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1.8.4Electricity, Energy and Waves

Heat Transfer and Changes in Temperature

5 min read 5 parts Lesson 44 of 54 Unit quiz

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Section 1 of 5What Controls a Temperature Rise

1.8.4 Heat Transfer and Changes in Temperature

When a substance is heated, four things decide how large the temperature rise will be.

Mass
  • A larger mass needs more energy for the same temperature rise because there are more particles to share the energy.
Material
  • Different materials store energy differently, so equal masses of two substances rise by different amounts when given equal energy.
Energy supplied
  • The more thermal energy transferred into the substance, the bigger the rise in its temperature, provided no state change occurs.
Temperature change
  • The energy needed is proportional to how many degrees the substance is heated, written as Δθ (final minus initial temperature).

Section 2 of 5Specific Heat Capacity

Specific heat capacity, c, is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.

Specific Heat Capacity
Equation
  • Q = mcΔθ, where Q is energy in joules, m is mass in kilograms and c is specific heat capacity in J/kg °C.
Rearranging
  • For higher tier work, c = Q ÷ (mΔθ), m = Q ÷ (cΔθ) and Δθ = Q ÷ (mc) all follow from the same equation.
Low value of c
  • The substance warms and cools quickly because little energy is needed for each degree of temperature change.
High value of c
  • The substance warms and cools slowly because a large amount of energy is needed for each degree of change.
MaterialApproximate c (J/kg °C)
Water4200
Aluminium900
Iron450
Copper390

Section 3 of 5Using the Idea in Everyday Life

Water as a coolant
  • Water has a very high specific heat capacity, so it carries away a lot of energy from an engine for a small temperature rise.
Storage heaters
  • Bricks or other dense materials with a high specific heat capacity store large amounts of energy overnight and release it slowly.
Cooking pans
  • Metal pans have a low specific heat capacity, so they heat up quickly and pass energy on to the food.
Coastal climates
  • Large bodies of water warm and cool slowly, which makes coastal areas milder than places far inland.

Section 4 of 5Worked Example

Question

  • A kettle heats 2.5 kg of water from 20 °C to 60 °C.
  • The specific heat capacity of water is 4200 J/kg °C.
  • Calculate the energy transferred to the water.

  1. Find the temperature changeΔθ = 60 − 20 = 40 °C.
  2. Write the equationQ = mcΔθ.
    What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
  3. Substitute the valuesQ = 2.5 × 4200 × 40.
  4. Calculate2.5 × 4200 = 10 500, then 10 500 × 40 = 420 000.

Answer Q = 420 000 J (4.2 × 10⁵ J)

Section 5 of 5Exam Checklist & Tips

1.8.4 – Key Points for the Exam

  • Specific heat capacity The energy needed to raise the temperature of 1 kg of a substance by 1 °C.
  • Thermal energy Energy transferred to or from a substance as a result of a temperature difference.
  • Temperature change (Δθ) The final temperature minus the starting temperature, measured in °C.
  • Joule (J)
    • The unit of energy;
      • one joule is the energy transferred when a force of 1 N moves an object 1 m.

Common Mistakes

  • Using the final temperature instead of the temperature change in Q = mcΔθ.
  • Leaving the mass in grams; the equation needs kilograms, so 500 g must be written as 0.5 kg.
  • Thinking a high specific heat capacity means a substance heats up quickly; it heats up slowly.
  • Forgetting that the equation only applies while the substance is not changing state.
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