Charging by friction, contact and induction Knowledge-Based Study

Study notes

1. Matter is fundamentally composed of atoms containing positively charged protons, negatively charged electrons, and neutral neutrons. In an electrically neutral object, the total number of protons equals the total number of electrons.

   Charging an object involves altering this balance by transferring mobile electrons:

   • An object that gains electrons acquires a net negative charge.
   • An object that loses electrons acquires a net positive charge.

   Law of Conservation of Charge: The net electric charge in an isolated system remains constant during any physical or chemical process. Charge is neither created nor destroyed; it is merely transferred from one body to another.

   Charging by friction occurs when two different neutral insulating materials are rubbed together. The mechanical work supplies the energy required to overcome the electron binding affinity of one of the materials.

   Key characteristics of charging by friction:

   • Electrons migrate from the material with lower electron affinity to the material with higher electron affinity.
   • Protons remain bound inside atomic nuclei and are never transferred.
   • The two objects acquire charges of equal magnitude but opposite sign.
   • Total charge is strictly conserved: $q_1 + q_2 = 0$.

   Example: Rubbing a glass rod with a silk cloth causes electrons to transfer from the glass to the silk. Consequently, the glass rod becomes positively charged and the silk cloth becomes equally negatively charged.

   Charging by conduction involves direct physical contact between a charged body and a neutral or differently charged conducting object.

   Key characteristics of charging by contact:

   • Electrons transfer between the surfaces across the contact point due to repulsive electrostatic forces.
   • If a negatively charged conductor touches a neutral conductor, excess electrons flow onto the neutral body, leaving both with a negative charge.
   • If a positively charged conductor touches a neutral conductor, electrons from the neutral body flow into the positively charged body, leaving both with a positive charge.
   • After separation, the bodies always share the same sign of charge.
   • For identical spherical conductors of radii $r_1 = r_2$, the total initial charge divides equally: $q' = \frac{q_1 + q_2}{2}$.

   Charging by induction allows a conductor to acquire an electric charge without direct physical contact with the charging object. This method relies on polarization followed by grounding.

   Steps in electrostatic induction:

   1. Polarization: A charged rod is brought near an insulated neutral conductor. Free electrons redistribute: they are attracted toward a positive rod or repelled from a negative rod, creating separated regions of opposite charge on the conductor.
   2. Grounding: While the charged rod remains in position, the conductor is connected to the Earth (grounded). The ground acts as an infinite reservoir or sink for electrons.
   3. Disconnecting Ground: The ground connection is broken first, trapping the excess or deficit of electrons on the conductor.
   4. Removing the Inducing Body: Finally, the charged rod is moved away. The trapped charge redistributes uniformly across the conductor.

   Crucial Rule: An object charged by induction always acquires a charge of the opposite sign to that of the inducing object, while the inducing object loses none of its own charge.

   A concise summary of how the three charging methods compare:

   • Friction: Involves two neutral insulators rubbed together. Results in equal and opposite charges on the two objects. Requires physical rubbing.
   • Contact (Conduction): Involves direct touch between conductors. Results in the objects acquiring charges of the same sign. The original charging body shares its charge.
   • Induction: Involves bringing a charged object near a conductor, grounding, and removing the ground before removing the inducing charge. Results in an opposite charge on the conductor without any charge loss from the inducing body.