Magnetic fields of a magnet

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material.

Image: Vecteezy. (PD)

Detecting magnetic fields

A magnetic field is invisible, but it can be detected using a magnetic compass. A compass contains a small bar magnet on a pivot so that it can rotate. The compass needle points in the direction of the Earth’s magnetic field, or the magnetic field of a magnet.

Magnetic fields can be mapped out using small plotting compasses or iron filings.

Activity 1

What you will need:

a bar magnet

a piece of paper

iron filings

What you will do:

1. Place the magnet under the piece of paper.  

2. Sprinkle iron filings over the piece of paper above the magnet.

Image: Openstax. BCIT Physics 0312 Textbook. (CC BY)

Iron filings near a magnet act like tiny compass needles, showing the shape of their fields. 

Activity 2

What you will need:

a bar magnet

a piece of paper

small compasses

What you will do:

1. Place the plotting compass near the magnet on a piece of paper.

2. mark the direction the compass needle points. 

Image: Flickr. Mason, D. (CC BY-NC-SA)

3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.

4. Join the points to show the field lines.

The needle of a plotting compass points to the south pole of the magnet.

Follow the link below:

https://www.physicsclassroom.com/Physics-Interactives/Magnetism/Magnetic-Field/Magnetic-Field-Interactive

The direction of magnetic field lines is defined to be the direction in which the north end of a compass needle points.

Magnetic field lines are imaginary lines. Magnetic field lines are a visual tool used to represent magnetic fields. They describe the direction of the magnetic force on a north pole at any given position.

The density of the lines indicates the magnitude of the field. Taking an instance, the magnetic field is stronger and crowded near the poles of a magnet. As we move away from the poles, it is weak, and the lines become less dense.

Images: Xaktly. (CC BY-NC-SA)

The diagrams show these key features:

• the magnetic field lines never cross each other
• the closer the lines, the stronger the magnetic field
• the lines have arrowheads to show the direction of the force exerted by a magnetic north pole
• the arrowheads point from the north pole of the magnet to its south pole

Magnetic shielding

Magnetic materials like iron concentrate the magnetic field lines and divert them out from the ends. Using this principle, these magnetic materials can act as magnetic shielding. Magnetic shielding is to prevent surrounding magnetic field lines from reaching the magnetic sensitive equipment (like MRI scanners) whose operation may be affected by the fields.

Magnetic shields work by redirecting the force lines away from the shielded object. Because of this, the materials used for magnetic shielding have to be able to sustain a strong magnetic field. Besides common materials such as iron, nickel and cobalt, there are several proprietary alloys commercially available that are especially designed for use as magnetic shields.

New technologies have provided some new magnetic shielding materials. For instance, nanotechnology has contributed magnetic shield materials that can be applied directly to the component like a coat of paint. While not always practical, superconductors, materials that lose all their electrical resistance at very low temperatures, are excellent magnetic shields.

Physics Lens. (2017). Magnetic shielding demonstrations. (CC BY)