Problem 118 - Master Resource Book in JEE Main Physics

Question

Assertion-Reason type. Each of these contains two Statements : Statement I (Assertion), Statement II (Reason). Each of these questions also has four alternative choice, only one of which is correct. You have to select the correct choices from the codes (a), (b), (c) and (d) given below (a) If both Assertion and Reason are true and the Reason is correct explanation of the Assertion (b) If both Assertion and Reason are true but Reason is not the correct explanation of the Assertion (c) If Assertion is true but Reason is false (d) If Assertion is false but the Reason is true Assertion During charging by rubbing, the insulating material with lower work function becomes positively charged. Reason Electrons are negatively charged.

Step-by-Step Solution

Answer

(b) Both are true, but Reason is not the correct explanation.

Step 1: Understanding the Assertion

The assertion states that during charging by rubbing, the insulating material with a lower work function becomes positively charged. The work function is the minimum energy required to remove an electron from a solid. Materials with a lower work function easily lose electrons, thus becoming positively charged when electrons are transferred away.

Step 2: Analyzing the Reason

The reason provided is that electrons are negatively charged. This is a factual statement, as electrons carry a negative charge.

Step 3: Evaluating Truth of the Assertion and Reason

Both the assertion and the reason are true. The assertion correctly describes the process of charging by friction based on work function, and the reason correctly describes the charge of electrons.

Step 4: Checking Explanation Relationship

While both statements are true, the reason does not directly explain why the material with a lower work function becomes positively charged during rubbing; the relation primarily depends on electron transfer principles rather than just the negative charge of electrons. Hence, Reason is not the correct explanation for the Assertion.

Key Concepts

Work Function in Physics

In physics, the work function is an essential concept to understand electron behavior in various materials. It denotes the minimum energy required to eject an electron from the surface of a solid material.
Materials with a lower work function tend to lose electrons more readily than those with higher work functions. This is because less energy is needed to release an electron from the atom's hold.

• **Significance of Work Function**: It influences electron emission in processes like the photoelectric effect, where light causes electron ejection from a material.
• **Material Dependence**: Metals typically have lower work functions, making them good conductors of electricity as they can easily relinquish electrons.
• **Role in Semiconductors**: Work function plays a crucial role in determining a material's electronic properties and efficiency in semiconductors.

Understanding the work function helps explain why certain materials become charged under different conditions, such as during frictional charging.

Charging by Rubbing

Charging by rubbing is a process where objects become electrically charged due to friction. When two different materials are rubbed together, electrons may transfer from one material to the other.
This movement is not random but depends on the properties of the materials, such as the work function.

• **Electron Transfer**: During rubbing, electrons are typically transferred from the material with a lower work function to the one with a higher work function.
• **Resulting Charges**: The material losing electrons becomes positively charged, while the one gaining electrons becomes negatively charged.
• **Example**: Common examples include rubbing a balloon on hair, where the balloon becomes negatively charged and can stick to walls owing to its acquired charge.

In essence, charging by rubbing is a demonstration of the principles behind electron movement and material interaction at a microscopic level.

Electron Charge

An electron carries a fundamental unit of negative charge. This property is key in understanding many principles of electricity and magnetism.
Electrons, smaller components of an atom, are negatively charged and orbit the positively charged nucleus.

• **Magnitude of Charge**: The charge of an electron is \( -1.602 \times 10^{-19} \) coulombs.
• **Impact on Electric Forces**: This negative charge allows electrons to interact electromagnetically with other charged particles, playing a vital role in forming bonds and electricity flow.
• **Role in Atom Stability**: Electrons balance the positive charge of protons within atoms, maintaining electrical neutrality.

Recognizing electron charge helps in comprehending how electrons are lost or gained during various charging processes, such as rubbing, affecting the object's electrical properties.