Edexcel IGCSE Physics · Spec 5.19-5.21
Gas Pressure and Kelvin Temperature
Covers link kinetic energy to Kelvin temperature, Use p1/T1 = p2/T2 and Convert Celsius to kelvin correctly.
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Gas Pressure and Kelvin Temperature
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Explained
Gas pressure, temperature, and the conversion that decides the marks
Gas pressure comes from collisions. The particles in a gas move constantly in all directions, and each time one strikes the wall of the container it exerts a small force. Pressure is the total effect of a very large number of those collisions spread over the area of the wall.
Why heating raises the pressure
The average kinetic energy of the particles is directly proportional to the absolute temperature, measured in kelvin. Heat the gas and the particles move faster.
Faster particles change the collisions in two ways at once. They reach the walls more often, so there are more collisions each second, and each collision involves a greater change of momentum, so each exerts a greater force. Both effects raise the pressure.
Answers that mention only one of the two usually earn part of the credit. Say more frequent collisions and greater force, and the point is complete.
The kelvin scale
Absolute zero is the temperature at which particles have the least possible kinetic energy. It sits at minus 273 degrees Celsius, and it is zero on the kelvin scale.
The two scales rise in equal steps, so converting is simple. Add 273 to a Celsius temperature to get kelvin, and subtract 273 to go back. Note that the unit is written K, with no degree symbol.
The equation
For a fixed mass of gas at constant volume, pressure divided by kelvin temperature stays constant, which gives p1 divided by T1 equals p2 divided by T2.
Rearranging it is straightforward. What is not straightforward, according to the examiners, is remembering to convert first.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Physics make the same point across different series. In one, only a third of all candidates knew to convert the temperatures into kelvin, and those who did not usually lost a mark. In another, many learners failed to convert temperatures into kelvin or did so incorrectly, and errors in rearranging the equation were also common, so many gained only partial credit, typically two marks instead of four.
This is the rare case where one habit protects several marks. Before you touch the equation, write both temperatures in kelvin. The rest of the calculation is ordinary arithmetic.
Why the conversion is not optional
The relationship only holds for a scale that starts at absolute zero. Doubling a Celsius temperature from 20 to 40 does not double the average kinetic energy, because 40 degrees Celsius is not twice as far from absolute zero as 20 degrees Celsius is. Those two temperatures are 313 K and 293 K, barely different. In kelvin the proportionality does hold, which is exactly why the scale exists.
Spec 5.19-5.21
What you need to know
- Link kinetic energy to Kelvin temperature
- Use p1/T1 = p2/T2
- Convert Celsius to kelvin correctly
Active recall
Quick check
Answer each question before opening the answer.
What temperature scale must gas laws use?
Kelvin
State the fixed-volume pressure law
p1/T1 = p2/T2
Convert 47 degrees C to kelvin
320 K
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