Magnetism Flashcards
Magnetism
• A force exerted by magnets when they attract or repel each other
• A fundamental force where materials either attract or repel each other due to the movement of electric charges within
• It is a property of matter within a material that results from orbiting electrons in atoms
• The orniting electrons generate magnetic fields as they orbit the nucleus – the key to this lies in the electron spin – the orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic momentum called spin
• Because of its spin – each electron acts like a tiny magnet creating a magnetic dipole -magnet with 2 poles (closed circulation of an electric current system. A simple example is a single loop of wire with constant current through it. A bar magnet is an example of a magnet with a permanent magnetic dipole moment
• In most materials these spin cancel each other out – but in some like iron – many of the spins align creating a net magnetic field
• In mri the concept of magnetism is used to our advantage as we use the magnetic properties of hydrogen atoms in the body and we exploit it
• When the hydrogen atoms are placed in the magnetic field they align with the field and can be manipulated by RF pulses and an image is created.
Magnetic poles
• We use the concept of magnetism as each small electron is a magnet. Each magnet will have a magnetic pole-a north and south pole
• The magnetic pole is the region where the external magnetic field is the strongest at each end of the magnet
• The bar magnet is an easy device to visualize the magnetic poles
• The two ends of a permanent magnet are called the poles of a magnet
• Therfore a magnetic dipole refers to a magnetic north pole and a magnetic south pole separated by a small distance
Magnetic field
An invisible area around each magnet where its magnetic power can be felt
Magnetic field lines
• Lines drawn by scientists to show where the magnetic force goes
• They go out from the magnet’s north pole and to into its south pole
• Only present for permanent magnets
• Will only be present when an electromagnetic magnet is on
Types of magnetic materials
- Ferromagnetic
- Paramagnetic
- Diamagnetic
- Super paramagnetic
Ferromagnetic materials
• Strongly attracted by magnetic field and can remain magnetised outside the external magnetic field
• When placed in a magnetic field, the strength is much stronger inside than outside the material
• Example – iron (FE), nickel (NI) and cobalt(Co), gadolinium and steel (contains iron)
• Can be hazardous within that environment
• The primary hazard related to the main magnetic field is the “missile effect” -the materials become projectiles
- when placed in a magnetic field the field strength is much stronger inside the material outside
Paramagnetic materials
• Weakly attracted by magnetic fields – paramagnetic materials do not retain magnetisation in the absence of an external magnetic field – unlike ferromagnetic objects
• Therefore paramagnetic materials exhibit a form of magnetism that attracts them by an external magnetic field.
• eg aliminium,platinum, air,hydrogen,magnesium,titanium, CoCrMo (cobalt, chrome, molybdenum) alloy
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Diamagnetic material
• Weakly repelled by magnetic fields
• Example – copper, gold, silver,water,superconductors,cortical bomne, de-oxygenated rbc,water and soft tissues
-generate an opposing magnetic field when exposed to an external magnetic field
Super paramagnetic materials
• Consist of individual domains of elements that have ferromagnetic properties in bulk
• Their magnetic susceptibility is between that of ferromagnetic and paramagnetic
• Example- ferumoxytol-feraheme- a class of MR contrast agents composed of nanoparticles of supermagnetic iron oxides (spios) coated in carbohydrates
• Contain stronger magnetic fields than paramagnetic materials ex hemosiderin and ferritin
Magnetic domains
• A region in which the magnetic fields of atoms are grouped together and aligned
• This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction
• A material can be a strong magnet if its magnetic domains align
How are magnetic domains stable?
• In ferromagnematerial, the magnetic moments (tiny magnetic fields) of atoms tend to align and create a large overall magnetic field
• However, if all the atoms in the material aligned in the same direction, the material would have a high amount of magnetistatic energy (the energy stored due it’s magnetic field)
• To lower this energy, the material splits into smaller regions called magnetic domains – where the magnetic moments are aligned in different directions
• This division helps reduce the overall energy of the material making it more stable
Paramagnetic materials- domains
• In paramagnetic materials the individual magnetic moments of the atoms do not interact with each other and hence do not naturally align into domains as the case with ferromagnetic materials i.e. they are completely random
• Once subjected to a magnetic field the magnetic moments align accordingly
• Paramagnetic materials such as gadolinium will increase nuclear spin relaxation rates and therefore we can use it as a contrast based product
Diamagnetic materials- domains
• In paramagnetic materials the individual magnetic moments of the atoms do not interact with each other and hence do not naturally align into domains as the case with ferromagnetic materials i.e. they are completely random
• Once subjected to a magnetic field the magnetic moments align accordingly
• Paramagnetic materials such as gadolinium will increase nuclear spin relaxation rates and therefore we can use it as a contrast based product
Summary-materials
• Diamagnetic- 0 magnetic moments but when subject to a magnetic field they will align but in an opposite direction to the magnetic field
• Paramagnetic – have a net magnetic moment but will be randomly aligned -when subjected to a magnetic field they will align with that field
• Ferromagnetic- net magnetic moment in a domain but still randomised. Once subjected to a magnetic field – those domains that will have moments parallel to the field will grow in size whil the others which are not parallel to the field will align and roatet to be aligned with that magnetic field – strong magnetisation in that direction
Magnetic field strength and units
• Magnetic field strengths are measured in Tesla – a derived Sinunit of magnetic flux density
• In an MRI, the magnetic field is stronger than the earths magnetic field
• A hugh strength is required in order to align the hydrogen atoms in our body
