Particle motion in gases
Gas molecules are in constant random motion
The temperature of a gas is related to the average kinetic
energy of its molecules: hotter means faster on average
Gas pressure is caused by molecules colliding with the walls
of the container. Each collision exerts a small force; the total force on
each square metre of wall is the pressure
Increasing the temperature of a gas at constant volume
increases its pressure :
The molecules have more kinetic energy and move faster
So they hit the walls more often (more collisions per second)
And each collision exerts a larger force
So the total force on the walls, and the pressure, is greater
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Why heating a gas raises its pressure cool gas few collisions per second, each one gentle hot gas many more collisions per second, and each one harder heat it → higher PRESSURE Pressure is caused by the particles colliding with the walls of the container. Each collision exerts a tiny force; the pressure is the total force per unit area. Heating a gas at constant volume gives the particles more kinetic energy, so they hit the walls more often AND harder — and the pressure goes up. Squashing a gas into a smaller volume at constant temperature also raises the pressure, because the same particles hit a smaller area more often: p × V = constant A box of gas particles at low and high temperature, arrows longer in the hot box, with more frequent, harder collisions on the walls labelled
The pressure answer is marked on three particle ideas and one
macroscopic one: faster particles; collisions more frequent (say
"per second" or "more often"); a greater force per collision ; so a
greater total force on the walls , and pressure is force per unit
area. "Particles vibrate more" scores nothing for a gas, and "more
collisions" without a time word does not score the frequency mark
If the volume is decreased at constant temperature, the speed does
not change: collisions are more frequent because the particles have
less distance to travel between walls. Saying the kinetic energy
increased loses the mark
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In the full topic
• Particle motion in gases• Pressure and volume [Physics only]• Doing work on a gas [Physics only, Higher Tier]Read the free physics topics →