Ferrofluid Preform Display Cell

Experiment 1: Magnetic Field of a Permanent Magnet

Purpose
Demonstrate the presence of an invisible magnetic field of a permanent magnet. The magnetic field around a magnet can be described by the concept of magnetic lines of force; the lines form closed loops originating at the north pole and ending at the south pole and also passing through the material. The space where the lines of force are crowded represent a region of high magnetic field compared with the region where the lines are further apart.

Note: Avoid shaking the Display Cell. Shaking will turn the ferrofluid into suspended drops that will settle quickly. Although the ferrofluid will still be affected by a magnetic field, it may develop into a frothy state and greatly diminish the "spiking" phenomenon. The ferrofluid will eventually return to its normal state. Do not leave the cell on the magnet in storage. It may form a residual film on the surface of the glass. However, the fluid will still be functional. DO NOT OPEN THE DISPLAY CELL. The Display Cell ferrofluid is specially formulated for that environment. Opening the Display Cell will damage the fluid and cause it to fail.

What You Need
You will need the Display Cell and both magnets provided with it.

Procedure
Use this demonstration as reinforcement to your unit on magnetism. Permanent magnets have a north and a south pole (N & S), and the magnetic lines of force produced by the invisible magnetic field travel from the north pole of a permanent magnet to its south pole.

1. Hold the Display Cell above the magnets to observe the spikes which form in the ferrofluid. The spiking phenomenon is actually the result of the ferrofluid attempting to follow the invisible magnetic lines of force as they travel from one magnetic pole of the permanent magnets to the other. The spikes are produced when the lines of force are perpendicular to the surface of the ferrofluid. The maximum effect is then observed. When the magnets are placed on top of the cell, the same spikes are formed. Note that the ferrofluid doe not fall under gravity until the magnets are moved much farther away from the Display Cell.

2. Turn the magnets so the lines of magnetic force are parallel to the surface of the ferrofluid. Notice the disappearance of the spikes in the Display Cell.

What's Happening?
Because of their properties, magnetic lines of force do not intersect one another and do not bend sharply. Instead, they will arc smoothly around to reach their opposite pole, and in doing so, extend themselves higher as they travel to the other side. When the magnetic field is perpendicular to the Display Cell, many spikes are created as the ferrofluid follows the lines of magnetic force.

Notice the different shape of the ferrofluid when the magnetic lines of force are parallel to it. The ferrofluid still follows the lines of force, but reveals a much different pattern. Use different shaped magnets with the Display Cell and detect their presence and the direction of their magnetic fields to learn about magnetism.

Experiment 2: Manipulating Lines of Force

Purpose
Demonstrate magnetic permeability of magnetic and nonmagnetic materials. Show how magnetic lines of force can be manipulated with a permeable material.

What You Need
You will need the Display Cell, a magnet, one fairly thick piece of stainless steel (e.g., the handle of a butter knife, a teaspoon, etc.), and one wooden pencil.

Procedure
Use this demonstration as reinforcement to your unit on magnetism. Place the stainless steel spoon on the top of the magnet and try to recreate the spikes in the Display Cell. If the spikes still appear, add another spoon to increase the thickness. The spikes are either very small or not there at all.

Next, place the pencil between the magnet and the Display Cell. The spikes should appear as if nothing were on top of the magnet.

What's Happening?
The reason the spikes disappear when the spoon is placed between the permanent magnet and the ferrofluid is because the spoon is a magnetic material and has a higher permeability than the surrounding air. Magnetic lines of force behave much like electricity in that they choose the path of least reluctance or resistance. Highly permeable materials provide a path of least reluctance or resistance for lines of magnetic force. Air is nonmagnetic with a permeability of 1. The stainless steel spoon on the other hand has a higher magnetic permeability, approximately 550 with less reluctance than air and provides an easy path for the lines of magnetic force. Iron provides an even lower path of reluctance because it has a very high permeability of approximately 8000. Note that magnetic lines of force crowd together in a high permeability material and no such effect is observed for a nonmagnetic material.

Try placing several different materials that are at least twice the size of the magnet (such as plastic, aluminum, cardboard, bolts, nuts, etc.) between the permanent magnet and the Display Cell. Small or no spikes indicate a material that is magnetic and permeable. Large spikes indicate a material that is nonmagnetic with low permeability.

Application Note: Did you know that audio speakers rely on concentrated lines of magnetic force to produce sound, and that ferrofluid is applied to over 50 million loudspeakers every year?