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Investigating electric potential between two parallel plates and around a charged sphere using a flame probe

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Introduction

Lam Cheuk Ying Regina

   F.6C (12)

Date: 11st March, 2010

Title: Investigating electric potential between two parallel plates and around a charged sphere using a flame probe

Objective: To investigate the vibration of the electric potential with distance using a flame probe

Apparatus:

1 flame probe, 1 goldleaf electroscope, EHT power supply, 1 ray-box, low voltage power 1 pair of capacitor plates, 1 slotted base, 2 polythene tiles, 2 bare copper wire 22 s.w.g. 0.5m, 1 retort stand and clamp, crocodile clips, connecting leads, 1 plastic ball about 0.2m in diameter, coated with a conducting paint and suspended by a nylon thread tied to a small screw on the ball

Procedure:

Calibrating the electroscope

  1. A piece of tracing paper was fixed on the window of an electroscope and a ray-box was used to cast a shadow of the leaf on it.
  2. The cap of the electroscope was connected to the + terminal of an EHT power supply and the metal case was connected to the – terminal. The – terminal was earthed.
  3. The voltage was increased in steps up to about 1500 V and the deflection of the leaf was marked on the tracing paper.

Potential between two parallel plates

  1. The apparatus was set up as above. The metal plates were placed 0.15 m apart.
...read more.

Middle

 shown remains 1.5kV.
  • When the flame probe is moved across from the earthed plate to the positive plate, the deflection of the leaf increased, the electric potential shown increased from 0 kV to 4.3 kV.
  • By E = dV/dr = △V/△r = (4.3-0.4)kV/(0.3-0.02)m = 13.9NC-1

Distance from earthed plate d/m

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

Electric potential kV/V

0.4

0.5

0.7

0.8

1.0

1.2

1.5

1.8

2.0

2.3

2.7

3.0

3.5

3.9

4.3

image00.png

A graph of V against d gives a straight line with positive slope passing through 0. It shows that the electric potential measured by the probe is directly proportional to the distance of the probe from the earthed plate.

  • Potential around a charged sphere
  • When the flame probe is moved around the sphere, keeping it at a 0.05m from the centre of the sphere, the deflection of the leaf remains unchanged, the electric potential shown remains 3.5kV.
...read more.

Conclusion

  • Sources of error:

When measuring the distance of the probe from the centre of the sphere, a piece of string with a mass attached was dropped from the sphere for locating the zero mark of the meter ruler. This may not be accurate as the sting may not be vertically below the centre of the sphere

  • Precautions:
  1. The wire which connects the needle and the electroscope cannot touch the bench or any other earthed conductor. This is to prevent charge leakage.
  2. The charged sphere must be well away from the walls and the bench top as the induced charges on these objects will upset electric field.

Conclusion:

  • Between two parallel plates:
  • the electric potential remains constant at constant distance from the metal plates
  • the electric potential is proportionally to the distance from the earthed plates
  • Around a charged sphere:
  • the electric potential remains constant at constant distance from the centre of sphere
  • the electric potential is inversely proportionally to the distance from the surface of sphere

...read more.

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