22.4:
Charging Conductors By Induction
Rub a plastic rod against fur to charge it negatively.
Take a conducting metallic sphere supported by an insulating plastic stand.
When the charged rod is brought close to the neutral sphere, free electrons inside the sphere are repelled away from it.
As more free electrons try to move, they are also repelled by those already moved. The electrons distribute such that there is no net force on any free electron.
More free electrons have accumulated on the far end of the conductor, leaving the closer end carrying a net positive charge. The two poles are oppositely charged or polarized. Such a charge distribution is called a dipole.
If the sphere is connected to the Earth via another metallic wire, the free electrons move to the Earth. When the wire is removed, the sphere carries a net positive charge. This process is called induction.
The negatively charged plastic rod does not lose or gain any charge. Induction results in a charge exchange between the conductor and the Earth.
22.4:
Charging Conductors By Induction
The Earth is a good conductor of electricity, and it is so big that it can be considered an infinite source or sink of charges. It can easily exchange charges with any matter.
Generally, conductors like metals do not allow any excess charge to be present on them. Any excess charge added to metals easily flows away, for example, when a metal is placed on the Earth. This process is called earthing.
However, conductors can be charged by a process called induction. For example, consider charging a glass rod, an insulator, by rubbing it against silk. The positively charged glass rod is brought close to the electrically neutral metallic sphere. Free electrons in the metal are attracted towards it and accumulate at the near end. As more accumulate, others are repelled by the accumulated electrons, finally creating a distribution where no electron feels attraction or repulsion.
The overall charge distribution is such that there are more negatively charged free electrons at the end close to the insulating rod and more positively charged atomic nuclei at the other end. Although the sphere has no net charge, the distribution of charges is such that it is now attracted to the insulating rod. It is said to be polarized, and the charge distribution is called an electric dipole.
If the sphere is then grounded, excess free electrons from the Earth flow to the positive end. If the grounding is removed, the negatively charged distribution close to the glass rod leaves the metallic sphere carrying a net negative charge. This process of charge is called induction.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson; section 21.2; page 692.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; section 5.2; page 189.