounding temperature is higher than the temperature of the liquid in the bulb, heat flows into the bulb through the glass. As the liquid absorbs heat, its temperature increases and it expands. This expansion forces the liquid to move upward through the narrow capillary tube. The higher the temperature, the higher the liquid column rises. If the surrounding temperature is lower, heat flows out of the liquid. The liquid then contracts, and the liquid column moves downward in the capillary tube. The level of the liquid column is compared with the calibrated scale marked on the thermometer. The point where the top of the liquid column stops indicates the temperature of the body or environment. Thus, the thermometer measures temperature by converting temperature changes into changes in the height of a liquid column. Conclusion A liquid-in-glass thermometer is a simple, reliable, and widely used temperature-measuring device. Its operation depends on the thermal expansion and contraction of a liquid such as mercury or alcohol. Due to its clear construction, sensitivity, and accurate scale, it has been used for many years in laboratories, medical measurements, and environmental temperature monitoring. For more amazing lectures please subscribe our channel. For more lectures of class 9th visit link below h #lectures of physics