O Level / IGCSE Physics · Thermal physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Everything is made of tiny particles that are always moving. In a solid they are close together in a regular pattern and only vibrate about fixed positions. In a liquid they are still close together but can slide past each other. In a gas they are far apart and move quickly and randomly.
Temperature tells you how fast the particles move. A higher temperature means a greater average kinetic energy. At absolute zero, −273 °C, the particles have the least kinetic energy possible.
A gas pushes on its container because its particles hit the walls. Each collision gives a small force, and the total force on each unit of area is the pressure.
Students say the particles get bigger or expand when a gas is heated. The particles stay the same size; they move faster and, if the gas can expand, they move further apart.
Answer: 60 kPa
Try each one, then open the answer.
Answer: C. Gas to liquid is condensation. Evaporation and boiling are liquid to gas; melting is solid to liquid.
Answer: A. Heating raises the average kinetic energy, so the particles hit the piston harder and push it out. In the larger volume they hit each unit area less often, and the pressure settles at its old value. The particles themselves do not change size.
Answer: C. Gas particles are far apart and move freely, so a gas fills its container, takes its shape and can be squashed into a smaller volume.
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