Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Heat Energy (i) Temperature and it's measurement
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Heat Energy (i) Temperature and it's measurement
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Subject: Physics
Class: Senior Secondary 2
Term: 1st Term
Week: 6
Theme: Conservation Principles
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Students should be ableto: Construct a device for measuring the temperature of abody.
Use the variation of:(a) pressure of a gas with temperature;(b) the expansion of solid, liquid or gaswith temperature;(c) electrical resistance ofa material to measurethe temperature of abody Distinguish betweenheat and temperatureand betweentemperature pointand temperatureintervals. Select those liquidswhich are suitable for use in liquid-in-glassthermometers from aglass from a given listof liquids and the irproperties.
Heat: Heat is a form of energy that is transferred from a region of higher temperature to a region of lower temperature due to a temperature difference. It is a measure of the total kinetic energy of the molecules in a substance. The SI unit of heat is the Joule (J). It depends on the amount of substance present (e.g., a bathtub of lukewarm water has more heat energy than a cup of boiling water).
Temperature: Temperature is a measure of the degree of hotness or coldness of a body. It is a measure of the average kinetic energy of the molecules within a substance. Temperature determines the direction of heat flow. The SI unit of temperature is the Kelvin (K), though Celsius (°C) is commonly used. Temperature is independent of the amount of substance (e.g., a cup of boiling water and a pot of boiling water both have the same temperature, 100°C).
Analogy: Think of water in a tank. The amount of water represents heat (total energy), while the pressure of water at the bottom represents temperature (average energy, determining flow direction).
Temperature Point: Refers to a specific value on a temperature scale, representing the hotness or coldness of a body at an instant. Examples include 0°C (freezing point of water) or 100°C (boiling point of water) or 37°C (normal human body temperature).
Temperature Interval: Refers to the difference between two temperature points. It indicates a change in temperature. For instance, the interval between 20°C and 30°C is 10°C. Temperature intervals are the same on the Celsius and Kelvin scales (e.g., a 10°C change is equal to a 10 K change). A thermometric property is a physical property of a substance that changes uniformly and measurably with temperature. These changes are used to indicate temperature.
Common thermometric properties include: (a) Expansion of Solids, Liquids, and Gases: Most substances expand when heated and contract when cooled.
Liquids: Mercury and alcohol expand uniformly when heated, making them suitable for liquid-in-glass thermometers.
Gases: Gases expand significantly with temperature changes. The volume of a gas at constant pressure, or the pressure of a gas at constant volume, varies linearly with temperature. This principle is used in gas thermometers, which are very accurate.
Solids: The expansion of solids is used in bimetallic strips (e.g., in thermostats for irons or ovens), where two different metals with different expansion rates are bonded together. When heated, the strip bends, completing or breaking a circuit. (b)
Electrical Resistance of a Material: The electrical resistance of most metals increases with temperature. This property is used in resistance thermometers (e.g., platinum resistance thermometer) and thermistors (semiconductor devices whose resistance changes significantly with temperature). (c)
Pressure of a Gas with Temperature: For a fixed mass of gas at constant volume, its pressure is directly proportional to its absolute temperature (Gay-Lussac's Law). This is the principle behind constant-volume gas thermometers. (d) Electromotive Force (e.m.f.): When two different metals are joined at two junctions, and these junctions are maintained at different temperatures, a small e.m.f. is generated. This is the Seebeck effect, used in thermocouples (e.g., in industrial furnaces, engine exhaust monitoring). (e)
Colour Change: Some materials change colour with temperature (e.g., liquid crystals in forehead thermometers, thermochromic paints). This is less precise but provides a visual indication. These are the most common type. They consist of a glass bulb connected to a fine capillary tube. The bulb contains a liquid (usually mercury or alcohol) which expands or contracts with temperature changes, causing the liquid column to rise or fall in the capillary tube.
Construction: Glass Bulb: Thin-walled to allow quick heat transfer.
Capillary Tube: Fine bore to ensure a significant rise/fall for small changes in volume, enhancing sensitivity.
Stem: Thick glass surrounding the capillary tube for protection and to hold the temperature scale.
Thermometric Liquid: Mercury or alcohol.
Constriction (Clinical Thermometers): A narrow bend in the capillary just above the bulb, which prevents the mercury from falling back immediately after removal from the body, allowing time for reading.
Working Principle: The liquid in the bulb expands when heated and contracts when cooled. The volume expansion is much greater than that of the glass, causing the liquid column to move along the calibrated scale.
Calibration (Fixed Points): Lower Fixed Point (Ice Point): The temperature at which pure ice melts at standard atmospheric pressure (0°C or 273.15 K).
Upper Fixed Point (Steam Point): The temperature at which pure water boils at standard atmospheric pressure (100°C or 373.15 K). The interval between these two points is divided into equal divisions (e.g., 100 divisions for the Celsius scale).
Healthcare and Public Health in Nigeria: Thermometers are indispensable in Nigerian hospitals, clinics, and homes.
Fever Detection: Clinical thermometers are used to measure body temperature to detect fever, a common symptom of malaria, typhoid, and other prevalent diseases. Accurate measurement ensures timely medical intervention.
Vaccine Storage: Vaccines require strict temperature control (e.g., 2°C to 8°C) in cold chains, especially important for remote areas in Nigeria. Thermometers in refrigerators and cool boxes are crucial to monitor and maintain these temperatures, preventing spoilage and ensuring vaccine efficacy.
Blood Bank Monitoring: Blood products also require specific temperature ranges for safe storage.
Agriculture and Food Preservation: Crop Storage: Farmers and agricultural industries in Nigeria use temperature monitoring in silos and storage facilities for grains (e.g., maize, rice) to prevent spoilage, pest infestation, and fungal growth.
Food Processing: In food processing factories (e.g., dairy, beverages, canned goods), temperature control is critical at various stages like pasteurization, cooking, and cooling to ensure food safety and quality.
Cold Storage Facilities: For perishable goods like meat, fish, and vegetables, cold storage using controlled temperatures helps extend shelf life and reduce post-harvest losses, supporting local markets and food security.
Industrial and Technological Applications: Automotive Industry: Vehicle engines need to operate within specific temperature ranges. Thermometers (e.g., in dashboards) alert drivers to overheating, preventing damage to the engine. In Nigeria, overheating is a common issue due to weather and poor maintenance, making temperature monitoring crucial.
Oil and Gas Industry: In Nigeria's vital oil and gas sector, temperature measurement is critical in exploration, refining processes, pipelines, and storage facilities to monitor and control reactions, ensure safety, and optimize efficiency. Thermocouples and resistance thermometers are commonly used here due to their robustness and accuracy.