Problem 121 - 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 No two electric field lines can intersect one another at any point in space. Reason Electric field lines always start from a positive charge and end on a negative charge.

Step-by-Step Solution

Answer

(b) Both Assertion and Reason are true but Reason is not the correct explanation of the Assertion.

Step 1: Understand the Assertion

The assertion states that no two electric field lines can intersect one another at any point in space. This is a property of electric field lines, as intersecting lines would suggest two different directions of the electric field at a single point, which is not possible. Thus, the assertion is true.

Step 2: Analyze the Reason

The reason given is that electric field lines always start from a positive charge and end on a negative charge. This is another key characteristic of electric field lines, indicating the direction of the electric field from positive to negative. Hence, the reason is also true.

Step 3: Determine the Explanation

While both the assertion and the reason are true, the reason does not explain why electric field lines cannot intersect. The inability of electric field lines to intersect is due to the specific directional nature of the electric field vector at any point, not because they start from positive and end on negative charges.

Step 4: Choose the Correct Option

Since both the assertion and the reason are true, but the reason does not explain the assertion, the correct choice is option (b).

Key Concepts

Electric Field Lines

Electric field lines are an imaginative and illustrative way to understand the direction and nature of electric fields. Imagine these lines as arrows that show how a positive charge would move if placed in the field. They start from positive charges and end at negative charges. This formation helps visualize the electric force on a charge.

Some unique properties of electric field lines include:

• The lines never cross each other. If they did, it would suggest two possible directions for the electric field at that point, which is not possible in physics.
• The density of the lines indicates the strength of the field. More lines suggest a stronger field.
• They can form closed loops, especially in the case of magnetic fields but typically not in purely electric fields where they go from positive to negative.

The concept of electric field lines helps us visually interpret how fields behave and interact, making complex ideas more intuitive.

Assertion Reason Questions

Assertion Reason Questions are a type of problem that tests the ability to understand and evaluate statements based on their logical connection. You have two statements: an assertion and a reason. The task is to determine if:

• Both are true, and the reason is the correct explanation of the assertion.
• Both are true, but the reason is not the correct explanation of the assertion.
• The assertion is true, but the reason is false.
• The assertion is false, but the reason is true.

For instance, consider the assertion that two electric field lines never intersect. The reason provided is that electric fields originate from positive charges and terminate at negative ones. While both statements are valid, the notion that field lines do not intersect is due to the singular direction of the field at any point, not strictly due to their origin. Therefore, in such questions, identifying the logical explanations can demonstrate a deep understanding of the concepts involved.

Physics Problem Solving

Physics problem solving is an essential skill in understanding the natural world. It involves not just finding the solution, but also applying relevant concepts and reasoning to arrive at that solution. Here are some steps to enhance your problem-solving skills in physics:

• Understand the Problem: Read and interpret what is being asked. Break down complex information into manageable parts.
• Identify Key Concepts: Recognize which physical principles apply, such as laws of motion, electromagnetism, etc.
• Plan your Approach: Decide on the strategy or formula you will use to tackle the problem.
• Carry Out the Plan: Execute your problem-solving steps efficiently and systematically.
• Check Your Work: Verify your results by considering units and logical reasoning, ensuring the answer makes sense physically.

Practicing these steps can build confidence and deepen your understanding of physics, setting a strong foundation for more complex problem-solving challenges.