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Internal energy (CIE A Level Physics)

Revision Note

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Ann H

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Ann H

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Defining internal energy

  • Internal energyU is determined by the state of the system
    • Temperature
    • The random motion of molecules
    • The state of matter: gases have the highest internal energy, solids have the lowest
    • Intermolecular interactions between the particles 
  • The molecules of all substances have both kinetic and potential energies
    • Kinetic energy is due to the speed of the molecules
    • Potential energy is due to the intermolecular forces between the molecules and varies with the separation of particles
  • The internal energy of a substance is defined as:

The sum of the random distribution of kinetic and potential energies within a system of molecules

Internal energy of water molecules

Internal Energy in Water, downloadable AS & A Level Physics revision notes

All molecules in a substance possess both kinetic and potential energies

  • The internal energy of a system can increase by:
    • Doing work on it
    • Adding thermal energy to it (heating it)

  • The internal energy of a system can decrease by:
    • Losing thermal energy to its surroundings
    • The system doing work on its surroundings (e.g. a gas pushing a piston)

Examiner Tip

When an exam question asks you to define “internal energy”, you can lose a mark for not mentioning the “random motion” of the particles or the “random distribution” of the energies, so make sure you include one of these in your definition!

Internal energy & temperature

  • The rise in temperature of an object is related to an increase in its internal energy
  • Molecules in solids and liquids are close together and bound by intermolecular forces, so have kinetic and potential energy
    • Molecules in a solids and liquids are tightly packed, so begin to vibrate more when heated
  • Molecules in a gas have no intermolecular forces so have no potential energy, only kinetic energy
    • When a container of gas molecules is heated, the molecules begin to move around faster, increasing their kinetic energy
  • The change in internal energy of an ideal gas is proportional to the change in temperature, and can be written as:

ΔU ∝ ΔT

  • Where:
    • ΔU = change in internal energy (J)
    • ΔT = change in temperature (K)

Molecular movement at low and high temperatures

Change in internal energy, downloadable AS & A Level Physics revision notes

As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy

Worked example

A student suggests that, when an ideal gas is heated from 50 oC to 150 oC, the internal energy of the gas is trebled. State and explain whether the student’s suggestion is correct.

 

Answer:

Step 1:           

Write down the relationship between internal energy and temperature

The internal energy of an ideal gas is directly proportional to its temperature

ΔU ∝ ΔT

Step 2:           

Determine whether the change in temperature (in K) increases by three times

The temperature change is the thermodynamic temperature ie. Kelvin

The temperature change in degrees from 50 oC to 150 oC increases by three times

The temperature change in Kelvin is:

50 oC + 273.15 = 323.15 K

150 oC + 273.15 = 423.15 K

fraction numerator 423.15 over denominator 323.15 end fraction space equals space 1.3

Therefore, the temperature change, in Kelvin, does not increase by three times

Step 3:           

Write a concluding statement relating the temperature change to the internal energy

The internal energy is directly proportional to the temperature

The thermodynamic temperature has not trebled, therefore, neither has the internal energy

Therefore, the student is incorrect

Examiner Tip

If an exam question about an ideal gas asks for the total internal energy, remember that this is equal to the total kinetic energy since an ideal gas has zero potential energy

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Ann H

Author: Ann H

Expertise: Physics

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.