Answer to Question #100359 in Electricity and Magnetism for Angela

Question

When a glass rod is rubbed with silk, the rod becomes positively charged, but when a rubber rod is rubbed with fur, the rubber becomes negatively charged. Suppose you have a charged object but don’t know whether its charge is positive or negative. Explain how you could use a glass rod and piece of silk to determine the sign of the unknown charge on the object.

Answer 1

When a glass rod is rubbed with silk, the rod becomes positively charged and as the sum of the total charge must be zero this means that silk has become negatively charged .

As we know that the same charges repel each other and repulsion is the surest test of charge so,

for identification of the sign of charge on the object we will bring the object near the glass rod, if it repels then charge on the object must be positive and if does not repels we bring the silk near the object and now, if it repels then the charge on the object must be negative.