Detection of Charges

The Structure of a Gold-leaf Electroscope

Describe the structure of a gold-leaf electroscope

The instrument used to detect the presence of electric charges is called gold leaf electroscope. It consists of an insulated brass rod with two pieces of thin gold foil at one end and a brass cap at the other end.

When the brass cap is touched with a charged object the leaves of the electroscope spread out. This is because the charge on the object is conducted through the brass cap and the brass rod to the leaves.

 

As they received the same kind of charge, the leaves repel each other and thus spread apart, this is charging by contact.

If you touch the brass cap with your finger, the charge is transferred through your body to the earth and the leaves of the electroscope then collapse together.

Function of an electroscope

1. Testing for the sign of the charge on the body.
2. Identifying the insulating properties of materials.
3. Detecting the presence of charge on a body.

The Sign of Charges

Determine the sign of charges

The true sign on a body has to be determined before use; the instrument that can be used to determine the presence of charge is called an electrophorus.

An electrophorus consists of a circular slab of insulating material (polythene) together with a brass disc (conductor) on an insulating handle.

An electrophorus works by electrostatic insulation and hence can be used to generate positive charges from single negative charges. The charge produced on the insulating slab is negative. The top disc is then placed on it. Since the surface is only in contact at relatively few points, a positive charge is induced on the lower surface and corresponding negative charge is produced on its top surface.

                                

The top of the upper disc is then touched briefly using a finger, hereby carrying away the negative charge to the earth; this is called EARTHING.

                         

 

Steps of Charging and Discharging of a Gold-leaf Electroscope

Identify steps of charging and discharging of a gold-leaf electroscope

The polythene slab is charged negative by rubbing it with fur. The brass disc is then placed on top of the slab so that the two charges become induced onto respective materials.

Note:Contact does not negatively charge the disc because it is not flat and makes contact with the slab at a few points only. When the brass disc is touched with a finger, electrons on the upper surface are repelled to the earth.

There is a force of attraction between the metal disc and the base. A spark (electric energy) is normally produced upon their separation. This spark can be used for lighting gas burners in laboratory.

The electrophorus can now be used to charge a gold leaf electroscope.

It can be used to charge a gold leaf electroscope by:

1. Contact
2. Induction

By contact

Here a positively charged electrophorus is made to touch the brass cap of the gold-leaf electroscope. The leaf of the gold-leaf electroscope diverges.

When a charged electrophorus is brought into contact with the electroscope, the latter gets charged and the leaves diverge. It acquires a negative charge. This is determined using the charged rods. When a positively charged glass rod is brought near the cap. It causes the leaf to collapse.