Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

On heating some polar crystals, weak electric current is produced. It is termed as (a) superconductivity (b) piezoelectricity (c) photoelectric current (d) none of these

Step-by-Step Solution

Answer

The correct option is (d) none of these.

Step 1: Understanding the Question

The exercise asks about a phenomenon that occurs when polar crystals are heated, resulting in the generation of a weak electric current. It's important to identify the characteristics described and align them with the given options.

Step 2: Identifying Relevant Phenomenon

When polar crystals are subjected to heat and produce an electric current, it may relate to piezoelectricity. However, it's worth noting that piezoelectricity typically involves mechanical stress leading to electricity rather than heat.

Step 3: Evaluating the Options

- **(a) Superconductivity:** Refers to the flow of electric current without resistance, which happens at low temperatures, not involving heating of polar crystals. - **(b) Piezoelectricity:** Generally involves certain crystals generating electric current when mechanical pressure is applied, not directly due to heating. - **(c) Photoelectric current:** Refers to the release of electrons due to light exposure, not related to heating crystals. - **(d) None of these:** If none of the situations fit, this may be the most accurate choice.

Step 4: Making an Educated Choice

None of the options directly describe the heating of polar crystals leading to electricity with the details provided. However, although piezoelectricity does involve crystals, it typically requires mechanical force rather than just heat.

Key Concepts

Polar Crystals

Polar crystals are fascinating structures with unique properties due to their asymmetrical arrangements of atoms. This structural characteristic results in permanent dipoles within the crystal. Here’s how they work:

• In polar crystals, the centers of positive and negative charges do not overlap, causing an inherent dipole moment.
• These crystals are usually composed of atoms arranged in an orderly fashion, yet their asymmetry gives them unique electrical properties.
• Common examples include quartz and certain ceramics, which are often used in electronic applications.

Understanding polar crystals is essential in grasping how they interact with different physical forces, such as pressure or temperature, to produce electrical effects. These crystals hold a key role in the production and manipulation of electric currents when subjected to various stimuli.

Electric Current Generation

The generation of electric current in polar crystals can occur under different circumstances. Typically, when polar crystals are subjected to mechanical stress, they can generate an electric charge — this is known as piezoelectricity.

• Piezoelectricity arises when the mechanical deformation of polar crystals creates a voltage across their surfaces.
• This phenomenon is widely used in applications such as sensors, actuators, and energy harvesting devices.
• However, the typical understanding of piezoelectricity involves mechanical pressure, not temperature changes.

It's important to understand that while heating polar crystals, as mentioned in the exercise, may not directly produce piezoelectricity, it still might induce other electrical phenomena due to changes in their internal structure.

Heating Effects on Crystals

When polar crystals are subjected to heat, several effects can alter their physical properties and behavior:

• Temperature changes can lead to physical expansion, potentially affecting the crystal lattice structure.
• In some cases, heating can alter the alignment of dipoles within the crystal, impacting its overall electric properties.
• While not typical for piezoelectric effects, heating could lead to changes in polarization, creating weak electric currents depending on the material.

The relationship between heating and electric current generation in crystals is complex and often material-specific. While piezoelectric effects primarily relate to mechanical stress, understanding the broader impacts of temperature on crystals helps in designing materials and technologies that capitalize on these unique properties.