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 answer is (b) piezoelectricity, although heating implies pyroelectric effects.

Step 1: Understand the Terms

Superconductivity refers to the property of zero electrical resistance in some substances at very low temperatures. Piezoelectricity is the generation of an electric charge in certain materials when they are mechanically stressed. Photoelectric current is the flow of electric current as a result of light hitting a material. Each term describes different phenomena.

Step 2: Analyze the Given Situation

The problem states that heating some polar crystals produces weak electric current. This suggests that the phenomenon is related to the alignment or re-alignment of charges within these crystals due to a thermal process.

Step 3: Identify Relevant Phenomenon

Polar crystals may generate current when subjected to specific conditions such as mechanical stress or change in temperature. Piezoelectricity occurs specifically under mechanical stress, not solely heating. This phenomenon could also relate to pyroelectricity, where temperature change influences surface charge, often observed in polar crystals.

Step 4: Conclude the Correct Option

Since the problem defines only options (a) superconductivity, (b) piezoelectricity, (c) photoelectric current, and (d) none of these, and piezoelectric effects can be mistakenly conflated with pyroelectric effects, option (b) piezoelectricity is the closest correct option among those listed even though it technically describes pyroelectricity due to the thermal context.

Key Concepts

Polar Crystals

Polar crystals are unique types of crystals that carry a permanent electric dipole moment. This means that within the crystal structure, there is an asymmetrical distribution of charges that creates a dipole, which is a pair of equal and oppositely charged or magnetized poles separated by a distance. In simpler terms, one part of the crystal has a positive charge and another has a negative charge.
These crystals are found in various materials, including ferroelectric materials like barium titanate.

• They are known for their ability to respond to external influences such as mechanical stress or temperature changes.
• This sensitivity to external stimuli is what leads to fascinating phenomena such as piezoelectricity and pyroelectricity.

Unlike non-polar crystals, which lack a permanent dipole moment, polar crystals exhibit properties crucial in many technological applications. This makes them particularly interesting in various fields, such as electronics and energy conversion, contributing to modern advancements in sensors and electrical devices.

Thermoelectric Effects

Thermoelectric effects refer to the direct conversion between thermal and electrical energy. When a temperature difference is applied to a material or a system, it can generate electric voltage. This phenomenon is reversible, where electrical energy can be used to produce a temperature difference. You might have heard of devices like thermocouples, which harness these effects.

• The Seebeck effect is a type of thermoelectric effect where a temperature difference between two dissimilar conductors or semiconductors results in a voltage difference.
• Another is the Peltier effect, where an electric current creates a temperature change at the junctions of two materials.

These effects are employed in a variety of applications such as power generation in space probes, where reliability and the absence of moving parts provide significant advantages. Additionally, thermoelectric coolers and heaters capitalize on these effects to achieve desired thermal conditions.

Electric Phenomena

Electric phenomena encompass the various ways in which electric charges interact with each other and with materials. This includes a wide range of behaviors and effects that arise from the presence and movement of electric charge. In polar crystals, electric phenomena can manifest as pyroelectricity, where heating causes a change in the polarization state, producing a temporary voltage. Other notable electric phenomena include:

• Piezoelectricity: Generates voltage through mechanical stress on materials that have asymmetric charge distribution.
• Photoelectric Effect: Ejects electrons from a material's surface when exposed to light, causing an electric current.
• Superconductivity: Exhibits zero electrical resistance and expulsion of magnetic fields below a critical temperature.

These phenomena are crucial for understanding fundamental principles in physics and finding innovative solutions in technology and industry.

JEE Main Chemistry

JEE Main Chemistry questions often test understanding of concepts like polar crystals and electric phenomena. These topics involve the study of how different materials react under a variety of circumstances and are essential for students aspiring to enter engineering and technical fields.

• Studying molecular structure helps predict a material's behavior in electric and thermal contexts.
• Understanding pyroelectric and piezoelectric effects is vital when dealing with advanced material technology.

The competitive exam may feature questions that assess a student's ability to identify the type of electric phenomenon based on clues about conditions such as mechanical stress or temperature changes. For instance, recognizing whether a certain phenomenon is piezoelectricity or pyroelectricity requires precise knowledge about how different materials respond to specific stimuli. Therefore, gaining an in-depth understanding of these concepts is crucial for success.