Temperature scales
Section: Temperature | Syllabus: Cambridge AS Level Physics 9702
Temperature Scales The Celsius scale was originally based on the properties of water (0 °C and 100 °C). However, the physical properties of substances do not always vary linearly with temperature, necessitating calibration against a standard scale.
The Thermodynamic (Kelvin) Scale To overcome the limitations of substance-dependent scales, we use the thermodynamic temperature scale . It is independent of the property of any particular substance. Absolute Zero (0 K) Calculated by extrapolating gas volume graphs, Absolute Zero (-273.15 °C) is the lowest theoretically possible temperature.
At this point, atoms and molecules would stop moving, a substance would have zero thermal energy, and an ideal gas would have zero volume. Figure 14.2: Absolute Zero Extrapolation Show a graph of gas volume (y) vs.
temperature in °C (x). The line is extrapolated backwards to hit the x-axis at -273.15 °C. Shifting the origin to this point makes volume directly proportional to temperature. T / K = θ / ^C + 273.15 A temperature interval of 1 ext K is exactly equal to 1 ^ extC.
Thermometers and Physical Properties A thermometer uses a thermometric property that varies with temperature. Not all properties vary linearly, meaning different thermometers can give different readings if not properly calibrated.
1. Liquid-in-Glass Uses change in density of a liquid (e.g., Ethanol or Mercury). Portable but lacks high accuracy and has limited range (Ethanol: -70 to 78 °C). 2. Gas Thermometers The constant volume gas thermometer uses gas pressure.
It is extremely accurate (pm 0.005 °C) and has a wide range (-200 to 500 °C) but is large and fragile. It is often used to calibrate other thermometers. 3. Resistance Thermometers (Platinum) Uses the fact that the electrical resistance of metals increases with temperature.
Platinum is used because its change is almost linear . Very sensitive (can detect 10^-3 °C). 4. Thermistors Semiconductor devices where resistance usually decreases with temperature. They give a rapid response but are highly non-linear , making them useful only in small ranges (e.g., -30 to +30 °C).
5. Thermocouples Uses the thermoelectric effect : an e.m.f. is generated between two junctions of different metals at different temperatures. They are robust and can measure up to 1300 °C. Thermometer Summary Type Physical Property Typical Range (°C) Liquid-in-glass Density of liquid -70 to 356 Gas thermometer Pressure or Volume -270 to 1500 Platinum resistance Resistance of metal -200 to 850 Thermistor Resistance of semiconductor -90 to 150 Thermocouple e.m.f.
across junctions -200 to 1300
Interactive revision notes, videos and practice questions load below.