Tp 14 specific latent heat (shared)
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Thermal physics A-level Physics Unit G484: The Newtonian World Specific latent heat
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Transcript of Tp 14 specific latent heat (shared)
Thermal physics
A-level Physics
Unit G484: The Newtonian World
Specific latent heat
Thermal physics
1. Define internal energy
2. Write a statement of the first law of thermodynamics.
3. Describe two ways in which the internal energy of a body can be
increased.
4. How would you identify a body whose internal energy had increased?
5. Is it possible to increase the internal energy of a body without
changing its temperature?
6. Energy can be added to a body without raising its temperature.
How is this possible?
Internal energy LOs
Thermal physics
Learning objectives
At the end of the lesson you will be able to:
• describe what is meant by the terms latent heat of fusion
and latent heat of vaporisation.
Lesson focus• Specific latent heat
Thermal physics
Learning outcomes
All of you should be able to
• explain why a substance has two latent heats;• give a full definition of the latent heat of fusion and of vaporisation;• recall the equation for latent heat;• solve straightforward problems involving latent heat.
Most of you will be able to
• solve more complex problems involving latent heat.
Thermal physics
Definition
A specific latent heat of a substance is the amount of energy needed
to change the state of _____ of the substance without changing its
________________ .
This energy may be absorbed or released depending on the direction
of the change of state.
Any substance will have two specific latent heats:
• latent heat of fusion (Lf ; solid liquid)
• latent heat of vaporisation (Lv ; liquid gas) [note spelling]
Unit: __________
Specific latent heat LOs
Thermal physics
Demonstration 1:
the latent heat of vaporisation of water
We are going to find LV for water by electrical heating.
• Set up appropriate word and symbol equations to represent the energy transfer that will take place.
electrical energy
supplied to water
energy needed to change
state of water (liquid to gas)=
i.e. E = mL where, m – mass of substance
L – specific latent heat
Specific latent heat LOs
Thermal physics
Specific latent heat
Demonstration 1: the latent heat of vaporisation of water
• Sketch a temperature – time graph to show the water being
heated towards boiling point.
• Describe and explain any changes to the internal energy of
the water.
• Now continue the graph as boiling starts.
• Describe and explain any further changes to the internal
energy of the water.
.
LOs
Thermal physics
Specific latent heat .
LOs
Demonstration 1:
the latent heat of vaporisation of water
The accepted value for water = 2.26 x 106 J kg-1
To do
1. Comment on, and explain the difference between this value and our experimental result.
2. Calculate the percentage difference between these values.
3. How can a more accurate determination of LV for water be made?
Thermal physics
To do
• On whiteboards sketch a graph showing the temperature of ice as it is
steadily heated from -10 °C to 110 °C.
• Describe and explain change to the internal energy in each section of
the graph.
temperature
time
Changing internal energy LOs
Thermal physics
Changing internal energy LOs
Figure from Hyperphysics
