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What is the Hall Effect?

sales@nationalimports.com — Tue, 24 Jan 2017 15:33:00 -0600

Current is produced by the movement of charge carriers within a material. Typically we think of negatively charged electrons as charge carriers. However, depending on the material, the charge carriers could also be positively charged holes or ions, or a mixture of different kinds of charge carriers. If a magnetic field (B) is applied perpendicular to the direction of the current (I), the charge carriers will experience a force known as the Lorentz force (FL) (see What is a Homopolar Motor?). This force acts perpendicularly to the direction of the charge carriers, causing their path to bend, meaning that opposite charges will collect on opposite edges of the material and this phenomenon is known as the Hall effect. The charge separation within the material results in the formation of an electric field and a voltage difference across the material. As the diagram below shows, the force due to the electric field (FE), opposes the Lorentz force. Once a field strong enough to balance the Lorentz force has been established, more charge carriers are prevented from moving to the edge of the material. This results in a constant voltage difference across the material, known as the Hall voltage.

The Hall effect was discovered by physicist Edwin Hall in 1879 and has since been used to reveal many fundamental principles regarding the nature of charge carriers. The sign of the Hall voltage gives the sign of the charge carriers in a material and the Hall experiments were the first to prove that currents in metals are generally carried by negatively charges electrons rather than positively charged protons. There are some situations, in semiconductors for example, where the Hall voltage is positive and in these cases, current is conducted by positively charged quasiparticles called holes. A hole can be thought of as the absence of an electron, so the movement of holes is actually caused by the movement of electrons. Therefore, you would probably expect that the Hall effect would be the same for both holes and electrons. However, there are cases where electrons behave as though they have negative mass, resulting in a positive Hall voltage, and it is easier to treat the charge carriers as positively charged holes.

The most common application of the Hall effect is as a sensor or magnetometer. Hall devices are used in a variety of situations, from measuring magnetic fields and investigating pipelines to use as rotating speed sensors in bikes and speedometers, to fluid flow sensors, current sensors, pressure sensors and even as compasses in smart phones and GPS devices.

When a current and a magnetic field is applied to the semiconductor crystal in a Hall probe, a Hall voltage is generated across the material. Because the Hall voltage is directly proportional to the size of the magnetic field, Hall devices can be used as magnetic field strength sensors. Conversely, as the output voltage varies in response to a magnetic field, if the strength of the magnetic field is already known, the level of the output voltage reveals the distance from the field. Hall devices have many advantages; they can be dust, dirt and water proof, making them very reliable and are also relatively cheap. Although one significant drawback is that they are very sensitive to stray fields, including the Earth’s magnetic field, making them useful as compasses but drastically reducing their accuracy. To overcome this problem Hall probes can be magnetically shielded, for example by placing the device in a ferrite ring which blocks out the stray fields.

A more complex application of a Hall device is in spacecraft propulsion systems. Hall effect thrusters have been around since the early 1970s when they were used to stabilize Soviet satellites and have the major advantage of being relatively low power devices. A radial magnetic field is used to trap electrons which then circulate and create an electric field due to the Hall effect. The energetic particles then ionize propellant particles and the positive ions and negative electrons are then ejected from the thruster creating thrust to move the spacecraft.

How Does an MRI Scanner Work?

sales@nationalimports.com — Sun, 24 Jul 2016 15:24:00 -0500

Magnetic resonance imaging (MRI) is a powerful imaging technique used to investigate the body. MRI scanners use very strong magnetic fields and radio waves, which interact with protons in tissues to create a signal that is then processed to form images of the body. The protons (hydrogen atoms) can be thought of as tiny bar magnets, with a north pole and a south pole, spinning on an axis – like a planet. Normally, the protons are randomly aligned, but when a strong magnetic field is applied, the protons will align with this field.

Applying a radio wave pulse at the correct frequency excites the protons, causing them to resonate and disturbing the magnetic alignment. The excited protons release the absorbed energy as a radio frequency signal and the emission is picked up by a receiver coil in the scanner. The radio frequency that causes the protons to resonate depends on the strength of the magnetic field. In an MRI scanner, gradient electric coils are used to vary the magnetic field strength across the body. This means that different sections of the body will resonate at different frequencies. So by applying different frequencies in sequence, you can image slices of the body separately and gradually build up a picture.

When the radio source is switched off, the protons will return to their original undisturbed state (aligned with the magnetic field), emitting radio waves as they do so which are picked up by the receiver coils. Different tissues will relax at different rates, for example fat and water have different relaxation times, so the relaxation time can reveal the type of tissue being imaged. There are two relaxation times which can be measured; T1 – the time taken for the magnetic alignment to relax and T2 – the time taken for the spin to return to its resting state.

Multiple radio pulse sequences can be used to highlight or suppress certain tissue types. For example, abnormalities are not usually found within fat, so a fat suppression pulse sequence can be used to remove the signal emitted by fatty tissues, leaving just the signal from areas which are more likely to contain irregularities.

MRI scanners need incredibly strong magnetic fields; generally around 1.5 Tesla, but they can be as large as 7 Tesla. For comparison, the Earth’s magnetic field is only 0.00005 Tesla. The magnet is composed of multiple coils of conductive wire through which a current is passed to generate the magnetic field. To achieve the high field strengths required, the magnet is cooled with liquid helium to below 10 Kelvin (-442oF / -263oC). This enables superconductivity, allowing current to flow through the coils without creating electrical resistance, meaning that when a magnet is super cooled, it is capable of conducting larger currents and therefore able to produce stronger magnetic fields.

MRI was invented by Paul Lauterbur in 1971 at Stony Brook University, Long Island. The technique was then developed by Sir Peter Mansfield and the first MRI body scan of a human being was produced in 1977. Although it wasn’t until the 1980’s that the first MRI scanner capable of creating clinically useful images was produced. This machine was designed by John Mallard, who is credited for the widespread introduction of MRI, and was used to identify several conditions afflicting a test patient, including a tumor in his chest, an abnormal liver and bone cancer. The ‘discoveries concerning magnetic resonance imaging’ won Paul Lauterbur and Sir Peter Mansfield the 2003 Nobel Prize in Physiology or Medicine.

MRI is widely used in medical diagnostics and unlike X-rays and CT scans, has the great advantage of not exposing the subject to ionizing radiation. However, the effects of the high magnetic fields on the body are still unknown. MRI scanners are particularly good for neurological scanning and are excellent for visualizing small tumors, dementia, epilepsy and other conditions of the central nervous system. A scan can take between 15 and 90 minutes, depending on the size of the area and how many images are taken. The machines are incredibly noisy and can produce sounds as loud as those generated by a jet engine.

MRI scanners can be very dangerous and strict safety procedures have to be followed in the vicinity of these machines, as several fatalities have been recorded. Due to the strong magnetic fields involved, this equipment cannot be used on patients who have pacemakers which could be disrupted, or metal implants or shrapnel which could be heated and moved by the magnets during the procedure. Furthermore, ferromagnetic objects can be strongly attracted to the magnets and pose a serious projectile risk. For this reason, such objects are banned from close proximity to the scanners.

What are Eddy Currents?

sales@nationalimports.com — Thu, 30 Apr 2015 09:54:00 -0500

Eddy currents are currents which circulate in conductors like swirling eddies in a stream. They are induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field. They can be created when a conductor is moving through a magnetic field, or when the magnetic field surrounding a stationary conductor is varying i.e. anything which results in the conductor experiencing a change in the intensity or direction of a magnetic field can produce eddy currents. The size of the eddy current is proportional to the size of the magnetic field, the area of the loop and the rate of change of magnetic flux, and inversely proportional to the resistivity of the conductor.

Like any current flowing through a conductor, an eddy current will produce its own magnetic field. Lenz’s Law states that the direction of magnetically induced current, like an eddy current, will be such that the magnetic field produced will oppose the change of magnetic field which created it. This resistance created by the opposing magnetic fields is exploited in eddy current braking, which is commonly used as a method of stopping rotating power tools and rollercoasters.

In the diagram below, the conductive metal sheet (representing the moving rollercoaster car or power tool for instance), moves past a stationary magnet. As the sheet moves past the left edge of the magnet, it will feel an increase in magnetic field strength, inducing counter-clockwise eddy currents. These currents produce their own magnetic fields and according to Lenz’s Law, the direction will be upwards i.e. opposing the external magnetic field, creating magnetic drag. At the other edge of the magnet, the sheet will be leaving the magnetic field and the change of field will be in the opposite direction, thus inducing clockwise eddy currents which then produce a magnetic field acting downwards. This will attract the external magnet, also producing drag. These drag forces slow the moving sheet, providing the braking. An electromagnet can be used for the external magnet, meaning it is possible to vary the strength of the braking applied by adjusting the current through the electromagnet’s coils. An advantage of eddy braking is that it is contactless, so results in no mechanical wear. However, eddy braking is not suitable for low speed braking and because the conductor has to be moving, eddy brakes cannot hold objects in stationary positions. Thus, it is often necessary to also use a traditional friction brake.

Eddy currents were first observed in 1824 by scientist and then Prime Minister of France, François Arago. He realized that it was possible to magnetize most conductive objects and was the first to witness rotary magnetism. Ten years later, Lenz’s Law was postulated by Heinrich Lenz, but it wasn’t until 1855 that the French physicist Léon Foucault officially discovered eddy currents. He found that the force needed to rotate a copper disk when its rim is placed between the poles of a magnet, such as a horseshoe magnet, increases and the disk is heated by the induced eddy currents.

The heating effect originates from the transformation of electric energy into heat energy and is used in induction heating devices, like some cookers and welders. The resistance felt by the eddy currents in a conductor causes Joule heating and the amount of heat generated is proportional to the current squared. However, for applications like motors, generators and transformers, this heat is considered wasted energy and as such, eddy currents need to be minimized. This can be achieved by laminating the metal cores of these devices, where each core is made up of multiple insulated sheets of metal. This splits the core in many individual magnetic circuits and restricts the flow of the eddy currents through it, reducing the amount of heat generated through Joule heating.

Eddy currents can also be removed by cracks or slits in the conductor, which break the circuit and prevent the current loops from circulating. This means that eddy currents can be used to detect defects in materials. This is called nondestructive testing and is often used in airplanes. The magnetic field produced by the eddy currents is measured, where a change in the field reveals the presence of an irregularity; a defect will reduce the size of the eddy current, which in turn reduces the magnetic field strength.

Another application of eddy currents is magnetic levitation. Conductors are exposed to varying magnetic fields which induce eddy currents within the conductor and produce a repulsive magnetic field, pushing the magnet and conductor apart. This alternating magnetic field can be caused by relative motion between the magnet and conductor (generally the magnet is stationary and the conductor moves) or with an electromagnet applied with a varying current to vary the magnetic field strength.

Power Walking – Smart Shoes Generate Power from Walking

sales@nationalimports.com — Wed, 01 Apr 2015 09:59:00 -0500

Wearable devices, such as health monitors and sensors, are of great interest. One of the major limitations in making these devices a practical reality has been the power supply – the need for large, heavy batteries. Movement generates power and a human can produce 10s of Watts just from walking and up to a kilowatt when sprinting. Harvesting this energy has the potential to overcome the power supply problem and German researchers have recently built power harvesting devices which are small enough to fit inside a shoe. The researchers have produced two devices, a swing harvester and a shock harvester. Both creations exploit the interaction between magnets and conductive coils to generate power.The aim was to create a self-lacing and unlacing shoe for the elderly, but this technology has many potential applications and could be used to power wireless devices.

As the name suggests, the swing harvester generates power when the foot is in motion. Multiple magnets are stacked within conductive coils. When the foot swings, this stack of magnets moves through the coils, inducing a voltage and causing current to flow though the windings. The amount of power produced depends on the speed of movement – the faster you run, the more power you generate.

The second invention, the shock harvester, produces power when the heel strikes the ground. This device is composed of a spring loaded magnetic circuit, containing three pairs of magnets, two coils and a magnetic suspension system. When the heel hits the floor, the two magnets in the magnetic suspension system begin to approach. This creates a repulsive force, pushing the magnetic circuit back up. The spring loading causes the magnetic circuit to oscillate, leading to power generation by resonant magnetic induction.

The energy output is small, up to 4 mW for the shock harvester. A smartphone requires around 5 W (5000 mW), but these devices are still capable of powering small sensors and wireless transmitters. As the output is directly related to the size of the system, making the devices larger would yield more power. Previous designs have produced up to 250 mW, but were impractically large. The same researchers are currently working on new models, which promise a significant increase in power without a large increase in size.

Details about the project entitled "Energy harvesting from human motion: exploiting swing and shock excitations" (Authors: K. Ylli, D. Hoffmann, A. Willmann, P. Becker, B. and Y. Manoli Folkmer) are published in IOP Publishing's journal "Smart Materials and Structures" at:

http://iopscience.iop.org/article/10.1088/0964-1726/24/2/025029/meta;jsessionid=A9298B422A8116BF0B2E7473862F0F6C.c4.iopscience.cld.iop.org

What is a ferrofluid?

sales@nationalimports.com — Sat, 31 Jan 2015 10:02:00 -0600

A ferrofluid is a liquid which becomes highly magnetized in the presence of a magnetic field. The distinctive ‘spikey’ shape of a magnetized ferrofluid is caused by the need to find the most stable shape in order to minimize the total energy of the system, an effect known as the normal-field instability. The fluid is more easily magnetized than the surrounding air, so is drawn out along the magnetic field lines, resulting in the formation of peaks and troughs. However, the extension of the ferrofluid is resisted by gravity and surface tension. The formation of the corrugations lowers the magnetic energy of the system but raises the gravitational energy and surface free energy. When these forces are balanced, the minimum energy configuration is achieved. Because ferrofluids are very easily magnetized (they have an incredibly high magnetic susceptibility), the peaks can be produced using a small bar magnet.

Ferrofluids are known as colloidal fluids and are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, usually water or an organic solvent like kerosene, and coated with a surfactant to stop them clumping together in the liquid. A typical composition would be 5% magnetic particles, 10% surfactant and 85% carrier fluid.

The particles in a ferrofluid have a diameter of 10 nanometers or less and are composed of a ferromagnetic, highly magnetically susceptible compound such as magnetite (Fe ₃O₄) or hematite (Fe ₂O₃). The particle size has to be small enough to allow them to be evenly dispersed through the liquid by Brownian motion (the random motion of particles in a liquid due to collisions which other molecules) but large enough for them to each make a significant contribution to the magnetic response of the fluid. Upon application of an external magnetic field, the nanoparticles align with the field. However, once the external field is turned off, the particles return to a random alignment. For this reason, ferrofluids are classed as superparamagnets rather than ferromagnets.

The surfactant's van der Waals forces stop the magnetic nanoparticles aggregating in the solution. Different surfactants work in different ways but the general principle is that the surfactant creates a layer around the particle which will repel other coated nanoparticles. The diagram below illustrates the principles of an ionic surfactant – the surfactant ions form a layer of charge around the nanoparticle, repelling other charged, surfactant coated particles. Whilst the addition of a surfactant is crucial, it has the negative effect of decreasing the viscosity of the fluid in the magnetized state and making it ‘softer’. As most applications require a ‘hard’ fluid in the magnetized form, this is an important factor to consider when choosing the ferrofluid composition.

In 1963, Steve Papell of NASA created ferrofluid for use as rocket fuel. His team of NASA scientists were investigating methods of directing fluids in space and realized that magnetic fluids could be completely controlled by the application and variation of a magnetic field. The ferrofluid was mixed with liquid fuel and drawn towards the ignition system with an external magnetic field. Ferrofluids have now found use in many applications from small electronic devices to space crafts to cancer treatments to art. In fact, ferrofluids are found in many common household devices, including hard drives where they are used to seal the interior of the device. When magnetized they form a barrier to dust and dirt which could damage the delicate plates.

Ferrofluids can have very high thermal conductivities and their heat transfer properties are exploited in devices such as loud speakers where they are used to cool the voice coil. In a loudspeaker, sound is produced when the voice coil vibrates but this also generates unwanted heat. Ferrofluids lose their magnetism as they are heated, fully losing their magnetic properties when heated to a high enough temperature, known as the Curie temperature. If ferrofluid is placed around the voice coil, a magnet placed near the coil will attract more cold ferrofluid than hot ferrofluid because the colder ferrofluid will be more strongly magnetized. This cold ferrofluid will absorb heat around the voice coil and then be moved towards a heat sink as it is replaced by cooler ferrofluid.

Ferrofluids are also the focus of current scientific research and have the potential to be used in many medical applications. In magnetic drug targeting for example, where drugs could be enclosed by ferrofluid and, once injected into the specific body area requiring treatment, a magnetic field could be applied to keep the drugs in this target area. The localization would limit exposure to the rest of the body and enable the dosage level to be decreased, reducing the adverse side effects experienced by the patient.

What is a homopolar motor?

sales@nationalimports.com — Wed, 31 Dec 2014 10:03:00 -0600

A homopolar motor is a direct current (DC) electric motor which produces constant circular motion. This device is easy to create and the figure below illustrates the basic concepts behind its operation. A permanent magnet is attached to one terminal of a DC power supply, in this case a AAA battery. A conducting wire connects the other terminal to the magnet, thus completing the circuit. This wire should be free to rotate whilst always maintaining contact with both the terminal and magnet. The current (I) flowing through the wire will produce a magnetic field. This field will interact with the magnetic field (B) produced by the permanent magnet, and a Lorentz force (F) will be exerted perpendicular to the directions of I and B. As you look at the diagram, the force on the left section of the wire is acting into the screen and the force on the right section is coming out of the screen. As the wire can move freely, these forces cause the wire to rotate in a clockwise motion. Because the polarities of the magnetic fields do not change (hence the name homopolar), the direction of the force will not change and the wire will rotate in a constant circular motion.

As mentioned above, it is the Lorentz force acting on the wire which causes it to turn. This force results from the interaction of the electric and magnetic forces in electromagnetic fields. In the presence of a magnetic field, a moving charged particle, such as an electron carrying the current in the wire, will experience a force.The direction of the force can easily be determined using the right hand rule; the index finger points in the direction of the current flow, the middle finger points in the direction of the magnetic field and the thumb reveals the direction of the force.

The discovery of the homopolar motor was surrounded by controversy. Michael Faraday was the first to successfully build the device in 1821. However, this achievement came after his discussions with the scientists Humphry Davy and William Hyde Wollaston, who had also been trying to create electric motors. Faraday’s design featured continuous circular motion that was produced by the magnetic force around a wire which stretched into a pool of liquid mercury containing a permanent magnet. Faraday presented his creation to the Royal Society and published his results without acknowledging the work of the other two scientists. This scandal damaged Faraday’s reputation at the Royal Society, limiting his subsequent involvement in electromagnetic research.

Based on the same general principles as the homopolar motor, rail guns have been of interest to the military since the early 1900s. A simple rail gun differs from a standard homopolar motor in that it does not use an external magnetic field. A pair of conducting rails connected to a power supply is joined by a sliding armature (or conductive projectile) to complete the circuit. A current flows down one rail, through the armature and back along the other rail. The current produces a magnetic field within the loop formed by the power supply, rails and armature. Similarly to the homopolar motor diagram above, a magnetic field will be produced perpendicular to the current and the plane of the armature and rails. As both the current and magnetic field act in opposite directions along each rail, the Lorentz force produced will accelerate the armature along the rails. This acceleration can launch a projectile to hypersonic speeds. However very large currents, and therefore large power supplies, are required and so much heat is generated that the rails can be eroded. Due to these practical complications, rail guns are still at the developmental stage. However, the U.S. Navy have been able to accelerate a 7 pound projectile to velocities of up to 8,600 km/h using experimental technology.

It is possible to turn a homopolar motor into a homopolar generator. If the conducting wire is physically turned, this motion will produce a current and a DC voltage between the battery terminals. This phenomenon was also discovered by Michael Faraday, who invented the Faraday disc. Whilst this device was incredibly inefficient and not suitable for use as a practical power source, it did pave the way for the development of the modern dynamos found in a variety of gadgets today, such as wind up radios and flashlights.

What is a Halbach Array?

sales@nationalimports.com — Sun, 30 Nov 2014 10:07:00 -0600

A Halbach array is a specific arrangement of a series of permanent magnets. The array has a spatially rotating pattern of magnetism which cancels the field on one side, but boosts it on the other. The main advantages of Halbach arrays are that they can produce strong magnetic fields on one side whilst creating a very small stray field on the opposite side. This effect is best understood by observing the magnetic flux distribution.

Strips of ferromagnetic materials (materials which can be permanently magnetized) with alternating magnetizations are combined such that the magnetic fields align above the plane of the composite structure, whilst below the structure the fields are in opposite directions and cancel out. More precisely, the alternating components of magnetization are p/2 or 90 ^o out of phase.

In the ideal case, shown above, this superposition would produce a field above the plane which is twice as large as if the structure were uniformly magnetized, and no field below the plane. However, in reality the ideal case is never observed and a very small field is produced on the underside. This arrangement can be continued indefinitely to produce large arrays.

These “one-sided flux” structures were first discovered by John C. Mallinson in 1973, who described them as “curiosities” with the potential to improve magnetic tape recording technology. However, their true potential wasn’t realised until the 1980s, when Berkley physicist Klaus Halbach independently rediscovered this magnetic phenomenon and created Halbach arrays for use in particle accelerators.Halbach produced the arrays using the ferromagnetic material cobalt to generate strong magnetic fields for focusing and steering the particle accelerator beams.

Halbach arrays now have many applications and are used in a range of systems of varying complexity. One of the simplest applications of Halbach arrays is in refrigerator magnets. In this case the one-sided flux properties are exploited in order to boost the holding power of the magnet. Variable arrays of magnetics rods can also be combined to create simple locking systems. If the magnetizations of the rods are arranged so the field is maximised above the plane and minimised below it, the flux confinement can be flipped by rotating each rod 90 ^o.

A more advanced example of a Halbach array in action is in a Maglev train track or Inductrack, where magnetic levitation is used to support the carriage. The magnetic arrays lift the train a small distance above the track and can support a weight of up to 50 times that of the magnet. The operation is based on the principle of induction; as the array is passed over the metallic track coils, the variations in the magnetic field induces a voltage in the track. The track then creates its own magnetic field and, similarly to when you attempt to push the two like poles of bar magnets together, when this field aligns with the field produced by the Halbach array, repulsion causes the train to levitate. Maglev trains do not suffer from many of the frictional forces which slow down traditional wheeled trains and are able to provide high speed transportation. In fact, the Japanese SCMaglev train system, which reached 361 mph in 2003, currently holds the Guinness World Record for the fastest rail transportation.

Halbach arrays are also used in advanced scientific experiments such as synchrotrons and free electron lasers (FELs), where they are known as Halbach ‘wigglers’. FELs have a very wide and highly tunable frequency range, and are used in many applications ranging from medical to military. A Halbach wiggler is one of the core components of a FEL, where the array’s magnetic field is used to periodically ‘wiggle’ a beam of charged particles (usually electrons). The wiggling effect causes a change in the direction and therefore a change in the acceleration of the particles. This in turn leads to emission of high intensity synchrotron radiation (photons) when combined with an external laser source.

It is also possible to create Halbach cylinders and rings, where the magnetic field is strong inside the ring or cylinder but negligible outside, or vice versa depending on the arrangement of magnets. These structures are typically used for brushless AC motors, where traditionally stray fields can reduce torque and efficiency. However, because Halbach cylinders are intrinsically shielded by their structure, with almost all flux contained within the centre, they are able to avoid this problem and produce higher torques.