Beginner · 10 min
How energy moves by heating
Distinguish conduction, convection, and radiation, and compare energy transfer rates.
Three transfer mechanisms
Conduction transfers energy through interactions between neighbouring particles, with free electrons also helping in metals. Convection involves bulk movement of a fluid: warmer, less dense regions can rise in a gravitational field. Thermal radiation transfers energy through electromagnetic waves and can cross a vacuum. An everyday object can exchange energy by more than one mechanism at the same time.
Worked example
A warm mug transfers energy through its wall by conduction, to moving air by convection, and to its surroundings by thermal radiation.
Incoming and outgoing energy
The rate at which an object gains or loses stored energy depends on the balance of transfers. If 70 J enters each second and 50 J leaves each second, the net gain is 20 J each second. Equal incoming and outgoing rates mean no net energy accumulation. This does not mean all transfer has stopped. In a steady situation without a change of state or other energy changes, temperature can stay constant.
Worked example
A model object receives 60 J each second and loses 40 J each second for 3 s. Net energy gain = (60 − 40) × 3 = 60 J
Follow energy around a warm object
- Draw a warm cup in a cooler room and label possible conduction, convection, and radiation transfers.
- For a model object receiving and losing 25 J each second, calculate the net energy change over 10 s. Explain why energy still crosses its boundary.