Ch 1 · Kinetic Particle Model of Matter · IGCSE Physics 0625
Topic 1 of 2 in Thermal Physics — IGCSE Physics 0625 — 20 lessons.
Introduction
Everything is made of tiny particles (atoms and molecules) that are always moving. This single idea — the kinetic particle model — explains why solids keep their shape, why gases can be squashed, why a football feels harder on a hot day, and even what the lowest possible temperature is. Examiners love "explain in terms of particles" questions, so this lesson focuses on giving complete, mark-worthy explanations.
Key ideas
Gases can be compressed because there is a lot of empty space between their particles; in solids and liquids the particles are already touching.
🧑🏫 Teacher's explanation
Imagine a crowded ballroom. In a solid, everyone is standing in a perfect grid, holding hands; they can wiggle in place, but they can't move from their spot. In a liquid, people are still packed close together, but they are walking around each other, weaving through the crowd. In a gas, everyone has run away from each other and is sprinting wildly around the room, only bumping into things occasionally.
📖 Definitions
🔍 Worked example — Easy
Convert a temperature of 45 ^circC to Kelvin.
🔍 Worked example — Standard
A gas has a volume of 400 cm^3 and a pressure of 100 kPa. If the volume is compressed to 200 cm^3 at a constant temperature, what is the new pressure?
🔍 Worked example — Exam-style
A gas is held at a pressure of 2.0 × 10^5 Pa at a temperature of 300 K. If the temperature is increased to 600 K while the volume remains constant, calculate the new pressure.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Practical notes
Worked example
Example 1 — temperature conversion. Liquid nitrogen boils at -196\ ^circC. Convert this to kelvin, and convert a room temperature of 300 K to degrees Celsius.
🔍 Worked example — Easy
Liquid nitrogen boils at -196 ^circC. Convert this temperature to Kelvin.
🔍 Worked example — Standard
A sealed container holds a gas at a pressure of 1.5 × 10^5 Pa and a volume of 2.0 m^3. If the volume is compressed to 0.5 m^3 at a constant temperature, calculate the new pressure.
🔍 Worked example — Exam-style
A gas in a cylinder has a pressure of 101 kPa at a volume of 500 cm^3. The gas is heated until the pressure increases to 250 kPa while the volume is kept constant. Calculate the final temperature in Kelvin, assuming the initial temperature was 300 K.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) Particles are close together (touching) in an irregular arrangement, and move randomly past one another. (2) 37 + 273 = 310\ K; 4 - 273 = -269\ ^circC. (3) Gas particles are far apart with a lot of space between them, so they can be pushed closer; particles in a solid are already touching. (4) Bright specks of smoke move in random, jerky paths; they are being hit unevenly by fast, randomly moving air molecules that are too small to see. (5) Pressure increases: the same particles are in a smaller volume, so they hit each unit area of the walls more frequently. (6) p_2 = 1.0 × 10^5 × 600/200 = 3.0 × 10^5\ Pa. (7) The volume doubles. (8) A particle bouncing off the wall has its momentum reversed; the change in momentum needs a force from the wall, so the particle exerts an equal and opposite force on the wall. Many such collisions per second give a steady force, and force ÷ area is the pressure.