Latent HeatLatent Heat

Thermal Physics Lesson 2Thermal Physics Lesson 2

Learning ObjectivesLearning ObjectivesDefine specific latent heat.Define specific latent heat.Perform calculations using ∆Q=ml.Perform calculations using ∆Q=ml.

Describe how specific latent heat of Describe how specific latent heat of fusion/vaporisation can be measured fusion/vaporisation can be measured in the lab.in the lab.

Recall Heating/Cooling CurvesRecall Heating/Cooling Curves

Remember from GCSE?

DemonstrationDemonstrationWhat tells you the water is boiling?What tells you the water is boiling?

So energy is being supplied but the So energy is being supplied but the temperature is not rising?temperature is not rising?What is going on?What is going on?

Work is being done to separate the Work is being done to separate the particles against intermolecular particles against intermolecular attractive forces. attractive forces.

shc vs. latent heat shc vs. latent heat (simple terms)(simple terms)

Specific Heat Capacity:-Specific Heat Capacity:-Energy needed to heat somethingEnergy needed to heat something

Latent Heat:-Latent Heat:-Energy needed to change phaseEnergy needed to change phase

Latent HeatLatent HeatThe 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.

Latent Heat FormulaLatent Heat Formula

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.

Q = m Lf or Q = m Lv Q = thermal energy m = mass L = heat of fusion or vaporization

DefinitionDefinitionThe specific latent heat (l) of fusion The specific latent heat (l) of fusion or vaporisation is the quantity of or vaporisation is the quantity of thermal energy required to change thermal energy required to change the phase of 1kg of a substance.the phase of 1kg of a substance.

Fusion (Fusion (liquid liquid solid solid ))Vaporisation (Vaporisation (gas gas liquid liquid))

EquationEquation

mlQ

where:-∆Q is the energy change in Jm is the mass of substance changing phase in kglv is the latent heat of vaporisation in J kg-1

lf is the latent heat of fusion in J kg-1

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

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

∆Q = ml = 0.65 kg × 330,000 J kg× 330,000 J kg-1 -1 = = 210,000 J (2 s.f.)210,000 J (2 s.f.)

mlQ

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

(resourcefulphysics.org)

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

P = 60 W∆ t = 5 minutes = (5 × 60)s = 300 s(5 × 60)s = 300 sm = m2 - m1 = 282g – 274g = 8g = 0.008 kg = 282g – 274g = 8g = 0.008 kglv = ?

∆Q = P∆ t = 60 W × 300s× 300s = 18,000 J= 18,000 Jlv = ∆Q/m = 18,000 J / 0.008 kg 0.008 kg = 2.3 × 10= 2.3 × 106 6 J kgJ kg-1 -1 (2 (2 s.f.)s.f.)

tEnergyP

(resourcefulphysics.org)

mlQ