# Magnetic Fields & Magnetic Field Lines

Show/Hide Sub-topics (Magnetism & Electromagnetism | O Level Physics)
Show/Hide Sub-topics (Electromagnetism | A Level Physics)

## Magnetic Field

Magnetic Field is the region around a magnet where other magnetic material will experience a force.

• OR A magnetic field is a region in which a moving charge or a current-carrying conductor will experience a force when it is placed in it. Hence it is a field of force.
• Magnetic fields are produced and experienced by moving charges. Magnetic field will not have any effect on a stationary charge.

A magnetic field can be graphically represented by magnetic field lines which indicates its strength and direction.

Note: Magnetic field is a vector quantity! (It has both magnitude AND direction!)

• Magnetic field lines show the direction of the magnetic field. (They emerge from the N pole and enter the S pole)
• When the field lines are close together at a point, the point is said to have a strong magnetic field.
• Arrows in the field lines outside the magnets show the direction in which a free north pole would move (from north pole to south pole).
• Field lines NEVER cross over  or touch one another.
• Parallel field lines indicate that the field is uniform
• Compass is used to find the direction and pattern of magnetic field. It has a permanent magnet needle which is free to rotate in a horizontal plane. The north pole of compass magnet (arrow head) will align and point along the magnetic field line direction.

IMPORTANT: Please note that for the last two diagrams, the field lines are NOT pushing against one another. Do NOT be tempted to say that the like poles repel because the field lines push against one another. It is NOT correct!

Interesting tidbits:

Magnetic field strength can be measured using a teslameter.

Magnetic field lines of a permanent magnet can be obtained by using a plotting compass.

• Under the influence of the magnetic field of the permanent magnet, the compass needle will align itself in the direction of the resultant magnetic field at that point.
• Detailed step-by-step guide is in the section below.

## Step-by-Step Guide to Plotting Magnetic Field Lines

By following this guide, you’ll be able to accurately plot the magnetic field lines of a bar magnet, providing visual insight into the invisible magnetic forces at play.

#### Materials Required:

• Bar Magnet: A standard bar magnet with clearly marked north and south poles.
• Plotting Paper: Graph or plain paper to plot the magnetic field lines.
• Plotting Compass: A small compass used for marking the direction of magnetic fields.
• Pencil: For marking positions on the paper.

#### Preparation:

1. Set Up Your Workspace: Choose a flat surface and place your plotting paper on it. Ensure you have enough space to work comfortably.

#### Procedure:

1. Position the Bar Magnet: Place the bar magnet horizontally at the center of your plotting paper. If your magnet has its poles marked, align it so that the north pole points towards the top of your paper.
2. Mark the Magnet’s Position: Outline the magnet’s shape on the paper with your pencil. This helps you remember the magnet’s position as you plot the magnetic field lines.
3. Starting Point: Position the plotting compass near the north pole of the magnet (or one end if not marked). Ensure the compass is close enough to the magnet to be influenced by its magnetic field but not touching it.
4. Mark the Compass Needle’s Position: Carefully observe the direction in which the compass needle points. Mark two small dots on the paper to represent the ends of the needle, labeling them as N (north) and S (south) accordingly.
5. Move and Mark Again: Lift the compass and place it down again so that the “S” end of the needle is directly over the dot you previously marked as “S”. The needle will align itself along the magnetic field line. Mark the new position of the “N” end of the needle with another dot.
6. Repeat the Process: Continue moving the compass and marking the new positions of the needle’s “N” end, following the magnetic field from one pole of the magnet to the other. Make sure each new position is accurately marked where the needle points.
7. Connect the Dots: Once you’ve marked a complete path from one pole of the magnet to the other, use a pencil to lightly connect the dots. This line represents a magnetic field line.
8. Plot Additional Field Lines: Repeat steps 3 to 7 to plot more magnetic field lines. Space the starting points of each line evenly around the poles to get a comprehensive map of the magnetic field. Ensure to plot lines on both sides of the magnet.

#### Finishing Up:

1. Review Your Field Lines: Once you have plotted several field lines, you should see a pattern that represents the magnetic field around the magnet. The lines should emerge from the north pole, curve around the magnet, and enter the south pole.
2. Clean Up: Erase any unnecessary marks or corrections, ensuring your final diagram is clear and accurate.

• Use a Ruler: For straighter lines or a neater diagram, use a ruler to connect the dots.
• Label Clearly: Mark the north and south poles of your magnet on the paper and label the magnetic field lines if necessary.
• Observe Patterns: Note that the field lines never cross each other and are denser near the poles, indicating stronger magnetic fields.

## Worked Examples

### Example 1: Conceptual Understanding

Explain why magnetic field lines never cross each other, using the concepts of magnitude and direction. Consider what the crossing of lines would imply about the magnetic field at the point of intersection.

Magnetic field lines represent both the direction and magnitude (strength) of the magnetic field at any point. If two field lines were to cross, it would imply that at the point of intersection, the magnetic field has two different directions and possibly two different magnitudes, which is impossible. A magnetic field at any given point in space can have only one direction and one magnitude. Therefore, the principle that magnetic field lines never cross is fundamental to understanding the vector nature of magnetic fields.

### Example 2: Practical Application

During an experiment to plot magnetic field lines around a bar magnet using a plotting compass, you observe that the compass needle points in unexpected directions at certain points near the magnet’s surface. What could be causing this phenomenon, and how would you correct it?

If the compass needle points in unexpected directions, it could be due to local distortions in the magnetic field near the magnet’s surface, possibly caused by irregularities in the magnet’s material or external magnetic influences. To correct for this, ensure the experiment is conducted away from other magnetic materials or electronic devices that could interfere with the magnetic field. Additionally, verify that the compass is functioning correctly and is sensitive enough to accurately indicate the magnetic field direction. Adjusting the distance between the compass and the magnet might also help in obtaining more accurate readings.

### Example 3: Critical Thinking

Consider a scenario where two bar magnets are placed with their like poles facing each other. How would the magnetic field lines be represented in this setup? Discuss the implications for the strength of the magnetic field at different points between and around the magnets.

When like poles of two bar magnets face each other, the magnetic field lines will emerge from the north pole of one magnet and curve around, avoiding direct entry into the north pole of the other magnet, due to the repulsive force between like poles. Instead, the lines will spread out and curve around to the south poles of the magnets. This setup creates a region of high magnetic field density (strength) between the magnets, where the field lines are closest together, indicating repulsion. Around the outer sides of the magnets, the field lines will be less dense, indicating weaker magnetic fields. This configuration illustrates the vector nature of magnetic fields, showing how they combine and interact in space.

### Example 4: Application of Knowledge

Using a teslameter, you measure the magnetic field strength at various points along the path of a magnetic field line around a bar magnet. You notice that the field strength decreases as you move away from the magnet. Explain why this happens and how it relates to the representation of magnetic field lines.

The magnetic field strength decreases as you move away from the magnet because the density of the magnetic field lines decreases with distance from the magnet. Near the poles, where the magnetic field is strongest, the field lines are closer together. As you move away from the magnet, the field lines spread out, indicating a decrease in the strength of the magnetic field. This phenomenon is consistent with the concept that the density of field lines represents the magnitude of the magnetic field; closer lines indicate stronger fields, and more spread-out lines indicate weaker fields. This observation underscores the utility of magnetic field lines in visually representing both the direction and the relative strength of magnetic fields.

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