Magnetic Field Lines MCQs Quiz | Class 10

Understanding Magnetic Field Lines

Magnetic field lines are a visual tool used to represent magnetic fields. They help us understand the direction and strength of the magnetic field in a given region. These imaginary lines are drawn to show the path a free isolated North magnetic pole would follow if placed in the field.

Key Properties and Representation

• Direction: Outside a magnet, magnetic field lines originate from the North pole and merge into the South pole. Inside the magnet, they travel from the South pole to the North pole. This forms continuous closed loops.
• No Intersection: Magnetic field lines never intersect each other. If they did, it would imply that at the point of intersection, the magnetic compass needle would point in two directions simultaneously, which is physically impossible.
• Density and Strength: The degree of closeness or density of magnetic field lines indicates the strength of the magnetic field. Where the lines are crowded together (e.g., near the poles of a bar magnet), the magnetic field is strongest. Conversely, where the lines are farther apart, the magnetic field is weaker.
• Magnitude: The relative strength of the magnetic field is directly proportional to the density of the field lines.

Magnetic Field Lines due to Current-Carrying Conductors

An electric current produces a magnetic field around it. The pattern of these field lines depends on the shape of the conductor:

• Straight Conductor: The magnetic field lines around a straight current-carrying conductor are concentric circles, with the center at the conductor. The direction of these circles can be determined using the Right-Hand Thumb Rule.
• Circular Loop: For a circular loop carrying current, the magnetic field lines are concentric circles near the wire, gradually becoming larger and more elliptical as they move away. At the center of the loop, the field lines are almost straight and perpendicular to the plane of the loop.
• Solenoid: A solenoid is a coil of many circular turns of insulated wire wound in a cylindrical shape. The magnetic field lines inside a solenoid are nearly parallel and straight, indicating a uniform magnetic field. This field is similar to that produced by a bar magnet, with one end of the solenoid acting as a North pole and the other as a South pole.

Right-Hand Thumb Rule

This rule is used to determine the direction of the magnetic field produced by a straight current-carrying conductor. Imagine holding the current-carrying straight conductor in your right hand such that your thumb points in the direction of the current. Then, the direction in which your fingers curl around the conductor gives the direction of the magnetic field lines.

Quick Revision Checklist

• Magnetic field lines are continuous closed curves.
• They never cross each other.
• Their direction is from North to South externally, and South to North internally.
• The density of lines indicates magnetic field strength.
• Right-Hand Thumb Rule is used for straight current-carrying wires.
• Field inside a solenoid is uniform.

Practice Questions

1. The magnetic field lines produced by a straight current-carrying wire are:
   (a) Straight lines parallel to the wire
   (b) Concentric circles perpendicular to the wire
   (c) Helical paths around the wire
   (d) Irregular patterns
2. Which property of magnetic field lines indicates a stronger magnetic field?
   (a) Lines are straight
   (b) Lines are curved
   (c) Lines are widely spaced
   (d) Lines are closely packed
3. Inside a bar magnet, the magnetic field lines move:
   (a) From North pole to South pole
   (b) From South pole to North pole
   (c) In any direction randomly
   (d) They do not exist inside
4. If a compass needle is placed at a point, its North pole will align in the direction of:
   (a) The electric field
   (b) The gravitational field
   (c) The magnetic field lines
   (d) Its South pole
5. The Right-Hand Thumb Rule can be used to determine the direction of the magnetic field around:
   (a) A moving charge
   (b) A stationary magnet
   (c) A straight current-carrying conductor
   (d) Earth’s magnetic field