B.1 · IB Physics
Heating and phase change
Heat a solid at constant power and watch its temperature climb, then stall at the melting and boiling points while the energy goes into breaking bonds, not raising temperature.
Predict first
Start from the default settings (water, 500 W, 1.0 kg). If you double the heater power to 1000 W, what happens to the total energy needed to run the curve all the way to the gas stage?
The energy needed is set by Q = mcΔT and Q = mL for the mass and substance. Power only sets how fast it is delivered, since t = Q ÷ P, so the time halves but the energy does not change.
Check it in the model: Move the Heater power, P slider from 500 to 1000 W and compare the Total energy for the full curve and Time elapsed readouts once the run finishes.
curve so far current point · the box shows the particles (dark: solid, gold: liquid, green: gas)
Q = m c ΔT (temperature changes)
Q = m L (melting or boiling, T constant)
Try this
- Choose water at 500 W and 1.0 kg. Compare the length of the melting plateau with the boiling plateau, then explain the ratio using L.
- Switch between 100 W and 1000 W with the box ticked for time. Which parts of the graph change, and which keep the same shape?
- Choose lead and then ethanol. Compare the slopes of the three rising sections and link each to the specific heat capacity.
Exam tip: on a plateau the temperature is constant and the energy goes into Q = mL; on a slope use Q = mcΔT, and a steeper slope means a smaller c. Do not mix up c and L.