Temperature (Edexcel IGCSE Physics)

• Imagine molecules of gas that are free to move around in a box
• The molecules in the gas move around randomly at high speeds, colliding with surfaces and exerting pressure upon them
• The temperature of a gas is related to the average speed of the molecules:
  • The hotter the gas, the faster the molecules move and vice versa
  • Hence, the molecules collide with the surface of the walls more frequently

Gas molecules move about randomly at high speeds

• This is because their kinetic energy increases

• Heating a system will change the energy stored in a system by increasing the kinetic energy of its particles
  • The Kelvin temperature of the gas is related to the average kinetic energy of the molecules

• This increase in kinetic energy (and therefore energy stored in the system) can:
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)

• The internal energy of a gas is the sum of the kinetic energy of all the molecules
• The higher the temperature, the higher the average kinetic energy of the molecules and vice versa
  • This means they move around faster

As the container is heated up, the gas molecules move faster with higher kinetic energy. The energy stored within the system - the internal energy - therefore increases

• If the temperature of a gas is increased, the particles move faster and gain kinetic energy
  • Therefore, they will collide more with each other and the container leading to an increase in pressure

• The temperature (in Kelvin) is proportional to the average kinetic energy of the molecules

T ∝ KE

ANSWER:  B

• • When evaporation takes place, the more energetic molecules are leaving the surface of the liquid
  • Since the more energetic molecules have left, the average kinetic energy per molecule must decrease
    • Therefore, A, C & D are not correct

  • Temperature is proportional to the average kinetic energy per molecule, therefore the temperature also decreases