OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(iii) an estimation of absolute zero using variation of gas temperature…
Full specification wording
Ideal gases: (iii) an estimation of absolute zero using variation of gas temperature with pressure
Absolute zero is estimated by measuring the pressure of a fixed mass of gas at constant volume as its temperature is varied.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Absolute zero is estimated by measuring the pressure of a fixed mass of gas at constant volume as its temperature is varied. Temperatures must be converted to kelvin. A graph of pressure p against thermodynamic temperature T is a straight line; extrapolating it to p = 0 gives the intercept on the T-axis, which is absolute zero (0 K or −273 °C). Alternatively, plot p against temperature in degrees Celsius; the intercept on the temperature axis is −273 °C. The method assumes the gas behaves ideally and does not liquefy. In a multiple-choice question, you may be asked to identify the correct graph, the value of absolute zero, or the effect of a systematic error such as a leak.
What earns the marks
- State that pressure is measured at constant volume for a fixed mass of gas.
- Convert Celsius temperatures to kelvin by adding 273.
- Plot a graph of pressure against temperature in kelvin.
- Extrapolate the straight line to p = 0 to find absolute zero.
- Recognise that the intercept on the temperature axis is approximately −273 °C.
- Identify that a leak or change in gas mass would invalidate the extrapolation.
Common mistakes and how to put them right
Using degrees Celsius directly on the temperature axis when the graph should be in kelvin.
Put it right: Convert to kelvin first.
Assuming the graph passes through the origin when using Celsius.
Put it right: The intercept is at −273 °C, not 0 °C.
Ignoring the dead volume of connecting tubes, which makes the measured pressure not directly proportional to temperature.
Extrapolating beyond the range of data without checking linearity.
Put it right: The gas may liquefy at low temperatures.
Exam tips
- Check the units on the axes; if temperature is in °C, the intercept is −273 °C, but if in K, the intercept is 0 K.
- Remember that absolute zero is the temperature at which pressure would be zero for an ideal gas.
- In multiple-choice questions, eliminate options that use Celsius without conversion or that give a positive intercept.
Try the lesson
The start of the Ideal gases: (iii) an estimation of absolute zero using variation of gas temperature with pressure lesson, made from checked teaching notes for this specification point.
Listen
Absolute zero is estimated by measuring the pressure of a fixed mass of gas at constant volume as its temperature is varied. Temperatures must be converted to kelvin. A graph of pressure p against thermodynamic temperature T is a straight line; extrapolating it to p = 0 gives the intercept on the T-axis, which is absolute zero (0 K or −273 °C). Alternatively, plot p against temperature in degrees Celsius; the intercept on the temperature axis is −273 °C. The method assumes the gas behaves ideally and does not liquefy. In a multiple-choice question, you may be asked to identify the correct graph, the value of absolute zero, or the effect of a systematic error such as a leak. A strong answer earns marks for these points. State that pressure is measured at constant volume for a fixed mass of gas. Convert Celsius temperatures to kelvin by adding 273. Plot a graph of pressure against temperature in kelvin. Extrapolate the straight line to p = 0 to find absolute zero. Exam tip: Check the units on the axes; if temperature is in °C, the intercept is −273 °C, but if in K, the intercept is 0 K.
Key words
Say what each word means before you turn the card.
Absolute zero
Try answering aloud or in your head before turning the card.
Pressure–temperature relationship
Try answering aloud or in your head before turning the card.
Graph extrapolation
Try answering aloud or in your head before turning the card.
Kelvin scale
Try answering aloud or in your head before turning the card.
See every definition
- Absolute zero
- The temperature at which pressure would be zero for an ideal gas, found as the intercept on the T-axis when the line is extrapolated to p = 0 (0 K or −273 °C).
- Pressure–temperature relationship
- Measuring the pressure of a fixed mass of gas at constant volume as its temperature is varied gives a straight line when pressure is plotted against thermodynamic temperature.
- Graph extrapolation
- Extending the straight line to p = 0 gives the intercept on the T-axis, which is absolute zero.
- Kelvin scale
- Celsius temperatures are converted to kelvin by adding 273, and the graph of pressure against temperature in kelvin is a straight line.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Ideal gases: (iii) an estimation of absolute zero using variation of gas temperature with pressure
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge