1. Thermal Physics Lesson 2
Latent Heat
Thermal Physics Lesson 2

2. Learning Objectives Define specific latent heat.
Perform calculations using ∆Q=ml.
Describe how specific latent heat of fusion/vaporisation can be measured in the lab.

3. Recall Heating/Cooling Curves
Remember from GCSE?

4. Demonstration What tells you the water is boiling?
So energy is being supplied but the temperature is not rising?
What is going on?
Work is being done to separate the particles against intermolecular attractive forces.

5. shc vs. latent heat (simple terms)
Specific Heat Capacity:-
Energy needed to heat something
Latent Heat:-
Energy needed to change phase

6. Latent Heat
The word “latent” comes from a Latin word that means “to lie hidden.”
When a substance changes phases (liquid  solid or gas  liquid) energy is transferred without a change in temperature. This “hidden energy” is called latent heat.
For example, to turn water ice into liquid water, energy must be added to bring the water to its melting point, 0 ºC. This is not enough, however, since water can exist at 0 ºC in either the liquid or solid state. Additional energy is required to change 0 ºC ice into 0 ºC water. The energy increases the internal energy of the water but does not raise its temp. When frozen, water molecules are in a crystalline structure, and energy is needed to break this structure. The energy needed is called the latent heat of fusion. Additional energy is also needed to change water at 100 ºC to steam at 100 ºC, and this is called the latent heat of vaporization.

7. Q = m Lf or Q = m Lv Latent Heat Formula
Q = thermal energy m = mass L = heat of fusion or vaporization
L is the energy per unit mass needed to change the state of a substance from solid to liquid or from liquid to gas. Ex: Lf (the latent heat of fusion) for gold is 6440 J / kg. Gold melts at 1063 ºC. 5 grams of solid gold at this temp will not become liquid until additional heat is added. The amount of heat needed is: (6440 J / kg) (0.005 kg) = 32 J. The liquid gold will still be at 1063 ºC.

8. Definition
The specific latent heat (l) of fusion or vaporisation is the quantity of thermal energy required to change the phase of 1kg of a substance.
Fusion (liquid  solid )
Vaporisation (gas  liquid)

9. Equation where:- ∆Q is the energy change in J
m is the mass of substance changing phase in kg
lv is the latent heat of vaporisation in J kg-1
lf is the latent heat of fusion in J kg-1

10. Worked Example 1
The specific latent heat of fusion (melting) of ice is 330,000 J kg-1. What is the energy needed to melt 0.65 kg of ice?

11. Worked Example 1
The specific latent heat of fusion (melting) of ice is 330,000 J kg-1. What is the energy needed to melt 0.65 kg of ice?
∆Q = ml = 0.65 kg × 330,000 J kg-1 = 210,000 J (2 s.f.)

12. (resourcefulphysics.org)
Worked Example 2
The power of the immersion heater in the diagram is 60 W. In 5 minutes, the top pan balance reading falls from 282g to 274g. What is the specific latent heat of vaporisation of water?
(resourcefulphysics.org)

13. (resourcefulphysics.org)
Worked Example 2
The power of the immersion heater in the diagram is 60 W. In 5 minutes, the top pan balance reading falls from 282g to 274g. What is the specific latent heat of vaporisation of water?
P = 60 W
∆ t = 5 minutes = (5 × 60)s = 300 s
m = m2 - m1 = 282g – 274g = 8g = kg
lv = ?
∆Q = P∆ t = 60 W × 300s = 18,000 J
lv = ∆Q/m = 18,000 J / kg = 2.3 × 106 J kg-1 (2 s.f.)
(resourcefulphysics.org)