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Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEE

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METHODS OF CHARGING
(i) Charging by friction: Charging by friction is the oldest form of charging. It was found that when an amber rod is rubbed with fur, the rod became negatively charged. The two bodies acquire opposite signs of electricity; one gets positively charged, while the other becomes negatively charged. 

Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEE

Fig. Charging by friction

When two bodies are charged by friction, they acquire the same magnitude of charge. Furthermore, the bodies retain these excess charges even when they are separated from each other.
Note: Charging involves transformation of mass.

(ii) Charging by conduction: In charging by contact, an uncharged body is brought into contact with a charged body. The uncharged body acquires the same sign of charge as the charged body. The total charge is distributed between the two bodies.   

Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEE

Fig. Charging by conduction

(iii) Charging by Induction 
Charging by induction occurs when we bring a charged object near a conductor. It is not for no reasons that we say near. The charged object does not physically touch the conductor. The charged object is just allowed to get close to the conductor.
As a result, the conductor will be charged. We say that a charge has been induced in the conductor. We will make this clear with an experiment.

Experiment
Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEStep 1: Consider the two spheres above made with a metal. Since metal is a good conductor, it is a good choice for this experiment.

  • They touch each other, so they become a single conductor.
  • We put them on insulated stands so charges or electricity does not travel to the ground.
  • The two spheres right now form a neutral system. This means that there is the same amount of electrons and protons in each sphere.

Step 2: We get a rod that is negatively charged and we put it next to the two spheres. The rod is shown on the right in green. The lines inside the rod represent negative charges. Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEElectrons in sphere B are repelled by the rod and move to sphere A to create an excessive charge called also net charge. This situation creates also a net charge in sphere B.
We say that a charge has been induced on the spheres.

Step 3: We can separate the spheres while the rod is still there.
Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEStep 4: Finally, we can remove the rod completely.
Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEThe spheres will keep their charges and this is what we mean by charging by induction. The charges on the spheres are equal and opposite.

Note: That the charges are equal because for each single electron that goes to A and therefore creates a single negative net charge, it leaves B with a single positive charge.

In our example above, let’s assume electrons went to A creating a situation where B has a positive charge with protons. Keep in mind that spheres A and B could have billions and billions of electrons. The reason that sphere A is charged now is because it has an excess of 3 electrons although this is a small charge. By the same token, the reason sphere B is charged is because it has 3 more protons now than electrons.

Charging by induction and grounding 
Use only 1 sphere this time and induce a charge again with a charged rod. Then, put your finger on the side where negative charge is induced i.e. away from the rod.
Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEWhen you touch the metal with your finger, electrons leave the sphere by means of your finger and enter the ground through your body.
Methods of Charging: Friction, Conduction & Induction Notes | Study Physics For JEE - JEEWhen we allow charges or electricity to leave a conductor by touching it, we call it grounding the conductor.

Note: That neutral body does not imply a ‘charge-less’ body. It means a microscopic balance of –ve and +ve charge in the body.

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