PH5 definitions

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Capacitor	A pair of conducting plates separated by an insulator. If a p.d. is placed across the plates they acquire equal and opposite charges.
Capacitance, C, of a capacitor	(Charge on either plate)/(p.d. between plates) units F(farad)
Dielectric	Insulator between plates of a capacitor, also serving to make the capacitance larger than if there were just empty space
Magnetic field strength B	A vector quantity.. Direction is that in which the North pole of a freely pivoted magnet points. Magnitude defined by F=BIL where F is the force on a length L of wire carrying a current I placed perpendicular to the direction of the field. Units T(tesla)
Hall voltage	When a magnetic field B is applied to a conductor carrying a current I at right angles to the field direction a so called hall voltage appears across the specimen at right angles to the B and I directions
Ampere	The ampere is that current which, when flowing through two infinite parallel wires, one metre apart in vacuum, produces a force between the wires of exactly 2x10^(-7) N per methre of length. Units A
Magnetic flux	If a single turn coil of wire encloses an area A and a magnetic field B makes an angle o with the normal to the plane of the coil, the magnetic flux through the coil is given by ABcos(o). Unit Wb(weber)
Flux linkage	If the above coil consists of N turns the flux linkage is given by N*magnetic flux. Unit Wb or Wbturn
Faraday's Law	When the flux linking an electrical circuit is changing an emf is induced in the circuit of magnitude equal to the rate of change of flux linkage.
Lenz's Law	The direction of any current resulting from an induced emf is such as to oppose the change in flux linkage that is causing the current.
Root mean square (RMS) value	If an alternating voltage is read at regular intervals throughout a cycle, giving the values V1,V2..VN the rms p.d. is defined as Vrms =root(1/n V1^2 + V2^2....+Vn^2)
Alpha radiation	Fast moving particles, helium nuclei, ejected from certain radioactive nuclei
Beta radiation	Electrons with speeds just less than the speed of light, ejected from certain radioactive nuclei
Gamma radiation	Photons of high energy (high frequency, short wavelength) ejected from radioactive nuclei
A/Z X notation	X is the chemical symbol of the element, A is the mass number and Z the atomic number
Half life of a nuclide	The time taken for the number of radioactive nuclei N to reduce to one half of the initial value. Unit s
Activity A	The rate of decay (number of disintegrations per second) of a sample of radioactive nuclei. Unit Becquerel (Bq)
Decay constant	The constant which appears in the exponential decay law N=Noe^(-lamdat) and determines the rate of decay (the greater lamda the more rapid the rate of decay) lamda is related to half-life by (ln2)/half life =lamda. Units /s
Radio-isotopes	Isotopes of an element have the same atomic number Z but a different mass number A; radio isotopes are simply isotopes which are radioactive.
Unified atomic mass unit u	The unified atomic mass unit is defined as exactly 1/12 of the mass of one atom of carbon -12. Thus one atom of carbon 12 has a mass of exactly 12u
Electron-volt	This is the energy transferred when an electron moves between two points with a potential difference of 1 volt between them. 1eV = 1.6*10^(-19)J. So for an electron being accelerated it is the KE acquired when accelerated though a p.d of 1V
Binding energy of a nucleus	The energy that has to be supplied in order to dissociate a nucleus into its constituent nucleons. (Not energy the nucleus possesses) unit J
Conservation of mass-energy	Energy cannot be created or destroyed, only transferred from one form to another. We can measure the energy in a body by multiplying its mass by c^2
Nuclear Fission	Certain nuclei of large mass number can absorb a neutron and will then split into two smaller, beta radioactive, nuclei. Two or more neutrons are also released. The fragments have larger kinetic energies. This is called nuclear fission.
Chain reaction	A chain reaction is repeated events of nuclear fission in a sample of fissile material, initiated by neutrons released in previous fissions
X-rays	High energy e-m radiation with photon energies between 100eV and 100keV
A-scan	Scan in which the strength of reflections of an ultrasound pulse from interfaces in the body is shown by the amplitude of a trace.
B-scan	Scan in which the strength of reflection of an ultrasound pulse from interfaces in the body is shown by the brightness of a trace. An array of transducers can produce a 2D image.
Acoustic impedance	The property of materials which determines the fraction reflected.
Coupling medium	Gel or oil used to exclude air between the skin and the ultrasound transducer. It reduces the mismatch in Z and enables more ultrasound to enter the body instead of being reflected off the skin.
Precession	The spin direction of protons rotate around the magnetic field direction, like a child's top.
Relaxation time	The time taken for the protons to flip back to their lower energy state.
ECG	Produced due to the nervous impulses associated with the heart beat being detected outside the body, a voltage time chart.
Atrium/atria	The top chambers of the heart
Ventricle	The lower chambers of the heart
Sino-atrial node	Sends a signal which triggers the muscles of the heart to contract. Located at the top right of the heart.
Surface potential	Voltage measured on the skin due to nerve signal within the body.
Contraction	Shortening of muscles in response to nerve stimulus.
Absorbed dose	The radiation energy absorbed per kilogram of tissue.
Dose equivalent	Q * absorbed dose in which Q is a factor which depends on the type of radiation and takes account of the biological effect of the radiation.
Quality factor (Q)	20 for alpha radiation and 1 for beta and gamma in equation: dose equivalent = Q *absorbed dose
Radioactive tracer	Chemical compounds with an atom replaced by a radioactive isotope - used to track the uptake of the compound by the body.
Tracer	Used to investigate function of particular organs
Electron capture	A method of decay where an element reacts with an electron to produce an isotope in an excited state which gives off gamma radiation.
Collimator	Device for producing a parallel beam
Scintillation crystal	Flashes of light are given out by certain crystals when high energy particles strike them.
Photomultiplier	An arrangement of electrodes with different voltages applied to them, so that electrons emitted from one electrode by the photoelectric effect are effectively multiplied in number to make a much larger current.
Positron	The antiparticle of an electron, the particle emitted in B+ decay.
Annihilation	When a particle and its antiparticle meet at low speed their mass energies are converted into photons

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	Criado por nicola.bowen mais de 10 anos atrás