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

(b) piezoelectricity.

Step 1: Understand the Term 'Polar Crystal'

Polar crystals are materials that have a permanent electric dipole moment, meaning they have distinct positive and negative electrical charges at different parts of the crystal.

Step 2: Know the Phenomenon Triggered by Heating

Heating some polar crystals causes them to generate a weak electric current due to the displacement of their internal charges. This is known as the pyroelectric effect, related to some specific properties of certain polar crystals.

Step 3: Match the Effect with the Options Given

The production of an electric current under mechanical stress or heating in polar crystals is termed piezoelectricity. However, strictly speaking, the electric generation due to heating alone is more accurately related to the pyroelectric effect, a special case often associated with piezoelectric materials.

Step 4: Identify the Closest Available Option

Piezoelectricity is commonly linked with these effects in common discussion, even as it traditionally refers to electricity produced from pressure rather than heat. Given the options, (b) piezoelectricity is the closest correct answer.

Key Concepts

Polar Crystals

Polar crystals are special types of materials that exhibit a peculiar property known as a permanent electric dipole moment. Imagine a crystal that is distributed in such a way that there is a positive charge at one end and a negative charge at the other.

This unique charge distribution means that polar crystals naturally possess an electric field. Think of it like a tiny battery being built right into the fabric of the material.

Polar crystals are essential components in many technological applications. They have particular electrical properties, which makes them valuable in fields like electronics and sensors.

• Common examples are tourmaline and quartz.
• These materials can react to external stimuli such as heat or mechanical stress.

Understanding this concept is crucial as it lays the foundation for studying other phenomena like the pyroelectric and piezoelectric effects.

Electric Dipole Moment

An electric dipole moment is at the heart of what makes polar crystals fascinating and functional. It is a measure of the separation of positive and negative charges in a system, and it can tell us a lot about the electrical properties of a material.

Visualize it as a tiny bar magnet, where one end is positive and the other is negative. This separation creates an entire electric field around the material, leading to some intriguing behaviors.

Here's what you need to know about electric dipole moments:

• The larger the separation of charge, the stronger the dipole moment.
• In a material like a polar crystal, countless dipole moments are aligned, resulting in a significant collective effect.
• Measurements of the dipole moment aid in understanding the material's response to external fields.

Electric dipole moments are foundational to the electric characteristics and reactions of polar materials, helping to explain both their everyday and specialized applications.

Piezoelectricity

Piezoelectricity is an exciting phenomenon where some materials generate an electric charge in response to applied mechanical stress. The term itself is derived from the Greek word 'piezein', meaning to press or squeeze.

This effect is most commonly associated with crystals like quartz. Under mechanical stress, these materials generate an electricity due to the displacement of internal charges.

Key features of piezoelectricity include:

• It is widely used in everyday applications such as in microphones, quartz watches, and ultrasound equipment.
• Piezoelectric materials are sensitive to mechanical changes, allowing them to convert physical movement into an electrical signal.
• It is closely related to the pyroelectric effect, but with a focus on mechanical stress rather than heat.

Understanding piezoelectricity is essential for harnessing the properties of materials that convert mechanical stress into useful electrical energy.

Electric Current Generation

The generation of electric current from various external inputs is a core theme within the study of polar crystals. Depending on the stimulus, these materials can generate electricity through different effects.

Typically, electric current generation in polar crystals occurs through either piezoelectric or pyroelectric effects.

Here's a closer look at electric current generation:

• Piezoelectric effect refers to electricity produced by mechanical stress.
• Pyroelectric effect involves current generation through changes in temperature.
• Both effects are similar due to their reliance on the displacement of internal charges within a material.
• These processes are essential in developing sensors and energy harvesting devices.

The study of electric current generation from polar materials continues to play a major role in advancing technology, offering sustainable ways to capture and utilize energy.