CAIE Physics A-Level Topic 11: Temperature Notes PDF

CAIE Physics A-level Topic 11: Temperature Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

CAIE Physics A-level Topic 11: Temperature Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 11 - Temperature (A-level only) 11.1 - Thermal Equilibrium Thermal energy is always transferred ​from an area of higher temperature to that of a lower temperature​, as shown in the diagram below. Energy will be transferred between objects in thermal contact of different temperatures until they reach the ​same​ temperature. This is known as ​thermal equilibrium​. It is important to note that if object A is in thermal equilibrium with object B, and B is in thermal equilibrium with object C, then object A and C must also be in thermal equilibrium. This is known as the ​zeroth law of thermodynamics​. 11.2 - Temperature Scales Many ​physical properties​ vary with temperature, meaning they can be used to measure temperature. Below are some examples: Change in resistance of a metallic conductor or semiconductor (such as a thermistor) Voltage produced across a thermocouple Change in volume of a liquid Change in volume of a gas at constant pressure Change in pressure of a gas at constant volume The ​thermodynamic scale​ or the ​Kelvin scale​ is an ​absolute scale of temperature that does not depend on the property of any substance​, whereas the Celsius scale for instance, is dependent on the melting point (0° C) and boiling point (100° C) of pure water at atmospheric pressure. The lowest possible temperature, ​absolute zero​, is represented as 0 K on the Kelvin scale. This is the temperature at which particles have ​no kinetic energy​ and the ​volume and pressure of a gas are zero​. All equations in thermal physics will use temperature measured in kelvin (K). A change of 1 K is equal to a change of 1°C , and to convert between the two you can use the formula: K = C + 273.15 ​Where K is the temperature in kelvin and C is the temperature in Celsius. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  Image source: ​OpenStax College​,​CC BY 4.0​, Image is cropped and fahrenheit scale is removed 11.3 - Practical Thermometers As shown above, many physical properties vary with temperature meaning many types of thermometers can be produced, each of which rely on a different property to measure temperature. A ​thermocouple ​is a type of thermometer and its structure is shown to the right. The reference junction is kept at a constant temperature, whilst the measurement junction is used to measure an unknown temperature. If there is a temperature difference between the junctions ​an emf is formed​, which is measured by the voltmeter. You can calculate the temperature from the recorded value of emf by using a calibration curve​. A ​thermistor ​is a semiconductor and its temperature decreases as its resistance increases. You can calculate the resistance of the thermistor and use a ​calibration curve​ to calculate temperature. There are advantages and disadvantages to using a thermocouple and thermistor to measure temperature. These are summarised on the table on the next page. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  Feature Thermocouple Thermistor Sensitivity Sensitivity is dependent on the choice of Very sensitive but only over a metals that make up the two types of narrow range of temperatures. wires (as shown above), therefore it can be made very sensitive. Range Large Narrow Response time Junctions are small so have a small Are larger in size so have a thermal capacity so response time is larger thermal capacity so have small. a larger response time. Stability The wires forming the junctions of the Are very stable and mostly thermocouple are subject to corrosion. unaffected by aging. Ease of use The reference junction must be kept at a Much easier to use. constant temperature, usually through the use of ice water, which may make measurements awkward. However, for more modern thermocouples this is not true. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc