Quantity of heat — KCSE Physics

Specific latent heat is the amount of heat required to change the state of a unit mass of a substance without changing its temperature. There are two types:

• Latent heat of fusion: This is the heat required to convert a unit mass of solid into liquid at its melting point.
• Latent heat of vaporisation: This is the heat required to convert a unit mass of liquid into gas at its boiling point.

To determine these values experimentally, you can use the following methods:

1. For latent heat of fusion: Measure the mass of ice being melted. Heat the ice using a calorimeter and record the heat supplied until all ice melts. Use the formula:

   [ L_f = \frac{Q}{m} ]

   where ( L_f ) is the latent heat of fusion, ( Q ) is the heat supplied, and ( m ) is the mass of ice.

2. For latent heat of vaporisation: Similarly, measure the mass of water being vaporised. Heat the water until it completely converts to steam. Use the formula:

   [ L_v = \frac{Q}{m} ]

   where ( L_v ) is the latent heat of vaporisation, ( Q ) is the heat supplied, and ( m ) is the mass of water.

The melting and boiling points of substances are influenced by several factors:

• Intermolecular Forces: Stronger forces lead to higher melting and boiling points.
• Molecular Weight: Heavier molecules generally have higher melting and boiling points.
• Pressure: Increasing pressure raises boiling points, particularly for liquids.
• Impurities: The presence of impurities can lower melting points (freezing point depression) and affect boiling points.

To solve problems involving heat transfer, we use the formulas:

• Q = mcΔT: This calculates the heat (Q) absorbed or released when a substance changes temperature, where:

  • m = mass (kg)
  • c = specific heat capacity (J/kg°C)
  • ΔT = change in temperature (°C)

• Q = mL: This calculates the heat (Q) involved in phase changes, where:

  • L = latent heat (J/kg)

Understanding these concepts is crucial for solving related problems effectively.

A pressure cooker functions by increasing the pressure inside the cooking chamber. This increased pressure raises the boiling point of water, allowing food to cook faster. The cooker has a sealed lid with a valve that regulates the steam pressure. When the pressure reaches a certain level, the valve releases excess steam to maintain safe cooking conditions. This efficient heat transfer results in quicker cooking times.

On the other hand, a refrigerator operates using the principles of evaporation and condensation. It contains a refrigerant that absorbs heat from the interior as it evaporates in the evaporator coils. The compressor then compresses this gas, raising its pressure and temperature. This hot gas moves to the condenser coils, where it releases heat to the surroundings and condenses back into a liquid. This cycle continues, keeping the inside of the refrigerator cool by removing heat from the stored items.