When a dielectric material is kept in an external electric field, which one of the following phenomena may be realized ?

Correct — B, (b) Polarization. A dielectric is an insulator: its electrons are bound to their molecules and cannot flow through the material as they do in a conductor. Put it in an external electric field and, since no current can be set up, the charges do the only thing left to them — they shift slightly within each molecule. In a non-polar material the field pulls the electron cloud one way and the nucleus the other, so every molecule acquires an INDUCED dipole moment. In a polar material such as water the molecules already carry permanent dipole moments pointing in random directions, and the field turns them so that they line up with it. Either way the material develops a net dipole moment per unit volume, and that is what POLARISATION means. Its visible consequence is a layer of bound charge on the two faces of the slab, negative on the face towards the positive plate and positive on the other, which sets up an internal field opposing the applied one. The field inside the dielectric is therefore reduced by the dielectric constant K, and this is exactly why sliding a dielectric slab between the plates of a capacitor multiplies its capacitance by K.

Why the others are wrong

• (a)Magnetization — Magnetisation is the exact counterpart of polarisation, but it belongs to the MAGNETIC story, not the electric one. A magnetic material placed in an external MAGNETIC field acquires a net magnetic moment per unit volume, which is what magnetisation means; the analogy with a dielectric in an electric field is close enough that the two are taught side by side, with permittivity answering to permeability and the dielectric constant to relative permeability. That parallel is precisely what makes this option tempting. But the stem specifies an external ELECTRIC field acting on a DIELECTRIC, and a dielectric slab in a static electric field acquires electric dipole moment, not magnetic moment.
• (c)Photoionization — Photoionisation is the ejection of an electron from an atom or molecule by an incident PHOTON whose energy exceeds the ionisation energy — the process behind the ionosphere, behind photoelectric devices and behind the damage done by ultraviolet and X-radiation. It requires light or higher-energy radiation, and it requires an electron to be torn away from its parent atom completely. Neither condition is met here: the stem describes a static external electric field, not incident radiation, and the whole point of a dielectric is that its charges stay bound and merely shift. If a field were made strong enough to tear charges loose the material would suffer dielectric breakdown and stop being an insulator, which is a different phenomenon again and not what this word names.
• (d)Circularization — This is not a phenomenon of dielectrics at all, and it names nothing that happens to an insulator in an electric field. There is no process by which the molecules or the charges of a dielectric become circular, nor any standard quantity in electrostatics of that name. The word is doing decoy work: it is long, it ends in the same suffix as polarisation and magnetisation, and it therefore sits comfortably in a list of four without meaning anything in this context. Options built to look like their neighbours are common on this paper, and the defence is to ask what the word would actually DESCRIBE rather than whether it looks like a physics term.

Materials are divided by what their charges can do in a field. In a CONDUCTOR the outer electrons are free, so an applied field drives them until the charge redistributes and the field inside falls to zero — electrostatic shielding. In a DIELECTRIC, or insulator, the charges are bound, so nothing flows and the field only distorts the charge distribution within each molecule. That distortion is polarisation, and it takes two forms: electronic or induced polarisation, in which the electron cloud is displaced relative to the nucleus in a non-polar molecule, and orientational polarisation, in which the permanent dipoles of a polar molecule such as water rotate into alignment against thermal agitation. The polarised slab carries bound surface charges that produce a field opposing the applied one, so the net field inside is E0 divided by K, where K is the dielectric constant or relative permittivity — about 1 for a vacuum or air, roughly 2 for polythene, about 5 for glass and around 80 for water. Three consequences are examined constantly. A capacitor filled with a dielectric of constant K has its capacitance raised K times, so dielectrics let a capacitor store more charge at the same voltage while also physically separating the plates and raising the breakdown voltage. Second, a polar dielectric in an ALTERNATING field has its dipoles turned back and forth continuously, and the frictional loss shows up as heat — dielectric heating, which is what a microwave oven does to the water in food. Third, if the field becomes strong enough, the bound charges are torn free and the insulator conducts suddenly: dielectric breakdown, the phenomenon behind a lightning stroke through air and behind the failure of insulation in equipment.

Key facts

• A dielectric is an insulator: its charges are bound and cannot flow through the material.
• In an external electric field a dielectric becomes POLARISED, acquiring a net electric dipole moment per unit volume.
• Non-polar molecules acquire induced dipoles; polar molecules with permanent dipoles rotate into alignment with the field.
• Polarisation produces bound charges on the surfaces of the slab, whose field opposes the applied field.
• The field inside a dielectric is reduced by the factor K, the dielectric constant or relative permittivity.
• Introducing a dielectric of constant K between the plates of a capacitor multiplies its capacitance by K.
• Magnetisation is the magnetic analogue — net magnetic moment per unit volume acquired in an external magnetic field.
• Dielectric heating in an alternating field, and dielectric breakdown in a very strong field, are the two further consequences most often examined.
• In a conductor, by contrast, free electrons redistribute until the electrostatic field inside becomes zero.

Common traps

• Picking magnetisation because it is the familiar word. It is the correct phenomenon for the wrong field and the wrong material.
• Assuming charges must flow for anything to happen. In a dielectric nothing flows; the charges only shift within their molecules.
• Believing polarisation makes the field inside larger. It opposes the applied field and makes the net field smaller.
• Confusing electrostatic polarisation of a dielectric with the polarisation of light, a different phenomenon that shares the word.
• Choosing an option because its ending sounds technical. Ask what the word would describe before accepting it.

• practice — not a real PYQ

  When a dielectric slab is introduced between the plates of a charged parallel-plate capacitor, its capacitance

  • (a)becomes zero
  • (b)decreases by a factor equal to the dielectric constant
  • (c)increases by a factor equal to the dielectric constant
  • (d)remains unchanged

  Answer(c) increases by a factor equal to the dielectric constant

• practice — not a real PYQ

  The electrostatic field inside a solid conductor placed in an external electric field is

  • (a)zero
  • (b)equal to the applied field
  • (c)greater than the applied field
  • (d)reduced by the dielectric constant of the conductor

  Answer(a) zero