PH5 definitions Flashcards
Capacitor
A capacitor is 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
Capacitance = charge on either plate / p.d between between plates
Unit: F (farad) [= C V-1]
Dielectric
Insulator between the plates of a capacitor, also serving to make the capacitance larger than if there were just empty space.
Magnetic field strength, B
Magnetic flux density
This is a vector quantity. Its direction is that in which the North pole of a freely-pivoted magnet points. Its magnitude is defined by B=F/il in which F is the force on a length l of wire carrying a current I, placed perpendicular to the direction of the field. Unit: T (tesla) [= NA-1m-1]
Hall voltage
When a magnetic field, B, is applied to 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.
Ampère A
The ampère is that current which, when flowing through two infinite, thin, parallel wires, one metre apart in vacuum, produces a force between the wires of exactly 2x10^-7N per metre of length. Unit: A.
Magnetic flux Capital Phi
If a single-turn coil of wire encloses an area A, and a magnetic field B makes an angle pheta with the normal to the plane of the coil, the magnetic flux through the coil is given by Captial Phi = AB cos pheta.
Unit: weber (Wb) =Tm2.
Flux linkage N * Captial Phi
If the above coil consists of N turns, the flux linkage is given by N*Captial Phi. Unit: Wb or Wb turn.
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. E= -(N*Captial Phi)/t
[Note: The sign is from Lenz’s Law, see below]
Lens 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., Vrms, is defined as:
sqrt(1/n)(V^2+V[2]^2 +… V[n]^2))
Ims, is defined similarly for current.
[ASteadyp.d.of magnitude Vrms(and a steady current ofIrms) would give the same power dissipation in a resistor as the
alternating p.d. and current.”]
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.
XAZ notation
X is the chemical symbol of the element, A the mass number (number of protons plus number of neutrons) and Z the atomic number (number of protons).
Half life of a nuclide T[1/2]
The time taken for the number of radioactive nuclei N (or the activity A) 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) = s-1.
Decay constant lambda
The constant which appears in the exponential decay law and determines the rate of decay (the greater lambda, the more rapid the rate of decay). lambda is related to half life by lambda = ln2/ T[1/2] Unit: s-1
Radio-isotopes
Isotopes (of an element) have the same atomic number Z but 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 one twelfth of the mass of one atom of carbon 12. Thus one atom of has a mass of exactly 12u.
(1u = 10-3 kg / NA = 1.66 *10-27kg)
Electron volt (eV).
This is the energy transferred when an electron moves between two points with a potential difference of 1 volt between them.
1 eV = 1.6 10-19 J
So for an electron being accelerated it is the K.E. acquired when accelerated through a pd of 1V.
Binding energy of a nucleus.
The energy that has to be supplied in order to dissociate a nucleus into its constituent nucleons. [It is therefore not energy which a nucleus possesses.] Unit: J [or MeV]
Conservation of mass-energy
Energy cannot be lost or gained, 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 (e.g. Uranium-235) can absorb a neutron and will then split into (usually) two smaller, beta-radioactive, nuclei. Two or more neutrons are also released. The ‘fragments’ have large kinetic energies. This is 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.
