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Physics Lesson 16.1.3 - Magnetic Field Lines

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Welcome to our Physics lesson on Magnetic Field Lines, this is the third lesson of our suite of physics lessons covering the topic of Introduction to Magnetism, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Magnetic Field Lines

The effect of magnetic field is understood better is we draw some imaginary lines (similar to those of electric field) to represent it. However, first we must do an experiment to determine the direction of magnetic field lines. This experiment is mentioned in the introduction part of this tutorial. When we place some iron filings on a paper and move slightly a bar magnet below the paper, the iron filings will align in the way shown in the figure:

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

This experiment is a demonstration that:

  1. Magnetic field lines (unlike electric field ones) are closed lines that start from one pole (North) and end to the other pole (South).
  2. The magnetic field lines are denser at the poles. This means that the magnetic field is the strongest at the poles.

We represent the magnetic field lines through arrows starting from the North Pole and ending to the South Pole of magnet as shown below.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

Magnetic field lines are not always curved. In U-shaped magnets for example, magnetic field lines in the shortest path from N-pole to S-Pole are straight, as shown in the figure.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

Like electric field lines, magnetic field lines have a very important property: they do not cross each other. This means the magnetic field changes gradually through a predictable process.

Remark! A magnet is a piece of metal that is coloured in the factory in which it is produced. The red and blue colour are just conventional signs; magnets are coloured to help the user identify which is the N-pole and which is the S-pole of magnet.

The following table summarizes the properties of magnetic field and comperes them to the properties of electric field discussed in Section 14.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

From the above table we can deduce other properties of magnets such as:

  • All magnets have two poles; unipolar magnets do not exist.
  • When a magnet is cut in two pieces, two new magnets with two poles each are obtained.

Example 1

The magnet in the figure is cut in four equal pieces.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

What is the polarity of the new magnets obtained?

Solution

Since the edges of the original magnet are not affected by the cutting process, they keep the original polarity and every new polarization is made by taking into account the fact that all magnets have two opposite poles. Therefore, the four small magnets will have the polarities as shown in the figure below.

Physics Tutorials: This image provides visual information for the physics tutorial Introduction to Magnetism

You have reached the end of Physics lesson 16.1.3 Magnetic Field Lines. There are 9 lessons in this physics tutorial covering Introduction to Magnetism, you can access all the lessons from this tutorial below.

More Introduction to Magnetism Lessons and Learning Resources

Magnetism Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
16.1Introduction to Magnetism
Lesson IDPhysics Lesson TitleLessonVideo
Lesson
16.1.1A Brief History of Magnetism. Natural and Artificial Magnets
16.1.2Magnetic Field of Magnets and Earth. Magnetic Poles
16.1.3Magnetic Field Lines
16.1.4How is a Magnet Produced?
16.1.5Permanent and Temporary Magnets
16.1.6The Earth as a Giant Magnet. Two North Poles.
16.1.7Magnetosphere
16.1.8Magnetism in Animals
16.1.9Magnetism in Use

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  1. Field Lines Feedback. Helps other - Leave a rating for this field lines (see below)
  2. Magnetism Physics tutorial: Introduction to Magnetism. Read the Introduction to Magnetism physics tutorial and build your physics knowledge of Magnetism
  3. Magnetism Revision Notes: Introduction to Magnetism. Print the notes so you can revise the key points covered in the physics tutorial for Introduction to Magnetism
  4. Magnetism Practice Questions: Introduction to Magnetism. Test and improve your knowledge of Introduction to Magnetism with example questins and answers
  5. Check your calculations for Magnetism questions with our excellent Magnetism calculators which contain full equations and calculations clearly displayed line by line. See the Magnetism Calculators by iCalculator™ below.
  6. Continuing learning magnetism - read our next physics tutorial: Magnetic Field Produced by Electric Currents

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