Magnetic Fields from Bar Magnets: What You Need to Know
Permanent magnets can come in a variety of shapes and magnetizations. Many sensing applications need only a simple bar magnet. This article talks about bar magnets. A bar magnet has two main characteristics.
- It has a simple shape (such as a cylinder or cube).
- It is uniformly magnetized. It has two well-defined poles.
The fields from all bar magnets have a similar shape. You need to become familiar with this.
Cross-section of Field from a Cylinder Magnet
This graph shows the magnetic field from a typical cylinder magnet.
- The arrows show the direction of the field.
- The green shading shows the strength of the field.
There are some things you should pay attention to.
- The field is strongest near the poles of the magnet.
- The field points out from the north pole and into the south pole.
- The field direction at the south and north poles is the same. Notice how the field is pointing upward at both poles in this graph.
- The field direction along the sides of the magnet is opposite that of the poles. Notice how it is pointing downward in this graph.
- Notice how the field "turns" as you get further out from the corners of the magnet.
All bar magnets have a field similar to this one. If you are serious about designing magnets and sensors, you need to become familiar with this field shape. The way this field shape interacts with sensors will determine how they function!
magnetic fields from magnets of different materials
Bar magnets can be made of different materials. Some materials are stronger than others.
The direction and shape of the field should not change much with material changes.
The strength of the field does change with material changes.
As a technical note, the field shape can change due to material changes. If the magnet is well designed, this will be a small effect. If the magnet is poorly designed, changing materials can noticeably change the shape of the field. You should be using well designed magnets in sensor applications!
MAgnetic fields from magnets of different shapes
Bar magnets can be made of different shapes from the same material.
Notice how both the strength and field directions change with the different shapes.
Notice too that the general shape is still the basic bar magnet field. The field points out from the north pole and into the south pole and back around the sides. This is common to all bar magnets.
Plotting Axis for Magnetic Field
The material used for bar magnets affects the field strength. The red line shows the line where the field will be measured and plotted below. Three different materials were simulated to give you an idea of how much the field can vary.
Note: You should not confuse the internal field of the magnet with the field outside of it. A quantity called Br ("B sub R" or remanence) is used to describe the internal field of magnetic material. For weaker materials Br can be on the order of a few thousand gauss. For the strongest available materials, it can be over 14,000 gauss. Some people mistakenly think that this Br value is the field strength outside of the magnet. This is not the case!
This plot shows the effect of different materials on the field. Each curve is the field from a magnet of different materials. Stronger materials will generate stronger fields; weaker materials will generate weaker fields.
The field strength drops off fairly rapidly as you move away from the pole face.
Designing Magnets for Sensor Use
Magnet shape is an important part of magnet design for use with sensors. Magnetic sensors respond to both the direction and strength of magnetic fields. You must properly design and orient a magnet to produce the correct direction and strength of field where the sensor operates.
There are important aspects to magnet design not discussed here. The main point of this article was to show the basic bar magnet field. Other articles will discuss magnet design in much more detail.