Specific Latent Heat

What you’ll learn

Calculate energy for a phase change at constant temperature.

គណនាថាមពលប្ដូរសភាពនៅសីតុណ្ហភាពថេរ។

Meaning and use

Latent heat is energy transferred during a phase change without the temperature change described by mcΔT. Specific latent heat is energy per unit mass for the stated change and conditions. Heating to a melting point and then melting are separate stages whose energies must be added.

កម្ដៅឡាតង់ជាថាមពលប្ដូរសភាពដោយសីតុណ្ហភាពប្រហែលថេរ។ L អាស្រ័យលើសារធាតុ និងប្រភេទការប្ដូរ។ កម្ដៅឡើងដល់ចំណុចរលាយ និងការរលាយជាជំហានផ្សេងគ្នា។

Formulas

Phase-change energy magnitude

\[Q=mL\]

Mass changed

\[m=\frac QL\]

Symbols and units

Symbol	Meaning	Unit or restriction
Q	Magnitude of phase-change energy	J
m	Mass that changes phase	kg
L	Specific latent heat for the stated transition	J/kg

Conditions and limits

• Use L for the correct material and transition at the stated conditions.
• The formula covers only the phase-change stage, not earlier or later heating.
• ការកក ឬកំណកអាចបញ្ចេញថាមពល; ត្រូវបញ្ជាក់ទិសផ្ទេរនៅពេលប្រើសញ្ញា។

Worked examples

Melting ice already at its melting point

Melt 0.2 kg ice using supplied L = 334000 J/kg.

1. Use \(Q=mL\).
2. Calculate \(0.2\times334000=66800\) J.

66800 J absorbed

ទឹកកកចាប់ផ្ដើមនៅចំណុចរលាយ ដូច្នេះមិនបន្ថែម mcΔT មុននេះ។

Finding changed mass

Energy 100000 J causes a phase change with L = 200000 J/kg.

1. Use \(m=Q/L\).
2. Calculate \(100000/200000=0.5\) kg.

0.5 kg

ប្រើតម្លៃ L ដែលបានផ្ដល់សម្រាប់ការប្ដូរសភាពនោះ។

Common mistakes

Instead of	Use	Why
Temperature must rise whenever heat enters.	During an ideal phase change it can remain constant.	Energy changes the phase rather than temperature in that stage.
Use the same L for melting and boiling without checking.	Use the value for the stated transition.	Specific latent heats differ.

Practice

Solve these before revealing the worked solutions. Include units where relevant.

សាកល្បងដោះស្រាយមុនបើកមើលចម្លើយ។ សូមបង្ហាញជំហាន និងឯកតាដែលពាក់ព័ន្ធ។

1. Find Q for m = 0.3 kg and L = 200000 J/kg.
2. Find changed mass for Q = 50000 J and L = 250000 J/kg.
3. A solid starts below its melting point. Is mL alone the total heating-and-melting energy?

Answers and steps

Show solutions / បង្ហាញដំណោះស្រាយ

1. 60000 J

   0.3 × 200000 = 60000.

2. 0.2 kg

   50000/250000 = 0.2.

3. No.

   Add the energy to raise its temperature to the melting point before the phase-change energy.

Quick recap

• Use the correct phase-change value of L.
• Separate heating and phase-change stages.
• State whether energy is absorbed or released.