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Cambridge IGCSE Physics · 0625

Chapter 2: Thermal physics — Part 2

Topic 2.1.3 · Gases and the absolute scale of temperature

Pressure changes in a gas (qualitative)

For a fixed mass of gas, in terms of particles:

Temperature up, volume constant
Particles move faster, so more frequent and harder collisions, so pressure rises.
Volume down, temperature constant
Particles hit the walls more often, so pressure rises.
Two particle-box sketches: same volume, higher temperature — faster particles, more wall hits; same temperature, smaller volume — more frequent hits.
Diagram 1: Two particle-box sketches: same volume, higher temperature — faster particles, more wall hits; same temperature, smaller volume — more frequent hits.

Kelvin and Celsius

Convert with T (in K) = θ (in °C) + 273.

Examples: 0 °C = 273 K; 27 °C = 300 K; 0 K = -273 °C.

Convert 27 °C to kelvin.

Worked example: 27 degrees Celsius plus 273 equals 300 kelvin
T = 27 + 273 = 300 K. Never use Celsius in pV = constant.

pV = constant

For a fixed mass of gas at constant temperature, pV = constant.

If volume halves, pressure doubles. A graph of p against 1/V is a straight line through the origin.

A fixed mass of gas at constant temperature has pressure 200 kPa and volume 0.030 m3. The volume is reduced to 0.012 m3. Calculate the new pressure.

Worked example: p1 V1 equals p2 V2 gives new pressure 500 kilopascals
p1V1 = p2V2 → p2 = 500 kPa.
Graph of p against 1/V for a fixed mass of gas at constant temperature, a straight line through the origin showing pV = constant.
Diagram 2: Graph of p against 1/V for a fixed mass of gas at constant temperature, a straight line through the origin showing pV = constant.

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