Magnetic Field Lines: Definition, Explanation and Q&A

Key Concepts

• Magnetic Field
• Magnetic Field Lines
• properties of magnetic field lines
• Uniform and non uniform magnetic lines

Introduction

Two magnets when placed close to each other attract and stick to each other. However, if we go on increasing the distance between them, the attraction between them reduces gradually to such an extent that they no longer seem to attract each other anymore. In this session, we will discover the reason behind such an event. 

Explanation

Magnetic field

There is a limited space around a magnet in which its influence is apparent, in which it is able to exert enough force for another magnet to move towards itself. 

The region surrounding a magnet in which the force of the magnet is detected is said to have a magnetic field. 

Activity: 

1. Fix a sheet of white paper on a drawing board using pins. 

2. Place a bar magnet at the center of the paper. 

3. Now, sprinkle some iron fillings uniformly throughout the paper. 

4. Tap the board gently a few times. 

In this activity, it will be observed that the iron fillings are seen to get arranged in a specific pattern around the magnet as shown in the pictures. This results from the force exerted by the magnet on the iron fillings. The force experienced by the iron fillings makes them arrange in such a pattern in the magnetic field of the bar magnet. The lines along which the iron fillings arrange themselves in the magnetic field are termed as the magnetic field lines or lines of force. 

Magnetic field lines

A magnetic field is represented by magnetic field lines. It is used to visualize the magnitude and direction of the magnetic field. 

Properties of magnetic field lines 

1. The closeness of the magnetic field lines in a particular area is directly proportional to the magnetic field strength there. This means that the magnetic field strength near the poles (where the field lines are densely packed) is greater than the area on top of the middle of the magnet. 
2. The direction of the magnetic field lines outside the magnet is from north pole to south pole. Whereas the direction of the magnetic field lines inside the magnet is from south pole to north pole. This means that the magnetic field lines are closed loops. 
3. The magnetic field lines never intersect each other. If they did, it would mean that there are two directions of magnetic field at the point of intersection, which is impossible. 
4. The magnetic field strength is a physical quantity which has both magnitude and direction. Hence, it is a vector quantity. Magnitude is given by the closeness of the magnetic field lines at a particular area. Direction is given by the tangent drawn at a particular point in the magnetic field or the direction in which a compass needle points when placed at that point. 

Questions and Answers

1. The magnetic field inside the rectangle ABCD is 

• Different at all the points (non-uniform) 

• Exactly the same at all the points (uniform) 

• Greater along AB as compared to that of CD 

• Greater along AD as compared to that of BC 

Answer: 

b. Exactly the same at all the points (uniform). 

Explanation: The magnetic field lines are equally spaced and parallel to each other giving rise to a uniform density throughout the area of the rectangle ABCD. 

2. The figure below shows the magnetic field lines due to a horseshoe shaped magnet and there are 6 different points marked on its field. Identify the point at which the magnetic field strength is the highest. 

Fig No. 2.2: Question 2 

Answer:  

The magnetic field strength at point D is the highest. This is because the density of the magnetic field lines is the highest near that point. 

Summary

• When iron fillings are sprinkled around a bar magnet, they arrange themselves in a
  specific pattern of lines under the influence of the magnetic force exerted by the field
  generated by the bar magnet around it.
• These lines are called the magnetic field lines, which help us visualize the field created
  by the magnet.
• The closeness of the magnetic field lines in a particular area is directly proportional to
  the magnetic field strength there.
• The magnetic field lines are closed loops.
• The magnetic field lines never intersect each other.
• The magnetic field strength is a physical quantity which has both magnitude and
  direction. Hence, it is a vector quantity.