Temperature scales
Section: Temperature | Syllabus: Cambridge A 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 ExtrapolationShow 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 / ^ + 273.15A 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 |
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