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Specific latent heat and thermal insulation

  • 3 things to know
  • Paper 6

Overview

The energy for a change of state from mass and specific latent heat, calculations for melting and boiling, and how trapped air, double glazing, loft insulation, low-conductivity walls and cavity foam reduce energy transfer.

What you need to know

Answer each one before you reveal it.

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Key term

Explain, in terms of particles and conduction, how a woollen jumper, double glazing, thick loft insulation and cavity foam reduce energy transfer.

Key term

State the equation for the thermal energy needed for a change of state.

Numbers

Calculate the energy needed to melt 0.50 kg of ice at 0 °C, given a specific latent heat of fusion of 3.34 × 10⁵ J/kg.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the equation for the thermal energy needed for a change of state.

    Model answer

    Thermal energy = mass × specific latent heat.

  2. 2

    Calculate the energy needed to melt 0.50 kg of ice at 0 °C, given a specific latent heat of fusion of 3.34 × 10⁵ J/kg.

    Model answer

    Energy = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J.

  3. 3

    Calculate the energy needed to boil away 0.20 kg of water at 100 °C, given a specific latent heat of vaporisation of 2.26 × 10⁶ J/kg.

    Model answer

    Energy = 0.20 × 2.26 × 10⁶ = 4.52 × 10⁵ J.

  4. 4

    Melting 2.0 kg of a solid at its melting point requires 6.8 × 10⁵ J. Calculate its specific latent heat of fusion.

    Model answer

    Specific latent heat = 6.8 × 10⁵ ÷ 2.0 = 3.4 × 10⁵ J/kg.

  5. 5

    A substance has a specific latent heat of 2.0 × 10⁵ J/kg. Calculate the mass changed in state by 5.0 × 10⁴ J.

    Model answer

    Mass = 5.0 × 10⁴ ÷ (2.0 × 10⁵) = 0.25 kg.

  6. 6

    Explain why no temperature term appears in the equation for the energy needed for a change of state.

    Model answer

    The temperature stays constant while the state changes, so only the mass and the specific latent heat matter.

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