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…
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.
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Q1: What happens to free electrons when a charged rod approaches a neutral conductor?
Free electrons inside the conductor are repelled away from the charged rod. As more electrons move, they repel others already displaced, creating a distribution where no electron experiences net force. This redistribution leaves the end closer to the rod positively charged and the far end negatively charged, establishing an electric dipole.
Q2: How does grounding enable a conductor to become permanently charged by induction?
When a polarized conductor is grounded via a metallic wire, free electrons flow to or from Earth, which acts as an infinite source or sink of charge. After removing the wire, the conductor retains a net charge opposite to the inducing charge. This permanent charge transfer distinguishes induction from temporary polarization.
Q3: Why is the charge distribution on an induced conductor called a dipole?
A dipole consists of two oppositely charged poles. During induction, the conductor develops a positive charge at one end and negative charge at the other, creating this two-pole configuration. Although the conductor has no net charge initially, the spatial separation of opposite charges defines the dipole structure.
Q4: What role does Earth play in the induction charging process?
Earth is a good conductor and effectively an infinite source or sink of charges due to its size. When a conductor is grounded, Earth exchanges charges with it, allowing excess electrons to flow away or additional electrons to arrive. This charge exchange is essential for converting temporary polarization into permanent net charge on the conductor.
Q5: Does the inducing charged object lose charge during the induction process?
No, the inducing charged object does not lose or gain any charge during induction. The negatively charged rod, for example, remains negatively charged throughout. Induction involves only charge redistribution within the conductor and charge exchange between the conductor and Earth, not transfer from the inducing object.
Q6: How does polarization differ from permanent charging in conductors?
Polarization occurs when a charged object redistributes free electrons in a nearby conductor without grounding, creating temporary opposite charges at each end. Permanent charging requires grounding, which allows charge to flow to or from Earth. Once the inducing object is removed, polarization disappears, but permanent charge remains on the grounded conductor.
Q7: Why do free electrons in a conductor reach an equilibrium distribution during induction?
Free electrons move until they reach a configuration where no electron experiences net attractive or repulsive force. Initially, electrons are attracted toward the positive inducing object, but as they accumulate, they repel incoming electrons. Equilibrium is reached when these opposing forces balance, stabilizing the charge distribution across the conductor.