Methods of Charging

Key Concepts

• Method of charging by rubbing
• Method of charging by induction
• Method of charging by conduction
• Uses of an electroscope

Introduction: 

The transfer of electrons from one place to another by rubbing causes an imbalance of positive and negative charges. This results in static electricity. Static electricity is the build-up of positive or negative electric charges on a material’s surface. 

Some materials that can be charged by rubbing are plastic comb, balloons, dry hair, cat’s fur, glass rod, silk cloth, ebonite rod, etc. The charges produced are static. They produce static electricity. But if we rub silver, copper, steel, or brass items we see that they cannot get charged by friction.  

When a balloon and hair are charged by rubbing, only the rubbed areas become charged, and there is no tendency for the charge to move into other regions of the material. 

In contrast, when materials like copper, aluminum, and silver are charged on rubbing in some region, the charge readily distributes itself over the entire surface of the material, then the charge gets transferred to the human body and the material again becomes neutral. 

Explanation: 

We can classify substances in terms of their ability to transfer charge. 

Charging a conductor by induction: 

When a charged body is brought near an uncharged conductor, the charged body will attract the opposite charges and repel similar charges present in the uncharged body. As a result of this one side of the neutral body (close to the charged body) becomes oppositely charged while the other is similarly charged. 

Charging an insulator by induction: 

When a charged body is brought near an insulator, the distribution of charge in atoms and molecules is shifted slightly. An opposite charge is induced nearer the charged rod, while the same charge is pushed. 

Conclusion: 

• The process of charging by redistribution of charges in a material is called electrostatic induction. 
• In the method of charging by induction, the inducing body neither gains nor loses charge. 
• The induced charge can be lesser or equal to the inducing charge but can never be greater. 
• In the process of charging by induction the charged body and neutral body are not kept in contact. 

Electroscope: 

An electroscope is a simple apparatus with which the presence of an electrical charge on a body is detected and the polarity of the charge can be determined. 

Detecting charge:  

When the metal knob is touched with a charged body, some charge is transferred to the leaves, which then diverge due to repulsion. When an uncharged body touches the metal knob the leaves don’t diverge. Thus, separation gives an idea of the amount of charge on the body. 

More charges à  More separation 

Fewer charges à Less separation 

Determining the polarity of charge:  

Using an electroscope, you can check the polarity of an electric charge. For example, let’s assume an electroscope in which the leaves are opened with some none type of charge (+ or –). 

If we bring the same type of charge close to the electroscope, the leaves will unfold more. 

On the other hand, if we bring the opposite charge close to the electroscope, the leaves will shrink.  

Experiment: 

An electroscope can be used to show experimentally induced charge separation, charging by conduction, charging by induction, and grounding. 

Grounding means any excess charge can be transferred to the ground using a conductor connected to the ground (earth). As the Earth is a huge conductor any amount of charge can be transferred to it or taken from it as shown in fig: b. 

Charging by induction: 

Procedure: 

We can take a charged rod of a different magnitude and different polarity positive or negative. 

On bringing the charged rod close to the metal nob of the electroscope, we see that it affects the electrons in the electroscope. 

This is charging by induction; the opposite charges get inducted in the metal nob whereas a similar type of charge gets induced in the leaves of the electroscope. The induced charge separation can be observed for the rough estimation of the amount of charging. 

Charging by conduction: 

Procedure: 

• We can take a charged rod of a different magnitude and different polarity positive or negative.  
• The electroscope can be charged positively or negatively by conduction by touching the rod to the metal nob of the electroscope. 
• Now after charging the electroscope if we disconnect the charged rod that will no longer affect the other electrons. We can see the new equilibrium of the electroscope. 
• The ground can be dragged and connected to the right side of the electroscope to remove any excess charge. 

Observations: 

1. Initially, the rod and the electroscope both are uncharged as shown in figure: A 

In electroscope  

No. of positive charges = No. of negative charges = 192 

2. The positively charged rod is kept in contact with the electroscope the positively charged rod attracts all the electrons in the electroscope so the leaves of the electroscope get positively charged and repel each other as shown in figure: B.  

In electroscope  

No. of positive charges = 192 

No. of negative charges = 103 

Net charge on electroscope = positive = 89 

3. When the rod is disconnected from the electroscope, the electrons get uniformly distributed and the electroscope remains charged, as shown in figure: C. 

In the electroscope 

No. of positive charges = 192 

No. of negative charge = 103 

Net charge on the electroscope = 192 – 103 = 89 positive charge. 

4. Now if we connect the electroscope to the ground it supplies negative charges to bring the electroscope back to its initial situation. 

Conclusion: 

• In the process of charging by induction, the electroscope remains electrically neutral. 
• In the process of charging by conduction, the electroscope gets positively or negatively charged by the transfer of electrons. 
• When the electroscope gets positively charged the electrons move from the electroscope to the positively charged rod. 
• When the electroscope gets negatively charged the electrons move from the negatively charged rod to the electroscope. 
• In the process of charging by conduction, the electroscope loses or gains some electrons so its mass gets affected.