Magnetic Linear Accelerator

This experiment is a simple way of demonstrating the distribution of the field inside a solenoid and how it effects ferrous metal objects.

This experiment simply consists of a plastic or cardboard tube with a coil of wire wrapped around one end. The coil can be powered by a set of standard batteries. the more batteries used the more powerful the magnetic field will be.

The tube used to for the coil around should be quite narrow. The case of a pen such as a biro is ideal. You can try using different sized batteries and different numbers of turns on the coil to produce different strength fields.

When a metal object is placed part way into the coil it is ready for firing. The metal should be ferrous (sticks to a magnet) and quite small. A metal rod of about 2 – 3mm wide and 10 – 20mm long is best. Something like a small nail or a screw with the head cut off should work fine. If a small rod magnet is used, it will work much better but make sure it’s inserted the right way around, or it could backfire.

To fire the coil gun you simply tap the switch. If you press it for too long, the projectile will either stop in the middle or come back out the wrong end. You can practice different methods and different coil and battery sizes to see what results you get. An alternative firing method would be to use a circuit such as the PWM-OCX which can give repeated pulses or a time you can set yourself.

For higher speed projectiles, it is possible to  use multiple coils and fire them in sequence so that each one will further accelerate the projectile. Each successive pulse must be shorter than the previous one due to the projectile spending less time in the acceleration region. If the pulses were too long, they would drag the projectile back, slowing it down. Our 3 channel time delay generator could be used for controlling the pulse timing to a transistor on each coil.

Thermal Effects on Resistance

The temperature affects the dimensions of the conductor; a higher temperature causes an expansion in a material while a colder temperature causes a contraction. And with this expansion/contraction a change in resistance occurs as a thicker wire has less resistance to current flow than a thinner one. Materials used as conductors typically tend to increase their resistance with a temperature increase while insulators have the adverse effect. Materials used as insulators often only exhibit a drop in resistance at very high temperatures meaning they usually don’t encounter temperatures high enough though typical use. Because of this these changes cannot all be attributed to the change in dimensions. In fact the resistance change is mainly due to the temperature affecting the atomic structure of the material, causing a change in the resistivity of the material.

The flow of current through a material is the movement of electrons. Electrons move under the influence of a magnetic field, they are negatively charged particles making them attracted by a positive electric charge. Therefore an electric potential can be applied to the conductor to move the electrons atom to atom towards the positive terminal. Not all electrons can migrate however, current is the movement of free electrons and the effectivity of an insulator or a conductor depends on the number of free electrons (a good conductor should have many free electrons while a good insulator should have few).

The effect heat has on an atomic scale is causing the atoms to vibrate, the higher the temperature, the more violent the vibration.

In a conductor the vibrations cause the many free electrons to collide with the captive electrons and other free electrons. These collisions use up some of the energy stored in the free electrons which in turn increases the resistance to current flow. Therefore increasing temperature of a conductor increases the resistance.

An insulator is different; the low number of free electrons means very little current can flow. Most of the electrons are tightly bound to their respective atom. Heating will still cause vibration; these vibrations however will cause significantly less collisions. If heated enough the vibrations may actually become violent enough to shake some electrons free, creating free electrons to carry a current. Therefore increasing the temperature of an insulator decreases the resistance.

This graph shows the measured resistance of a solenoid under varying temperatures.

Any normal conductor will see a drop in resistance with a drop in temperature. With a small range of temperatures like shown here the effect is almost linear. The wiggles in the graph are due to inaccurate data generated by the measurement process.

To demonstrate this you need a solenoid of at least several hundred turns, an ohm meter or multi meter, and some freezer spray.

The resistance of the solenoid used for this test was 6.3 Ohms at room temperature. To increase the temperature of the coil it can simply be connected to a battery and allowed to heat up. The temperature was measured using an infrared thermometer.

You could cool the solenoid in a standard freezer to about -20, but it would take a while so we used some freezer spray to get quicker results. The lowest resistance from our coil was just 4.8 Ohms whereas the highest was 8.2 Ohms.

Reducing the resistance of a coil means that a higher current can be drawn from the same source of EMF (volts). This means that the magnetic field it produces can be much stronger. When the temperature of a conductor drops below a certain level its resistance will suddenly drop to near zero. Under these conditions this is known as a superconductor. Superconducting electromagnets are used in MRI (Magnetic Resonance Imaging) machines so that ultra-strong magnetic fields can be produced. These usually have to be cooled with liquid nitrogen and require a lot of power.

FerroFluids

Ferrofluids are made from a suspension of tiny magnetic particles in a liquid such as water or oil. Such a mixture creates a liquid that can be attracted by a magnetic field. NASA discovered Ferrofluids at one of their research centers in the 1960’s while they were looking for different methods of controlling liquids in space.

The magnetic materials used are often made from iron or cobalt particles, but compounds such as manganese zinc ferrite are also used. The most common form of ferrofluid is made using particles of a type of iron oxide known as magnetite (Fe₃3₄). Making a stable Ferrofluid is not quite as simple as mixing tiny particles into a liquid. First of all the particles must be very small. The average size is around 10nm (0.00000001 meters). These particles can not be made by crushing or grinding a material, but are precipitated out of a solution during a chemical reaction.

During the precipitation the particles would naturally amalgamate (come together) due to magnetic and Van der Waals forces. To prevent this the mixture is heated so that thermal motion of the magnetite particles prevents them from sticking together. In order to prevent the particles from amalgamating after the reaction they must be kept apart from each other. This can be archived by coating each particle with another material known as a surfactant (surface active agent) to produce electrostatic or steric repulsive forces between the particles.

In an oil based ferrofluid, cis-oleic acid can be used as a suffricant. This is a long-chain hydrocarbon with a polar head that sticks to the surface of the magnetite particles. The long molecules stick out in all directions around each magnetite particle preventing them from getting close enough to stick together.

Water based (aqueous) ferrofluids often use ionic sufficants such as tetramethylammonium hydroxide. The negative hydroxide ions stick to the surface of the magnetite, and the tetramethylammonium cations form a positively charged layer around the outside. This means that the magnetite particles are held apart by the electrostatic repulsive force of the surrounding molecules.

Ferrofluids have several uses due to their magnetic properties. They can be used inside a magnetized bearing like an o-ring seal so that rotating shafts can pass from high to low pressure zones and vise versa. This is a much more efficient method than using solid seals as there is significantly less friction. This makes them ideal for use in submarines, rotating anode x-ray machines, disk drives, and vacuum chambers with external manipulators.

A more every day use of ferrofluid is in high quality loudspeakers. The fluid is pored into the magnetic cavity so that it surrounds the coil. This acts as a thermal conductor allowing more heat to be dissipated so that the speaker can be used at higher power. The fluid also helps to damp unwanted resonant vibrations giving an better overall sound quality.

These images show some ferro fluid in a container with a strong magnet placed underneath. The leftmost image shows a few large spikes that are formed as the magnet approaches the container. The other images show a large number of tiny spikes produced by the intense field of a magnet up close.

The spikes form in a manner as if they are following the field lines. In a stronger magnetic field there are more filed lines hence more spikes in the ferrofluid.

If you try this yourself make sure you don’t need the container again as the ferrofluid is very staining.

Video Clips

This Video Clip shows how the spikes of fluid change as a magnet is brought closer and then taken away again. This force is so strong that a normal heavy object such as a penny would appear to float on the fluid because displaced by the liquid moving underneath.

This Video Clip shows how the spikes of ferrofluid over a magnet change as a the magnetic field is oscillated using a coil surrounding the container. Its is possible to tune the vibrating fluid to resonance causing a fine jet to be ejected upwards from the centre. The electromagnetic coil is being powered by a PWM-OCX

Magnetorheological Fluid

A Magneto-rheological fluid is similar to a ferrofluid in the way that there are magnetic particles suspended in a fluid medium. This type of fluid does not use nano sized particles, but they must be small enough to remain suspended in the liquid. They are typically 2 or 3 times larger than the particles in Ferrofluids and are on the micrometer scale.

The particles in a magnetorheological fluid are magnetically polarisable. This means that when an external field is applied the micron sized particles will line up and form chain like structures. the alignment of the particles will increase the viscosity of the fluid.

A simple magnetorheological fluid can be made at home. Micron sized ferrous particles can be collect from sand or lake beds. By placing a magnet in a plastic bag and dragging it through sandy sediment many particles will be separated out. Turning the bag inside out and removing it from the magnet prevents the particle from becoming permanently stuck to its surface.

These particles can be mixed with a small amount of oil such as vegetable oil. By holding a magnet to the outside of t he container and poring off excess oil you will be left with a basic magnetorheological fluid. This fluid will not remaining stable for long periods due to the lack of a suffricant, but it serves well to demonstrate the scientific principles involved.

Magnetorheological fluids are being used mostly for controlled damping of oscillations. They are ideal for use in the suspension in large vehicles. In its liquid state it will provide limited damping, but when a magnetic field is brought near to the fluid it will greatly dampen any oscillations. This means that a large mechanical force can be controlled with a much smaller mechanical force.