PHYSICS GAS LAW 小問題

You do the experiment by measuring the absolute pressure at various temperatures and plot pressure (in 10^5 Pa) against temperature (in Celsius). When you have marked your experimental results on the graph for temperature between 0°C and 100°C, they will appear to be in a straight line. Draw a straight line through the plotted points.

At 0°C the pressure of gas is not zero, but as the temperature falls the pressure decreases. Extend the graph backwards by continuing the straight line to find out the temperature at which the pressure becomes zero. This is the temperature where the line touches the horizontal temperature axis. ( x-axis). The x-intercept is found to be -273°C

The process of extending a graph beyond the range of experimental results is called extrapolation. This process should always be used with caution because information it provides is not fully supported by experimental evidence. If you work on the temperature between 0°C and 100°C (in school lab.), the graph does not tell you what happens below 0°C and we assume that gases behave the same way below 0°C as they do above 0°C.

-273°C is the temperature known as absolute zero on Kelvin scale. It is the temperature at which the pressure of the gas is zero ~ the lowest temperature attainable.That is why x-intercept has fundamental importance.

We use the term “Expected Value” because -273°C is not an observed experimental value, not part of experimental data. That is the value you expected when you extended the straight line backward until the line hits the x-axis.

Different gases follow the extrapolated graph quite closely. However, all gases liquefy at low temperature (oxygen liquefies at -183°C) and graph cannot be extended below liquefaction point. Nonetheless, the graph is essentially a straight line and it projects until it crosses the x-axis at about -273°C.

If you plot pressure (10^5 Pa) against temperature in Kelvin, you have a straight line passing through the point of origin.

The graph you mentioned is the variation of pressure (in Pa) with temperature (in degree Celsius). The line so plotted follows the Pressure Law. The Law states that the pressue of a constant volume of ideal gas is proportional to its temperature in the absolute scale.

Hence, the Pressure Law can be expressed mathematically by the following equation:

P/T = constant
where P is the pressure of gas in Pa, and T is the gas temperature in absolute scale (K).

Since 0'C = 273 K
hence, P/(t + 273) = D, where D denotes the constant, and t is the gas temperature in degree Celsius
i.e. P = 273D + Dt

Therefore, a plot of P against t is a straight line, with slope D and y-intercept 273D
The x-intercept (found by setting P = 0 Pa) is given by,
0 = 273D + Dt
i.e. 273 + t = 0
or t = -273 K

Therefore, it could be easily seen that at t = -273'C, the gas pressure would be reduced to zero. Since the pressure of a gas cannot be negative, the temperature -273'C is thus the lowest temperature that can be attained by a gas. This temperature, -273'C is therefore called the absolute zero, as temperature below -273'C would give negative pressure to a gas and which is physically impossible.

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2011-06-05 13:21:46 補充：
"expected value" is not a technical term, so just use its ordinary meaning in English, i.e. the value that you would anticipate to find.