Episode 217: Conservation of energy

The Principle of Conservation of Energy lies behind much of the physics studied at this level. Although we are concerned with conservation of mechanical energy in this episode it is useful to extend the principle to its wider context in order to give the students a full appreciation of the overarching nature of the principle when they meet it in different guises.

Lesson Summary

• Discussion: Examples of energy conservation (5 minutes)
• Demonstration: Energy transfers (10 minutes)
• Student questions: Including calculations (15 minutes)

Discussion: Examples of energy conservation

Introduce different ways that energy can be stored, and discuss the physical process by which it is transferred. Concentrate on energy stored kinetically, gravitationally and thermally. Introduced power as energy transferred per second (measured in Watts). A Sankey diagram can be used to show where the energy is dissipated.

Note that some students may think that the conservation of energy is an idealized notion, and that in practical situations, energy is not conserved. This is an incorrect idea. Energy is always conserved; in practical situations, some energy may be dissipated, so stored thermally when we do not want it to be; however, correct accounting will show that the total amount of energy is still constant.

Demonstration: Energy transfers

In transport systems, it is vital to minimize energy dissipated. This demonstration draws attention to transfers that result in dissipation.

An alternative approach would be to ask a group of students to prepare this as a presentation, which they could then make to the class.

Episode 217-2: Free transport? (Word, 136 KB)

Student questions: Including calculations

These questions make use of the idea that, in a frictionless system, energy stored gravitationally is then stored kinetically when an object falls; i.e. mgh = 12mv².

Episode 217-3: Energy conservation (Word, 38 KB)