Em06 Equipotentials

31/10/2009

abstract

Introduction

The aim of this experiment is to investigate the electrostatic field pattern in a perspex dish of tap water, by measuring the lines of equipotential created by various electrode configurations, using a technique based on the bridge connection of resistors.

Theory

Within an Electric Field a force is exerted upon electrically charged particles. This force is equivalent to the product of the field strength (E) and and charge of the particle (q). The formula thus, is F = qE.

The electric field is a vector field, as it has both strength (E) and direction (directing charges from positive to negative).

The electric field can be illustrated graphically in the following manner:

A diagram showing the electric field between two charges plates.

where each field line has an arrow illustrating its direction.

Equipotential lines always lie perpendicular to the electric field.

The experiment uses a bridge circuit in order to eliminate the error associated with the voltmeter. This is because the potential of the water will not be measured directly, instead it will be compared with a point of known voltage within the bridge circuit.

List of Apparatus

  1. Perspex dish of tap water
  2. Graph paper x2
  3. Two sets of Electrodes
  4. Audio frequency Oscillator (3000Hz)
  5. Oscilloscope
  6. Wire probe
  7. Bridge circuit of Resistors
  8. Metal bar

Experimental Diagrams

Electrode Set 1

Electrode Set 2

Method

  1. Place the perspex dish with approximately 2.5cm of tap water on top of the first sheet of graph paper, ensuring that the edges of the paper are parallel with the edges of the dish.
  1. Place the electrodes (beginning with 'set one') in the perspex dish, ensuring they are connected to the output terminals of the audio oscillator.
  1. Set up an alternating voltage at a frequency of 3 kHz to avoid polarizing effects in the water.
  1. Connect the oscilloscope, ensuring that it is connected with the potential at its ground remains VB . The reading on the oscilloscope is now the potential difference between X and B.
  1. Move the wire probe about the tank, finding the points where Vx = VB = fV, for f = 0.1 through to 0.9, at intervals of 0.1. Plotting each equipotential line on the second piece of graph paper, corresponding to the first sheet (under the dish).
  1. Draw the error bars on the graph paper for various representative points spread across the tank. And ensure the position of the electrodes is clearly marked on the graph paper.
  1. For electrode 'set one', place the metal bar parallel to the lower electrode, measuring how the presence of the bar changes the lines of equipotential.
  1. Repeat this with the bar perpendicular to the lower electrode.
  1. Sketch the electric field lines.

Safety Procedures

This experiment is very safe provided and there is no risk of even minor accident if it is conducted properly and common sense is used at all times.

References

  1. “Equipotentials”, experimental brief provided by the department.

• Results (this section has to include the final results of the analysis, i.e. the coefficients

and the errors produced by the three methods. This section will also contains the data

table and the plot (from the computer fitting). All calculations and the manual graph

plotting results will be made in the labbook***. The later will also contain the exercises on

the error propagation.)

• Discussion (discuss the similarities/differences between the three different mehods of

analysis. Compare the results and their errors, explain the differences. This section should

also contain the answers to the questions given in different sections of the practical.)

• Conclusions. (brief summary of the analysis)

• References (list the sources of any supplementary materials used.)