Internal energy & graphs

• INTERNAL ENERGY:


  The total energy that particles in a system have from their movement & position.
• Internal energy is made up of:

  • KINETIC ENERGY STORES
  • Due to the VIBRATION or MOVEMENT of particles.
  • POTENTIAL ENERGY STORES:
  • Due to the POSITION of particles within the system.
• • Particles are always moving (vibrating or rotating) and this movement contributes to the system's KINETIC ENERGY.
• Particles have POTENTIAL ENERGY, because of the forces between them.

  • Their position within the field of these forces determines their potential energy.
• Kinetic energy + Potential energy = total internal energy
• HEATING a system will INCREASE its internal energy.

  • As it transfers energy to the particles, increasing their KINETIC ENERGY, which can result in a CHANGE IN TEMPERATURE.
• If the system's temperature changes, it can also lead to a CHANGE IN STATE - from solid to liquid or liquid to gas.

  
• The extent of temperature change from a given energy input depends on the substance’s MASS and SPECIFIC HEAT CAPACITY.

  
• Heating & cooling graphs:

  Provide a visual representation of temperature changes in a substance as energy is either ADDED to or REMOVED from it.

  • Show the points of CHANGES OF STATE occur.

  
• The graphs typically follow a similar pattern:

  During heating:

  • Energy is transferred to the substance.
  • Increasing the molecules' KINETIC ENERGY

  Cooling involve:

  • Energy being transferred away.
  • Decreasing the molecules' KINETIC ENERGY

  
• Heating Graphs:

  Depicts the relationship between temperature & time for a substance as it changes state from solid to liquid to gas.

  
• A to B:

  • Internal energy increases
  • Temperature increases as it absorbs heat.
  • Causes its particles to vibrate more energetically.
  • Solid state

  
• B to C:

  • Represents the MELTING
  • Internal energy increases
  • Temperature is constant despite the continued input of heat.
  • Because the energy is used to BREAK forces that hold the solid particles in place, than increasing their KINETIC ENERGY.
  • This phase change from solid to liquid occurs at a specific temperature known as the MELTING POINT.

  
• C to D:


  • After the substance has completely melted into a LIQUID state.
  • Internal energy increases
  • Temperature increases with further heat input.
  • shown by the upward slope following the horizontal section.
  • The liquid's particles gain KINETIC energy & move more freely.

  
• D to E:

  • The second HORIZONTAL line shows The phase change from liquid to gas at a specific temperature known as the boiling point.
  • Temperature does not increase during this phase change.
  • As the energy goes to Breaking forces that hold the liquid particles together, than increasing the KINETIC ENERGY.

  
• E to F:

  As the phase change completes

  • Internal energy increases
  • Temperature increases
  • As shown by the final upward slope meaning the substance is a GAS.
  • The particles moves rapidly from high kinetic energy from the absorbed heat.

  
• P to Q:

  Starting from the left, the substance begins as a GAS.

  • As it loses energy, the temperature & internal energy decreases.
  • shown by the downward slope.

  
• Q to R:

  • The first HORIZONTAL line means the process of CONDENSING.
  • where the gas turns into a liquid.
  • The temperature remains constant despite the continuous loss of energy.
  • Because the energy removed is being used to change the state of the substance & MAKE forces between the particles than lowering the temperature.

  
• R to S:

  The temperature of the liquid, decrease with further energy loss.

  • Shown by the downward slope.

  
• S to T:

  • The second HORIZONTAL line represents FREEZING
  • The liquid turns into a solid.
  • Temperature does not change during this phase transition.
  • As The energy is being used to change the state, by 
  • CREATING forces that turn the liquid into a solid.

  
• T to U:

  • After the substance has completely frozen into a SOLID.
  • The energy loss will decrease temperature.
  • Continuing the downward trend on the graph.

  
• The mean kinetic energy of the particles increases, which increases the internal energy of the water.

  
• AB:

  • changing state from solid to liquid at steady temp.

  BC:

  • Temp. of liquid rises, until it reaches boiling point.

  
• The figure shows this as:

  • The gradient for ice is steeper than the gradient for water, meaning less energy is needed to increase the temp. by a fixed amount.

  
• The specific latent heat of fusion of ice is less that the specific latent heat of vaporisation of water, the figure shows this as:

  • water took more time to vaporise than the ice took to melt.
  • meaning less energy is needed to change the state from solid to liquid, than from liquid to vapour.