General Science

PHYSICS: MAGNETISM

Overview

• The term magnetism describes how materials respond to an applied magnetic field
• All materials are influenced to a greater or lesser extent by the presence of a magnetic field. Some are attracted (paramagnetism) while some are repulsed (diamagnetism)
  
• Substances that are negligibly attracted by magnetic fields are called non-magnetic materials. Eg: copper, aluminium, water, glass
• The magnetic state of a material depends on its temperature, with the result that a substance may exhibit different magnetic characteristics depending on its temperature
• Magnetism can arise from either intrinsic magnetic moments contained in particles, or by electric currents applied to the substance
• Magnet is a material that produces a magnetic field
• Permanent magnet is a material that retain its magnetic field

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Types of magnetism

• Diamagnetism
  • Diamagnetism is the tendency of a material to oppose a magnetic field
  • It appears in all materials. However, in a material with paramagnetic properties, the paramagnetic behaviour dominates
    
  • Diamagnetic materials do not have unpaired electrons
  • Superconductors are diamagnetic materials
• Paramagnetism
  • Paramagnetism is the tendency of a material to be attracted to an applied magnetic field
  • Paramagnetism only occurs in the presence of an externally applied magnetic field. When the external field is removed, the magnetisation will drop to zero
    
  • Paramagnetic materials have one unpaired electron, allowing it to orient in the direction of the magnetic field
    
  • Oxygen, myoglobin are examples of paramagnets
• Ferromagnetism
  • Ferromagnetism is the only type of magnetism that can produce forces strong enough to be felt, and is responsible for the magnetic phenomena in everyday life
  • Ferromagnetic materials have unpaired electron, but unlike paramagnets, they remain oriented even after the external magnetic field has been removed
  • Ferromagnetic materials remain magnetized even after the external applied magnetic field has been removed
  • All permanent magnets are either ferromagnets or ferrimagnets
  • Eg: refrigerator magnets
• Antiferromagnetism
  • Magnetic moments of electrons point in opposite directions
  • Anitferromagnets have zero net magnetic field
  • They are not very common and usually occur only low temperatures
    
  • Antiferromagnetism disappears above the Neel Temperature and the material becomes paramagnetic
  • Examples include hematite, chromium, iron manganese
    
• Ferrimagnetism
  • Neighbouring pairs of electrons point in opposite direction
  • However, ferromagnetic materials retain their magnetisation in the absence of the magnetic field
  • Example is magnetite

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Electromagnets

• Electromagnet is a magnet whose magnetic field is produced by the flow of electric current
• The magnetic field disappears when the current ceases
• The electromagnet was invented by William Sturgeon (Britain) in 1824
• Electromagnets are widely used in electrical devices such as motors, generators, loudspeakers, particle accelerators
• Magnetic Levitation (MAGLEV) trains run on electromagnetic suspension produced by electromagnets

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Earth’s magnetic field

• The Earth’s magnetic field, which extends several tens of thousands of km into space is called the magnetosphere
• The earth’s magnetic field is explained by dynamo theory. The theory explains the mechanism by which celestial bodies like the earth, or a star generate magnetic fields. According to the theory, earth’s magnetic field is produced by electric currents in the liquid outer core
  
• The magnetic north pole of the Earth is located near the geographic south pole, and the magnetic south pole is located near the geographic north pole. This can be explained by understanding that the north pole of a suspended magnet points towards the north, indicating that the geographic north pole should have south polarity
  
• The earth’s magnetic poles move with time due to magnetic changes in the earth’s core. Currently, the magnetic north pole lies near Ellesmore Island in northern Canada, while the south pole is near Wilkes Land, Antarctica. The north pole is moving northwest by about 64 km/year and the south pole is moving northwest by 10-15 km/year
  

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CHEMISTRY: PROPELLANTS

Overview

• A propellant is a material that is used to propel an object
• The object is usually expelled by the pressure created by a gas
• This pressure may be created by a compressed gas or by a gas produced by a chemical reaction
• Propellants may be solids, liquids, gases or plasmas
• Common chemical propellants consist of a fuel and an oxidiser

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Types of propellants

• Aerosol sprays
  • Aerosol spray is a dispensing system that creates an aerosol (fine) mist of liquid particles
  • In aerosol sprays, the propellant is simply a pressurised gas in equilibrium with its liquid form
  • As some gas escapes to expel the payload, more liquid evaporates thereby maintaining an even pressure
  • The aerosol spray can was invented by Erik Rotheim (Norway) in 1927
  • Aerosol sprays are typically used to dispense insecticides, deodorants and paints
• Propellants used for propulsion
  • Rockets typically use bipropellants, which contain a combination of a fuel and an oxidiser. Tripropellants, which are not used commonly, use liquid hydrogen as a third component to provide additional efficiency
  • Propellants are usually made from low explosives, which deflagrate (burn) rather than detonate (explode)
  • The controlled burning of the propellants produces thrust by gas pressure which is then used to accelerate a rocket, projectile or other vehicles
  • Propellants are commonly used in rockets, firearms and artillery

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Solid propellants

• Solid propellants are used for rockets, firearms and artillery
  
• Examples of solid propellants include gunpowder (sulphur + charcoal + potassium nitrate), nitrocellulose and cordite 
  
• Single based propellants: They have nitrocellulose as its chief ingredient. Stabilizers and other chemicals may be added for chemical stability
• Double based propellants: they contain nitrocellulose with nitroglycerin or other liquid nitrate explosives added. Nitroglycerin reduces smoke and increases energy output. Used in small arms, cannons, mortars and rockets
• Triple based propellants: consist of nitrocellulose, nitroquanidine, and nitroglycerin or other nitrate explosives. Used in cannons
• Composite propellants: consist of a fuel such as metallic aluminium, a binder such as synthetic rubber and an oxidiser such as ammonium perchlorate. Used in large rocket motors such as spacecraft
• Solid propellants have been used since the 11^th century to power rockets based on gunpowder
• Solid fuel rockets offer ease of handling, reliability and long storage periods
• Solid fuel rockets are used for missiles due to their long storage periods and reliability of launch on short notice
• Currently, solid fuel rockets are not used for space explorations, but are commonly used as booster rockets to launch spacecraft

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Liquid propellants

• Liquid propellants are usually used in combinations of fuel and oxidiser
• Common liquid propellant combinations include
  • Liquid oxygen and liquid hydrogen
  • Liquid oxygen and kerosene
  • Nitrogen tetraoxide and kerosene
• Liquid fuel rockets are desirable because they offer higher energy output, they can be throttled and shut down and can be reused
• Liquid fuel rockets are used to power space shuttles
• A variant of liquid fuel engine is cryogenic fuel engine – these are engines that use gases which are super-cooled into their liquid forms

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Propellants used in the PSLV

• The Polar Satellite Launch Vehicle (PSLV) has a four stage propulsion system, using solid and liquid propellants alternately
• First stage: solid – Hydroxyl terminated polybutadiene (HTPB)
• Second stage: liquid – unsymmetrical di-methyl hydrazine (UDMH) as fuel and nitrogen tetraoxide as oxidiser
• Third stage: solid – HTPB
• Fourth stage: solid – mono methyl hydrazine as fuel and mixed oxides of nitrogen as oxidiser

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Propellants used in the GSLV

• The Geosynchronous Satellite Launch Vehicle (GSLV) is a three stage launch vehicle using solid, liquid and cryogenic propellants
• First stage – solid – HTPB
• Second stage – liquid – UDMH as fuel and nitrogen tetraoxide as oxidiser
• Third stage – cryogenic – liquid hydrogen and liquid oxygen

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BIOLOGY: STEM CELLS

Overview

• Stem cells are cells that can renew themselves.
• Stem cells renew themselves through mitotic cell division and can differentiate into a diverse range of specialised cell types
  
• Stem cells are found in most multi-cellular organisms
  
• There are two types of stem cells in mammals
  
  • Embryonic stem cells
    
  • Adult stem cells
    
• Stem cells are mainly found in blood from the umbilical cord and the bone marrow
• Due to their self-renewing nature, stem cells are very important for treatment of diseases
  

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Importance of stem cells

• For a cell to be characterised as a stem cell, it must exhibit the following properties
  
  • Self renewal: the ability to go through numerous cycles of cell division while maintaining the undifferentiated state
    
  • Potency: the capacity to differentiate into specialised cell types
    
• In developing embryos, stem cells can differentiate into all of the specialised embryonic tissues
• In adult organisms, stem cells act as a repair system for the body, replenishing specialised cells
• Stem cells also maintain the normal turnover of regenerative organs such as blood, skin or tissues
• Stem cells can be grown and transformed into specialised cells of various tissues such as muscles and nerves using cell culture
• Stem cell treatment holds the potential of transforming human medicine, wherein stem cells introduce new cells into damaged tissue in order to treat a disease or injury
• The ability of stem cells to self renew and differentiate offers the potential to replace diseased and damaged tissue without the risk of rejection or side effects

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Current stem cell treatments

• Currently, stem cell treatment is available to treat the side effects of chemotherapy on cancer patients, such as leukaemia or lymphoma
• During chemotherapy most growing cells are killed by cytotoxic agents
• These agents kill not only the leukaemia cells but also healthy haematopoietic stem cells in adjacent bone marrows.
• Using stem cell therapy, healthy bone marrow stem cells are used to reintroduce healthy stem cells to replace those lost in the treatment

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Potential stem cell treatments

• Stem cells can be potentially used to treat a number of serious diseases. These include
  • Brain diseases such as Parkinson’s and Alzheimer’s
  • Cancers
  • Spinal cord injury
  • Heart damage
  • Haematopoiesis (blood cell formation)
  • Baldness, missing teeth
  • Blindness, deafness
  • Diabetes
  • Neural damage
• Almost all these treatments are still in the research stage
• In Jan 2009, the US Food and Drug Administration (FDA) gave clearance to Geron Corporation for the first clinical trials of an embryonic stem cell therapy on humans. The trial will evaluate the efficacy of the drug GRNOPC1 on patients with spinal cord injury
  

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Important milestones in stem cell research

• 1963: Ernest McCullogh (Canada) and James Till (Canada) illustrate the presence of self renewing cells in the bone marrow
• 1968: Bone marrow transplant between two siblings successfully treats Severe Combined Immunodeficiency (SCID)
• 1978: haematopoietic stem cells discovered in human blood
• 1998: James Thomson (USA) derives the first human embryonic stem cell line
• 2001: Scientists at Advanced Cell Technology (USA) clone first early human embryos for the purpose of generating embryonic stem cells
• 2006: Scientists at Newcastle University (England) create first every artificial liver cells using umbilical cord blood cells
• 2008: Robert Lanza and colleagues at ACT create first human embryonic stem cells without destruction of the embryo