3. A swinging pendulum If you hold a pendulum to one side and release it, it will swing for a while and eventually come to a stop. Gravitational potential energy is converted to motion energy on the downswing of the pendulum. On the upswing, motion energy is converted to gravitational potential energy. The height of the swing will become slightly lower with each swing. This is because some of the motion energy is dissipated as heat energy to the surrounding air. This is due to air resistance. Eventually all the motion energy will dissipate and the pendulum will return to a resting position.
30.5 Energy efficiency
We can use a Sankey diagram to show how much energy is changed into useful forms and into forms that are not useful and which dissipate. The thicker the arrow in the diagrams below, the greater the amount of energy involved. Let us take a look at two light bulbs to illustrate energy changes and dissipation. 1. Old-type filament light bulbs
Input energy
Useful output energy
Highest swing point. No motion energy. Maximum gravitational potential energy.
Fig. 13 A swinging pendulum.
Maximum motion energy. Minimum gravitational potential energy.
2. Modern energy-saving bulbs
Input energy
Dissipated energy
Useful output energy
Dissipated energy
Fig. 14 Sankey diagrams show how much input energy is changed into useful forms and how much is dissipated.
The efficiency of a device is determined by how much of the input energy changes into useful output energy. The more energy changed into useful energy, the more efficient the device is.
Calculating efficiency:
Useful energy output (J) x 100 Total energy input (J)
