Electric Charge & important facts about it.

Electrostatics :  Electrostatics is the branch of Physics, which deals with static electric charges or charges at rest. The charges in a electrostatic field are analogous to masses in a gravitational field. These charges have forces acting on them and hence possess potential energy. 

Electrostatics – frictional electricity : The substances which acquire charges on rubbing are said to be ‘electrified’ or charged. These terms are derived from the Greek word elektron, meaning amber. The electricity produced by friction is called frictional electricity. If the charges in a body do not move, then, the frictional electricity is also known as Static Electricity.

Two kinds of charges :

1. If a glass rod is rubbed with a silk cloth, it acquires positive charge while the silk cloth acquires an equal amount of negative charge.

2. If an ebonite rod is rubbed with fur, it becomes negatively charged, while the fur acquires equal amount of positive charge. This classification of positive and negative charges were termed by American scientist, Benjamin Franklin.

Thus, charging a rod by rubbing does not create electricity, but simply transfers or redistributes the charges in a material.

Like charges repel and unlike charges attract each other– experimental verification :

A charged glass rod is suspended by a silk thread, such that it swings horizontally. Now another charged glass rod is brought near the end of the suspended glass rod. It is found that the ends of the two rods repel each other (Fig 1.1). However, if a charged ebonite rod is brought near the end of the suspended rod, the two rods attract each other (Fig 1.2). The above experiment shows that like charges repel and unlike charges attract each other.

The property of attraction and repulsion between charged bodies have many applications such as electrostatic paint spraying, powder coating, fly−ash collection in chimneys, ink−jet printing and photostat copying (Xerox) etc.

Note :

Gold-leaf electroscope : A simple apparatus to detect charge on a body is the gold-leaf electroscope [Fig.]. It consists of a vertical metal rod housed in a box, with two thin gold leaves attached to its bottom end. When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and they diverge. The degree of divergance is an indicator of the amount of charge.

Fig. : Gold-leaf Electroscope