IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Electric Potential in a Capacitor

## Objective

To study the variation of electric potential in a capacitor.

## Introduction

The textbook talks of large parallel plate capacitors in which the electric field is uniform in the space between the plates and is zero outside. In finite capacitor, fringing of field makes the field look different near the ends. In this experiment you study the potential variation in and around the capacitor.

## Apparatus

A plastic box with two aluminium plates which can be held at a distance and parallel to each other, a battery or power supply, multimeter, graph paper etc.

## Procedure

Place the plates in the box and fill the box with water up to about half the height. Connect the battery to the plates through a switch. Connect the common of multimeter to the plate connected to negative terminal of the battery. Make the multimeter mode DC Voltage. Now wherever you put the other probe of the multimeter, you will get the potential of that point with respect to the negative plate of the capacitor.

Measure the potential perpendicular to the plates in three regions (a) near the middle, (b) near 1/4 of the length of a plate, and (c) at the edge. Each time measure it at a regular interval of say 5 mm and present the data on distance from the negative plate and the potential there.

Measure the potential parallel to the plates in three regions (a) near the middle, (b) near 1/4 of the distance between the plates, and (c) close to a plate. Each time measure it at a regular interval of say 5 mm and present the data on distance from the negative plate and the potential there.

Make more observations close to the plates as there could be sharper variation.

Draw graphs for these variations.

## Questions

1. Is the electric field components perpendicular to the plates constant everywhere in the capacitor?
2. Is the electric field components parallel to the plates zero everywhere in the capacitor?