Common misconceptions › Class 11 › Physics › Kinetic Theory Class 11 Physics · Chapter 12 · NCERT (keph205)
Kinetic Theory: common misconceptions
See the tempting mistake students make in Kinetic Theory, why it feels right at first glance, and a worked visual example that shows the right idea.
Worked visual “If your child says…” (8) Teach & practice The tempting mistake & worked visual Does equal temperature mean equal molecular speed? Wrong
Helium and oxygen molecules have the same average speed at the same temperature.
Why it feels right. Temperature measures molecular motion, so I expect it to fix speed itself.
Right
At the same temperature, ideal gases have the same mean translational kinetic energy per molecule.
Worked example
O₂ speed relative to O₂ 1
He speed relative to O₂ 2.83
Normalize oxygen's root-mean-square speed to 1. At equal temperature, helium's is √8, about 2.83 times as large. O₂ molecules have about eight times the mass of He atoms. At equal temperature, the root-mean-square speed of He is √8 times that of O₂. If your child says… (8 concept checks in this chapter)
As you work through Kinetic Theory , listen for these everyday misunderstandings and use the prompt alongside each one.
If your child says…
“The molecules themselves fill the whole container.”
Ask together Which shrinks in this diagram: the molecules or the gaps? What’s true A dilute gas fills its container, but its molecules occupy only a small fraction. If your child says…
“Equal masses of different gases give equal pressure shares.”
Ask together Which equal mass contains more molecules: hydrogen or oxygen? What’s true Each gas's pressure share follows molecule count, and these shares add to give total pressure. If your child says…
“A crowded, colder gas is more ideal.”
Ask together If cooled vapour becomes liquid, can we ignore attractions between its molecules? What’s true Usually, crowding and cooling make molecular size and attractions harder to ignore. If your child says…
“The wall gets only the molecule's incoming momentum.”
Ask together After stopping the molecule, would sending it backwards require another push? What’s true Reversing momentum requires twice the change needed merely to stop the molecule. If your child says…
“Equal temperature means equal speeds.”
Ask together Does equal energy always mean equal speed? What’s true Equal energies allow different speeds. If your child says…
“A vibration gets energy only for movement.”
Ask together When the bond is stretched but momentarily still, where is its energy? What’s true A fully active vibration shares its average energy equally between movement and stored stretching energy. If your child says…
“Heating gas always needs the constant-pressure heat capacity.”
Ask together Which piston can rise and let its gas do expansion work? What’s true A sealed rigid vessel cannot expand, so its gas uses constant-volume heat capacity. If your child says…
“The time between collisions is just one divided by speed.”
Ask together At the same speed, would the longer marked flight take the same time? What’s true Travel time depends on both distance and speed. Use this chapter three ways Previous chapter Thermodynamics Next chapter Oscillations
In the app, your child answers a short check-in with confidence taps, and you see which of these misunderstandings showed up—and which ones are gone.
Start free 
