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.

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 forms of energy and discuss the interchanges. Concentrate on kinetic, gravitational potential and thermal energy. A Sankey diagram can be used to show where the energy goes when it is ‘lost’, such as the example shown here of energy changes per second (measured in Watts) in a motor vehicle.

Sankey diagram

 

Episode 217-1: Sankey diagram for a motor vehicle (Word, 30 KB)

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 transferred as heat energy 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 losses. This demonstration draws attention to these energy losses.

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, gravitational energy is converted to kinetic energy when an object falls; i.e. mgh = ½mv2.

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

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Episode 217: Conservation of energy (Word, 193 KB)