Energy Diagrams

In quantum mechanics the concept of energy is frequently used to help us describe events and motion. Instead of describing changes in motion in terms of forces and accelerations, we use changes in energy. A convenient way to describe changes in energy is to sketch a graph of the value of an object's energy at various locations. When we create such a graph for energy, we call it an energy diagram.

Using energy to describe motion is somewhat different from the typical approach. However, it is a very powerful technique that can be used for both small and large objects.

To prepare you for learning about atoms and other small objects you will first describe the motion of a toy car using potential energy diagrams. Toy cars, magnets and "hotwheels" track have been included in your course materials for this purpose. You will see that you will be able to describe the motion of a moving object without actually seeing that object or its motion. All you need to know is the energy.

An example of a potential energy diagram is shown in Figure 1. On the horizontal axis is the location of the car while the vertical axis shows the value of the potential energy at each location. It shows us how the potential energy changes along the path of the car. The potential energy is highest for locations between 0 and 0.2 meters, is zero from 0.2 to 0.4 meters, and rises to an intermediate value for 0.4 to 0.6 meters. We shall see we can learn a lot about the car's motion from this diagram.

The potential energy is associated with the position of the object. This energy can arise from variety of interactions including elastic, gravitational, chemical, etc. In this unit, the potentials are generated with the help of magnets. However, the interactions between atoms and nuclei are of different origin. Therefore, the comparison with the atom that you will attempt later on, will only be concerned with the shape of the potential energy curve, not with the origin of the interactions.

In these four activities you will arrange magnets along a track and observe the interaction between these magnets and a magnet on the car. By changing configurations of the magnets along the track you will obtain potential energy diagrams of various shapes. In this activity you will arrange the magnets in attractive mode.

The friction between the wheels of the car and the track is relatively high. If we were to account for that friction, it would have made the energy diagrams complicated. However, for the purposes of our qualitative analysis we will try to think about what would have happened if the friction acting on the car was very small.