Tutorial details

Students work through a series of questions that form a demanding review of many electrostatic concepts (charge density, field, circuits). They begin by finding the charge density of a large thin conducting plate with a given net charge. Using the principle of superposition or Gauss’ law, students determine the electric field at various locations. A second conducting plate with the same magnitude charge, but opposite sign, is then held near the first and students determine the distribution of charge on the two plates.

The second and third sections of the tutorial are based on situations similar to those presented in the pretest. Students first consider two conducting plates with given surface charge densities that are placed on insulating stands a given distance apart. They determine the electric field between and outside the plates and the potential difference between the plates. They are guided to recognize how moving the plates closer together would affect the electric field and potential difference. They also consider how the ratio of charge to potential difference is affected by the total charge on each plate. Next students imagine that the plates are connected to different terminals of a battery. They work through similar questions that help them relate the potential difference to the electric field between the plates and the charge on each plate. They also consider how moving the plates closer together would affect these quantities.

Some students who try to answer the questions above based solely on formulas often have difficulty in recognizing which variables are changing and which remain the same as the plate separation is changed. They should be led to relate the formulas to the basic physical situation.

In the last section, students reflect on the previous two cases—in which the plates were set on insulating stands and then connected to a battery—to compare and contrast the situations. They are led to recognize that the ratio of the charge on the plates to the potential difference between the plates is the same for each situation. This result motivates the definition of capacitance.

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Prerequisite tutorials

The Charge, Electric fields and flux, Gauss' law, and Electric potential difference tutorials are prerequisites to Capacitance.

Other prerequisites

Students are expected to have worked through previous tutorials Charge, Electric field and flux, Gauss’ law, and Electric potential difference. No knowledge of capacitors or capacitance is assumed.

Equipment

List

• white board or large sheet of paper with markers

Discussion