Which of the following statements are correct?
a. Electric lines of force are just imaginary lines
b. Electric lines of force will be parallel to the surface of conductor.
c. If the lines of force are crowded, them field is strong
d. Electric lines of force are closed loops

Options

A. both a and c
B. both b and d
C. only a
D. all

Answer

To determine which statements about electric lines of force are correct, we will analyze each statement one by one based on the principles of electrostatics.

### Step-by-step Solution:

1. **Statement a: Electric lines of force are just imaginary lines.**
- **Analysis:** Electric lines of force are indeed imaginary constructs used to visualize electric fields. They help us understand the direction and strength of the electric field but do not physically exist.
- **Conclusion:** This statement is **correct**.

2. **Statement b: Electric lines of force will be parallel to the surface of a conductor.**
- **Analysis:** In electrostatics, the electric field inside a conductor in electrostatic equilibrium is zero. Therefore, the electric lines of force must be perpendicular to the surface of the conductor, not parallel. This is because if they were parallel, it would imply a potential difference along the surface, which contradicts the definition of an equipotential surface.
- **Conclusion:** This statement is **incorrect**.

3. **Statement c: If the lines of force are crowded, then the field is strong.**
- **Analysis:** When electric field lines are close together (crowded), it indicates a strong electric field in that region. Conversely, when the lines are spaced further apart, the electric field is weaker. This is a fundamental property of electric field lines.
- **Conclusion:** This statement is **correct**.

4. **Statement d: Electric lines of force are closed loops.**
- **Analysis:** Electric lines of force do not form closed loops. They originate from positive charges and terminate at negative charges. They do not return to the same charge, hence they cannot form closed loops.
- **Conclusion:** This statement is **incorrect**.

### Final Summary:
- **Correct Statements:** a and c.
- **Incorrect Statements:** b and d.