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

# Direction of Electric Field

## Introduction

We study in electrostatics that if we move in the direction of the electric field, the electric potential decreases. This demonstration is a useful way to understand it as we can see the change of potential in the multimeter. There is no need to visualize it in our minds.

## Apparatus

Two aluminium plates, a glass/plastic trough, water, a battery, a multimeter and some connecting wires and a little plasticine.

## Procedure

1. Fix the two aluminium plates in plasticine and put it on the two edges of the glass/plastic trough. Pour water into the trough.
2. Connect the two plates with the two terminals of the 9 V battery. Connect the left plate to the positive terminal of the battery and the right plate to the negative terminal.
3. Now connect the black lead (common terminal) of the multimeter to the aluminium plate on the right.
4. Move the other terminal (red lead) of the multimeter from left to right. Note the voltage in the multimeter. It decreases as we move towards right. Why?
5. Now move the multimeter lead parallel to the plates. Do you see any change in voltage of the multimeter. Why?

## Discussion

The left aluminium plate is connected to the positive terminal of the battery so the direction of electric field is from left to right. But as we move in the direction of the electric field the potential decreases. So the multimeter shows a decrease in the voltage which is the potential at that point with respect to the potential of the common terminal.

When we move parallel to the plates there is no change in potential because we are moving on an equipotential surface.

Point of discussion: The presence of water in the trough is mandatory for the multimeter to show any voltage. Water being a conductor (a poor one though with dielectric constant = 80) allows the current to flow through the multimeter enabling it to show the voltage.