Physics · Ch 7 — Thermal Properties of Matter
Absolute zero and absolute temperature
Absolute zero and absolute temperature
Experiments on gases at low density show that, at constant pressure, the volume of a fixed quantity of gas is directly proportional to its temperature measured in °C; likewise, at constant volume, its pressure is directly proportional to temperature in °C. Graphed against , both the volume-temperature curve (Fig. 7.2(a)) and the pressure-temperature curve (Fig. 7.2(b)) are straight lines, for every gas -- equal increases in temperature give equal increases in volume (or pressure). The lines for different gases have different slopes and, importantly, do NOT pass through the graph's origin.
If we imagine (idealised, since no real gas actually behaves this way at very low temperature -- real gases liquefy first) extending these straight lines backwards to lower and lower temperatures, every one of them -- regardless of which gas, or its particular slope -- crosses the temperature axis (i.e., reaches zero pressure or zero volume) at exactly the SAME point: . This universal, substance-independent crossing point is called absolute zero. It is the theoretical lower limit of temperature; no real system has ever been cooled to reach it exactly. …
What this figure shows. A graph with volume V on the vertical axis and temperature TC (in degrees Celsius) on the horizontal axis, for a fixed mass of gas held at constant pressure. Several straight, upward-sloping lines are drawn (one per gas/gas sample), each showing volume increasing linearly with temperature. None of the lines pass through the graph's origin -- each has a positive, non-zero volume-axis intercept -- and the lines have different slopes from each other (different gases expand at different rates per degree). When extended backwards (dashed/extrapolated) to lower temperatures than shown by real data, all the lines converge and cross the temperature axis (V=0) at the SAME single point, at TC = -273.15 °C, even though they ha …
What this figure shows. A graph with pressure P on the vertical axis and temperature TC (in degrees Celsius) on the horizontal axis, for a fixed mass of gas held at constant volume. Like Fig 7.2(a), several straight, upward-sloping lines are drawn (one per gas), each with a non-zero positive pressure-axis intercept and different slopes for different gases, none passing through the origin. Extrapolated backwards, all the lines again converge to the same single temperature-axis crossing point at TC = -273.15 °C (P=0), reinforcing that this is a universal property of gases, not specific to any one gas …
What this figure shows. Three thermometers drawn side by side, each marked with a different scale -- Kelvin (K), Celsius (°C) and Fahrenheit (°F) -- all placed in the same fixed-temperature bath (human body temperature) so that the mercury column in each shows the identical physical height/rise, corresponding to the SAME real temperature read off on three different numerical scales. The Kelvin thermometer reads a value near 310 K, the Celsius thermometer near 37 °C, and the Fahrenheit thermometer near 98.6 °F, at the same mercury level -- illustrating that the three scales are just different numberings of the same physical hotness, related by the linear conversion equation. The figure's caption notes the thermometer …