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Specific heat capacity and latent heat
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Heating a material either raises its temperature or changes its state. Two equations tell you how much energy is needed.
Specific heat capacity
The specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
ΔQ = m × c × Δθ: energy (J) = mass (kg) × specific heat capacity (J/kg °C) × temperature change (°C).
Water has a high value: about 4200 J/kg °C.
ΔQ = m × c × Δθ: energy (J) = mass (kg) × specific heat capacity (J/kg °C) × temperature change (°C).
Water has a high value: about 4200 J/kg °C.
Specific latent heat
During a change of state the temperature stays the same; the energy breaks or makes bonds between particles.
Q = m × L: energy (J) = mass (kg) × specific latent heat (J/kg).
Latent heat of fusion: melting. Latent heat of vaporisation: boiling.
Q = m × L: energy (J) = mass (kg) × specific latent heat (J/kg).
Latent heat of fusion: melting. Latent heat of vaporisation: boiling.
Heating curves
On a temperature-time graph for heating ice, the flat parts are changes of state (melting, then boiling). The sloping parts are where the temperature rises.
Internal energy is the total kinetic and potential energy of the particles. Heating increases it.
Internal energy is the total kinetic and potential energy of the particles. Heating increases it.
How much energy is needed to heat 2 kg of water from 20 °C to 70 °C? (c = 4200 J/kg °C)
- Δθ = 70 − 20 = 50 °C
- ΔQ = 2 × 4200 × 50
Answer: 420 000 J
ΔQ = m c Δθ for temperature change; Q = m L for change of state. The temperature stays constant while a substance melts or boils.
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