10.4 – Internal Energy of a System

Internal energy of a system is the sum of the total kinetic energy and potential energy of all the particles (atoms or molecules) in the system.

Internal energy = total k.e. + total p.e. of all particles

  • kinetic energy – related to the temperature of the system
  • potential energy – related to the physical state (solid, liquid or gas) of the system

 

  • The transfer of thermal energy in and out of a body (or a system) can cause a change in temperatureor a change in state of the body
    • Transfer of thermal energy into a body increases the internal energy of the body
      • Increase in internal energy – either increase in k.e. or p.e. (or both)
    • Transfer of thermal energy away from a body decreases the internal energy of the body
      • Decrease in internal energy – either decrease in k.e. or p.e. (or both)
Example 4
Thermal energy is supplied continuously to a kettle to keep the water boiling. As the water boils, molecules escape as vapour.

The water vapour above the kettle is at the boiling point of water.

How do the molecules of water in the vapour compare with the molecules in the liquid water?

A Their average kinetic energy is greater and the force between them is smaller.

B Their average kinetic energy is greater but the force between them is the same.

C Their average kinetic energy is the same and the force between them is the same.

D Their average kinetic energy is the same but the force between them is smaller.

ANSWER: D – The temperature is the same (100 ºC) and thus the average kinetic energy of both will be the same. In the vapour the molecules are far apart and the forces between them are very small.
Example 5

The water in a swimming pool has a temperature of 28 °C and the hot water in a kettle has a temperature of 80 °C.

Where does the water have the greater total internal energy (IE) and where do the water molecules have the greater average kinetic energy (KE)?

A Kettle has greater total IE, Kettle has greater average KE.

B Kettle has greater total IE, Pool has greater average KE.

C Pool has greater total IE, Kettle has greater average KE.

D Pool has greater total IE, Pool has greater average KE.

ANSWER: C – The pool having a very large volume (mass) of water will contain more thermal energy than the smaller kettle. However, the water in the kettle being at a higher temperature will contain molecules with a higher average kinetic energy.

 

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