PH5 definitions

Question 1

Capacitor

Answer 1

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.

Question 2

Capacitance, C, of a capacitor

Answer 2

Capacitance = charge on either plate / p.d between between plates

Unit: F (farad) [= C V-1]

Question 3

Dielectric

Answer 3

Insulator between the plates of a capacitor, also serving to make the capacitance larger than if there were just empty space.

Question 4

Magnetic field strength, B

Magnetic flux density

Answer 4

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]

Question 5

Hall voltage

Answer 5

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.

Question 6

Ampère A

Answer 6

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.

Question 7

Magnetic flux Capital Phi

Answer 7

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.

Question 8

Flux linkage N * Captial Phi

Answer 8

If the above coil consists of N turns, the flux linkage is given by N*Captial Phi. Unit: Wb or Wb turn.

Question 9

Faraday’s Law

Answer 9

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]

Question 10

Lens Law

Answer 10

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.

Question 11

Root-mean-square (rms) value

Answer 11

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.”]

Question 12

Alpha Radiation

Answer 12

Fast moving particles, helium nuclei, ejected from certain radioactive nuclei.

Question 13

Beta radiation

Answer 13

Electrons with speeds just less than the speed of light, ejected from certain radioactive nuclei.

Question 14

Gamma radiation

Answer 14

Photons of high energy (high frequency, short wavelength) ejected from radioactive nuclei.

Question 15

XAZ notation

Answer 15

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).

Question 16

Half life of a nuclide T[1/2]

Answer 16

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.

Question 17

Activity A

Answer 17

The rate of decay (number of disintegrations per second) of a sample of radioactive nuclei.
Unit: Becquerel (Bq) = s-1.

Question 18

Decay constant lambda

Answer 18

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

Question 19

Radio-isotopes

Answer 19

Isotopes (of an element) have the same atomic number Z but different mass number A; radio-isotopes are simply isotopes which are radioactive.

Question 20

Unified atomic mass unit u

Answer 20

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)

Question 21

Electron volt (eV).

Answer 21

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.

Question 22

Binding energy of a nucleus.

Answer 22

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]

Question 23

Conservation of mass-energy

Answer 23

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.

Question 24

Nuclear Fission

Answer 24

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.

Question 25

Chain reaction

Answer 25

A chain reaction is repeated events of nuclear fission in a sample of fissile material, initiated by neutrons released in previous fissions.