Electric Charge ( Electrostatics )

Electrostatics:

"The study of stationary electric charges or fields as opposed to electric currents"

Electrostatics is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges

Electrically charged objects have several important characteristics:

Like charges repel one another; that is, positive repels positive and negative repels negative.
Unlike charges attract each another; that is, positive attracts negative.

Charge is conserved. A neutral object has no net charge. If the plastic rod and fur are initially neutral, when the rod becomes charged by the fur, a negative charge is transferred from the fur to the rod. The net negative charge on the rod is equal to the net positive charge on the fur.

A conductor is a material through which electric charges can easily flow. An insulator is a material through which electric charges do not move easily, if at all. An electroscope is a simple device used to indicate the existence of charge. As shown in Figure 1, the electroscope consists of a conducting knob and attached lightweight conducting leaves—commonly made of gold foil or aluminum foil. When a charged object touches the knob, the like charges repel and force the leaves apart. The electroscope will indicate the presence of charge but does not directly indicate whether the charge is positive or negative.

Figure 1

An electroscope reports the presence of charge.

A large charge near a neutral electroscope can make the leaves move apart. The electroscope is made of conducting material, so the positive charges are attracted to the knob by the nearby (but not touching) negatively charged rod. The leaves are left with a negative charge and therefore deflect. When the negative rod is removed, the leaves will fall.

Now, consider touching the electroscope knob with a finger while the charged rod is nearby. The electrons will be repulsed and flow out of the electroscope through the hand. If the hand is removed while the charged rod is still close, the electroscope will retain a charge. This method of charging is called charging by induction (see Figure 2).

Figure 2

Charging an electroscope by induction.

When an object is rubbed with a charged rod, the object shares the charge so that both have a charge of the same sign. In contrast, charging by induction gives an object the charge opposite that of the charged rod.

Even though the charges are not free to travel throughout the material, insulators can be charged by induction. A large charge nearby—not touching—will induce an opposite charge on the surface of the insulator. As shown in Figure 3, the negative and positive charges of the molecules are displaced slightly. This realignment of charges in the insulator produces an effective induced charge.