Monday, June 7, 2010

Types of Magnetism

Types of Magnetism

Before discussing the different types of magnetism we need clear up a few definitions which are used to categorise the types of magnetism.

Magnetic Suseptibily

As the magnetic field increases, the magnetic flux increases. We denote magnetic field intensity by H and magnetic flux by B the constant of proportionality is μ0 this is known as the magnetic permitivity

In a vacuum, μ0 has a value of 4π x 10-7 H m-1 in SI units

For other materials this proportionality is expressed with the relative permitivity, μr

The susceptibility χ is defined in terms of the relative permitivity. χ = (μr - 1)


Ferromagnetism  alignment of electron spins



Ferri magentic material have two sets of magnetic dipole moments pointing in opposite directions. The magnetic moments do not cancel each other out becauses the dipole moment in one direction is smaller than the other. On a B-H graph ferrimagnetism is like ferromagnetism.


Diamagnetic materials are composed of atoms that have no net magnetic moments. However, when exposed to a field a weak negative magnetisation is produced which causes repulsion instead of attraction. Diamagnetic materials have negative susceptibilty with magnetitude of around -10 to -10-4

Many common materials are diamagnetic and hence In the presence of very strong magentic fields the repulsion caused by diamagnetism can cause objects to levitate even frogs.



Paramagnetic materials such as liquid oxygen and aluminium show a weak magnetic attraction when placed near a magnet. Some atoms or ions in the material have a net magentic moment due to unpaired electrons in partially filled orbitals. In the presence of a field, there is a partial alignment of the atomic magnetic moments in the direction of the field resulting in a net positive magnetisation and positive susceptibility.

In a strong magnetic field, paramagnetic materials become magentic and will stay magnetic while the field is present. When the strong magnetic field is removed the net magnetic alignment is lost and the magnetic dipoles relax to a random motion.


Introduction Of Magnetism

The ancient Greeks knew that a type of rock with magnetic properties known as lodestone or magnetite attracted iron. The compass, an important device for navigation, has a suspended magnet which aligns parallel to the magnetic field produced by the Earth and as a result points to the  North Pole. The compass was documented as early as 1040. The Ching Tsung Yao describes how iron can be magnetised by heating and quenching in water. It is known that the Vikings used Lodestone to navigate. By the end of the twelfth century, Europeans were using this simple compass to aid navigation. A steel needle stroked with such a "lodestone" became "magnetic" as well.

In 1600, William Gilbert (also known as Gilberd) of Colchester proposed an explanation in his work De Magnet for the operation of the compass and that The Earth itself was a giant magnet, with its magnetic poles some distance away from its geographic ones (i.e. near the points defining the axis around which the Earth turns). He made an experimental model of the earth by creating a .

Properties of Magnets

William Gilbert also experimented on bar magnets and found the following properties:

A magnet will always have two poles which we call arbitrarily North and South. I the magnet is broken in two this will create two new magnets with North and South poles. If a bar magnet is broken in two, at the fracture new north and south poles are formed at the point of fracture.

  • Like pole repel each other. If a N pole is brought close to the north pole of a second magnet a repulsive force will be felt. Similarly if a South pole is brought close to the South pole of another magnet, the two magnets will repel each other.
  • Unlike poles attract and will stick together.
  • Magnets attract iron rich materials and like poles and the repulsion between like poles can be reduced if a strip of iron is placed between them.



Comets are very beautiful when they pass close to the sun but they are merely giant balls of frozen water mixed with cosmic dust. There is also a small percentage of ammonia, methane, and CO2. These chunks are theorised to have originated from the forming of giant planets like Saturn, Jupiter, Neptune, and Uranus, and each comet is usually the size of one of the Rocky Mountains. There are four parts to a comet: nucleus, coma, plasma tail, and dust tail. The nucleus is the only permanent part of the comet. Although it has never been seen, there is no doubt of its existence. The brightness of the comet can depend on how large the nucleus' diameter is since these spherical forms can scatter sunlight like a mirror.

Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper Belt, or its associated scattered disc,[1] which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a cloud of icy bodies in the outer Solar System that were left behind during the condensation of the solar nebula. Long-period comets plunge towards the Sun from the Oort Cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories.

Comets come from the outermost regions of the solar system called the Oort cloud, some 50,000 AU from the Sun.

The Sun's gravitational field is the dominant force out to around 2 light-years although beyond the Oort cloud there is still much that is not known beyond.

The structure of a comet

The Solar System

What is in the Solar System?

The Sun

The Sun is our nearest star. The Sun contains approximately 98% of the mass of the solar system and provides the gravitational force required to keep the planets orbiting around it. Solar energy is created deep within the core by nuclear fusion. Every second 700 million tons of hydrogen are converted into Helium. And the energy produced by this conversion is what gives energy to almost everything that lives on the Earth.

The  Planets

In order of distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The inner planets (Mercury, Venus, Earth and Mars) are formed from solid material while the outer planets (Jupiter, Saturn, Uranus and Neptune) are formed from gas and liquid.

The Discovery of Neptune

(Since antiquity, the planets Mercury, Venus, Mars, Saturn, Jupiter).

Neptune could have been discovered a lot earlier than it was, Galieo had seen it

Newtons law of gravity allowed the solar system to be modelled with great accuracy, however observations of Uranus showed that it did not behave entirely as predicted. The only planet to be discovered was Uranus in 1781 by Music Teacher, William Herschel who believed it to be a comet and named it Georgium Sidus (George's Star) in honour of King George III.

 Accurate data for the orbits of the other planets was available but the data for Uranus did not fit with the calculated results. The anomalous were caused by another large object and Neptune was discovered by calculating where a planet would have to be to produce the anomalous behavior of Uranus. This was the first planet to be discovered by mathematics.

The first accurate predictions of Uranus' motion were published in 1792. Within a few years, it was obvious that there was something wrong with the motion of the planet, as it did not follow the predictions. Attempts to calculate improved tables using the latest mathematical techniques were unable to fit all the observations, ancient and modern, to a single orbit. A commonly accepted suspicion during the next few years was that Newton's Law of Gravity, although accurate out to the orbit of Saturn, might not work in the same way at greater distances; but by the late 1830's it seemed at least equally likely that there was an unknown body lying beyond the orbit of Uranus.

Neptune was discovered on September 23, 1846 by the astronomer Johann Galle and his student Heinrich d'Arrest, after only thirty minutes of searching the sky, within a degree of the position predicted by the Urbain Le Verrier. This was the high-water mark of Newtonian physics: to be able, given the laws of physics and the peculiar motion of one object, to reach out into the depths of space and uncover a previously hidden object and caused an even greater sensation than the discovery of Uranus

There were a number of observations of Neptune prior to its discovery as a planet, but once again, mostly at times when it was near a stationary point, and hence nearly motionless relative to the stellar background. In fact, the first observations were made by Galileo, he even noted that the object which we now know as Neptune seemed to have moved relative to the nearest star, but failed to follow up his observations, and hence lost the opportunity to discover Neptune 168 years before Herschel discovered Uranus.

A planet had been discovered by mathematics, was it possible that other planets might be found in this way? A search at both ends of the solar system followed. Close to Mercury, the precession of the line of apsides (the imaginary line that runs along the major axis of the orbit). Unfortunately, even in the 1840's, it was obvious that the calculated rate of precession for the line of apsides was too small however, it might have conceivebly been caused by a smaller planet than Mercury that orbited on an orbit closer than Mercury. The planet was christened Vulcan (hence the name for the hot planet that Spock supposedly came from in the Star Trek series). The planet does not exist even though it has been observed on two occassions. The precession an effect of special and general relativity. It is ironic, given the early suposition that Uranus that the orbits errors were due to a flaw in Newtonian physics and the subsequent discovery that they were due to Neptune, that in the case of Mercury, the situation was exactly opposite. Initial speculations centred on a unknown planet and the solution involving a failure of Newtonian physics.

Around some of the planets there are moons which orbit the planet.

Dwarf Planets

At the time of writting, the IAU has officially reconises three Dwarf planets however there are other objects which may be designated as dwarf planets.

Ceres - located in the main asteroid belt was discovered in Jan 1st, 1801 by Giuseppe Piazzi. It is the smallest of the dwarf planets.

Pluto is not now considered to be a planet but a dwarf planet

2003 UB313 formerly known as Xena is now officially named Eris - the largest known dwarf planet

Sedna - the coldest most distant place known in the solar system; possibly the first object in the long-hypothesized Oort cloud.

Types of Capacitors

Types of Capacitors

The below is the Types of Capacitors

Electrolytic Capacitors

Aluminum electrolytic capacitors are made by layering the electrolytic paper between the anode and cathode foils, and then coiling the result. The process of preparing an electrode facing the etched anode foil surface is extremely difficult. Therefore, the opposing electrode is created by filling the structure with an electrolyte. Due to this process, the electrolyte essentially functions as the cathode.

Electrolytic capacitors are soaked in a liquid or paper impregnated with a liquid which is not a dielectric but when a voltage is applied creates a layer of aluminium oxide which acts a the dielectric. The reaction is dependent on the polarity of the applied voltage. If the polarity is reversed the capacitor will produce a gas and is likely to explode or burst because of the pressure inside and so are not suitable for alternating current applications. 

MEMs Capacitors

Micro Electro-Mechanical System(MEMs) are small devices manufactured from Silicon. Plate capacitors can be fabricated which show small changes in capacitance a the separation of the plates is increased or decreased. The small devices can be used as sensors and gyroscopes

Common types of devices are the parallel plate capacitors for position sensing. Also, interpenetrating comb-like structures, in which the capacitance may be altered using by moving one comb relative to another, either in the transverse direction or longitudinal direction. Because of their small size, the variation in capacitance is very small, of the order 10-15 F. (femto-Farads).

Tantalum Capacitors

Tantalum capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small so they are used where a large capacitance is needed in a small size such as mobile phones or laptop computers. These capacitors have increasingly become an important as the demand for ever smaller electronic gadgets has grown. Columbite-tantalite - coltan, for short, the ore from which tantalum is refined is mined in Australia, Egpyt. The high demand for the ore has also financed civil wars in the Democratic Republic of the Congo. A UN security council report charged that a great deal of the ore is mined illegally and smuggled over the country's eastern borders by militants from neighboring Uganda, Burundi and Rwanda providing the revenue to finance the military occupation of the Congo. 

Super capacitors

Super capacitors are capacitors which have the ability to store large amounts of charge, and therefore energy, in a very small volume. Energy storage is by means of static charge rather than of an electro-chemical process that is inherent to the battery. Applying a voltage differential on the positive and negative plates charges the super capacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. The super capacitor was first conceived in 1957 but now research is focused on using these as a light weight power sources as an alternative for batteries. the super capacitor crosses into battery technology by using special electrodes and some electrolyte. Super capacitors could find applications such as temporary back-up power supplies in the electrical power grid or providing the initial burst of energy to get electric cars moving. 

Sunday, June 6, 2010

Thermodynamic Potentials

Thermodynamic Potentials

Thermodynamic potentials are used to measure the energy of a system in terms of different variables because often we can only measure certain properties of the system. For example we might know the pressure and temperature of system but not the volume or the entropy. Thermodynamic potentials allow us to measure more state variables of the system.

The thermodynamics potentials are the  internal energy,enthalpy, Gibbs Free Energy and the Helmholtz Free Energy.

These different potentials can be remembered by writing them in the form of a grid.

From the first law of thermodynamics, the internal energy, is given by

dU = dQ + dW

dU = T dS + P dV

From H = U + P V, then dH = dU + dP V + PdV (using the chain rule).

The first column of the first row contains just the internal energy U, to obtain the enthalpy, we must add P V. In the second row, the Helmholtz Free F energy is the internal energy, U minus T S, finally in the second column of the second row, the Gibbs Free Energy G we must add P V and take away T S.

We wish to have a number of expressions for the energy of a system in terms of different variables, for we shall sometimes know the pressure and temperature of a system, but not the volume or entropy; or we might know the volume and temperature but not the pressure and entropy. In most cases, we shall know only a few relevant quantities and will wish to find out many more. This leads to the following definitions two or more thermodynamic quantities:


H = U + P V

F = U - T S

G = U + P V - T S

Using Thermodynamic Potentials

From the first and second laws of thermodynamics, dU and dQ are not obviously measurable. Therefore we need thermodynamic manipulation to get useful properties from what is measurable, i.e., P, V and T. Thermodynamics quantities are more commonly defined in terms of partial differentials. For example:

Example of Thermodynamic Potentials

Good examples of thermodynamic potentials include electrolysis or

They are thermodynamic potentials because each can be found as a result of differentiating.

Heat Engines

Heat Engines

A heat engine is any machine which converts heat into useful work for example, a steam engine or a car engine. Real heat engines are complex and there are many ways of converting heat energy into useful work. We can abstract and generalise the workings of any heat engine into three parts:

  • The Hot Resevoir - heat energy is created by some process such as combustion of a fuel to provide the heat energy.
  • The working body - converts the heat energy into work. In real heat engines, the conversion process is never 100% efficient, so the work output is always less than the heat energy supplied. However we frequently idealise and assume reversibility.
  • The cold resevoir - the energy that cannot be turned into work is dumped and goes to heat up the cold resevoir. In practice, the cold resevoir is usually the atmosphere. We also assume that the temperature of the cold resevoir does not increase, it has an infinite heat capacity.

Schematic diagram of a heat engine.

Assume that a heat engine starts with a certain internal energy U, intakes ΔQi heat from a heat source at temperature Ti , does work ΔW , and exhausts heat ΔQf into a the cooler heat reservoir with temperature Tf. With a typical heat engine, we only want to use the heat intake, not the internal energy of the engine, to do work, so ΔU=0. The first law of thermodynamics tells us:

ΔU=0 = ΔQi - ΔQf - ΔW

To determine how effectively an engine turns heat into work, we define the efficiency, η, as the ratio of work done to heat input:

η = ΔWQi = (ΔQiQf)/ΔQi

= 1 - ΔQfQi

Because the engine is doing work, we know that ΔW >0, so we can conclude that ΔQ > 0. Both and are positive, so the efficiency is always between 0 and 1:

Efficiency is usually expressed as a percentage rather than in decimal form. That the efficiency of a heat engine can never be 100% is a consequence of the Second Law of Thermodynamics. If there were a 100% efficient machine, it would be possible to create perpetual motion: a machine could do work upon itself without ever slowing down.