Detection of charges

Electrophorus

Is the instrument that can be used to determine the presence of charge

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.

Charging an Electrophorus

Electrophorus is charged by induction. The polythene is given a negative charge by rubbing it with fur causing positive charge to be induced on the upper part of the brass plate, and a negative charge on the lower part of the brass plate will leave the polythene plate charged negatively. The electrophorus is left with an excess positive charge

 

Gold Leaf Electroscope

Gold leaf electroscope is an instrument for detecting/identify the presence of an electrical charge on an object. The gold leaf electroscope consists of gold leaf and a brass rod of metal held by insulating materials

The brass cap, brass rod and the gold leaf form the conducting part of the electroscope.

Charging an Electroscope

It charged by contact and Induction

Charged by Contact

When charge and uncharged body contact the charge always move from charged body to another because like charge repel and unlike charge attract.

Therefore, the charged one when brought to contact with metal cap result to charge metal cap and became charged

Charged by Induction

A negatively charged rod is brought near the brass cap of the gold leaf electroscope. Induced charges are formed with brass rod acting as conductor.

Uses of Gold Leaf Electroscope

A gold leaf electroscope has a number of

functions the follows are some of them

         i.            Test for the sign of charge on a body

       ii.            Identify the insulating properties of material

     iii.            Detect the presence of charge on a body

 

Test for the sign of charge on a body

         i.         It used to test whether charge is positive or negative.  In order to identify the charge of a body we should use a charged electroscope. Let us say the electroscope is negatively charged.

Now bring the unknown charged body 'X' near the cap of the electroscope.

       ii.        If the leaves diverge more the charge in 'X' must be negative. Suppose that the leaves close a little when 'X' is brought near 'C', then 'X' may have a positive charge or it may not have any charge.

 

     iii.        In order to confirm the positive charge, you must bring the rod 'X' near the cap of a positively charged electroscope.

If the leaves diverge more, then 'X' has positive charge and vice versa.

Table of summary of charges and their effect

Charge on electroscope	Charge brought near	Effect on leaf divergence
+	+	Increase
-	-	Increase
+	-	Decrease
-	+	Decrease
+ or -	Uncharged body	Decrease

Identify the insulating properties of material

An electroscope that is positively charged can used to test for the insulating properties of material.

If the material that as placed near the cap of an electroscope is a conductor, then the metal leaf will converge and a divergence will show that the material is an insulator