Apparatus Used
- White plastic ruler
- Cloth
- Gold-Leaf Electroscope
- Lighter
Observations
The length of the plastic ruler was rubbed continuously with cloth for about 10 seconds. The cloth was then removed, and the ruler was brought close to the surface of the electroscope provided. Upon close proximity with the surface of the electroscope, the gold leaf displaces, with one side of the gold foil rising up, forming an angle with the other side of the gold foil. A finger was then placed on the surface of the electroscope, opposite to that of the ruler and it was then observed that the previously displaced leaf fell back to its original position, with both gold foil hanging vertically downwards with no angle between either of them. The finger was then removed, and following that, the ruler as well. This time, the leaf was observed to rise yet again, forming a similar angle with the other leaf as in the case where the ruler was placed in close proximity with the surface of the electroscope. A lighter flame was then brought close to the surface of the electroscope, and the gold leaf fell back into its original position.
Explanation
Rubbing the plastic ruler with cloth would cause both objects to be charged eletrostatically. Electrostatic forces of attraction between positive and negative charges on both objects would cause a movement, or a transfer of electrons between the objects, resulting in a disparity in the number of charges in both objects. In this case, the plastic (Perspex) ruler is known to be charged positively due to a transfer of electrons from the ruler to the cloth; the consequent increase of electrons within the cloth would cause it to be negatively charged. This is known as charging by friction, where the constant rubbing motion of the cloth provides heat energy to electrons held within the atoms of both the ruler and the cloth, resulting in a rise in kinetic energy of the electron and thus allowing the electron to break free from the nuclear charge exerted by the nucleus. These electrons are thus mobile, and can be transferred to the other object through electrostatic attraction between the electron and the nucleuses of the other object.
The positively charged ruler is brought within close proximity to the surface of the electroscope. This causes an inductive effect, where electrons within the electroscope are attracted to the positively charged ruler and move to the top of the electroscope, while positive charges are similarly repelled towards the bottom of the electroscope. As both sides of the gold foil are positively charged, they repel each other, causing one side of the gold foil to rise, resulting in a formation of an angle between them.
When a finger is placed on the electroscope on the opposite side of the ruler, the deficiency of electrons on the opposite side of the ruler as electrons are attracted to the positively charged ruler would cause electrons to flow from the earth, through the experimenter and onto the slightly positively charged side of the surface of the electroscope. This discharges the electroscope it as some of these electrons move down towards the gold foil, neutralizing the positive charges of both gold foil, causing the gold leaf to fall. Upon removal of the positively charged ruler, the electrons previously held at the surface of the ruler would flow back down into the now net negatively charged electroscope, negatively charging both gold foil and causing it to repel and separate again.
When the lighter flame is brought close to the electroscope, the high heat from the flame ionizes the air around the electroscope. Essentially the flame is made up of plasma, where the high heat ionizes the air within the flame, causing it to split into positive and negative ions. When positive ions come into contact with the electroscope, the net negative charge of the electroscope is offset by the increase in positive ions, causing the electroscope to be discharged and the leaves to fall.
