Key Equations

Thermal Energy Transfer — AQA A-Level Physics

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Thermal energy change
$$\Delta Q = mc\Delta\theta$$
  • Where:
    • $\Delta Q$ = change in thermal energy (J)
    • $m$ = mass (kg)
    • $c$ = specific heat capacity (J kg^{-1} \(K^{-1}\))
    • $\Delta \theta$ = change in temperature (K or degrees C)
Delta theta is the same in kelvin and Celsius. No conversion needed.
Continuous flow calorimetry
$$c = \frac{Q_2 - Q_1}{(m_2 - m_1)\Delta\theta}$$
  • Where:
    • $c$ = specific heat capacity (J kg^{-1} \(K^{-1}\))
    • $Q_1$ = electrical energy in run 1 = I_1 V_1 t_1 (J)
    • $Q_2$ = electrical energy in run 2 = I_2 V_2 t_2 (J)
    • $m_1$ = mass of fluid in run 1 (kg)
    • $m_2$ = mass of fluid in run 2 (kg)
    • $\Delta\theta$ = temperature rise (same in both runs) (K)
Derived by subtracting two runs of IVt = mc delta theta + E_lost. E_lost cancels.
Energy for change of state
$$Q = mL$$
  • Where:
    • $Q$ = thermal energy for change of state (J)
    • $m$ = mass (kg)
    • $L$ = specific latent heat (J kg^{-1})
L is either L_fusion (solid to liquid) or L_vaporisation (liquid to gas). Temperature does not change.
Electrical energy input
$$Q = IVt$$
  • Where:
    • $Q$ = electrical energy (J)
    • $I$ = current (A)
    • $V$ = potential difference (V)
    • $t$ = time (s)
Used in continuous flow and other heating experiments. Equivalent to Q = Pt.
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