All Definitions Needed For A Level

Alternating Current:

The root-mean-square (r.m.s.) value of an alternating current is equivalent to the steady direct current that converts electrical energy to other forms of energy at the same average rate as the alternating current in a given resistance.

Rectification is the process in which an alternating current is forced to only flow in one direction.


Band gap is the energy difference between top of valence band and bottom of conduction band. It is also a range of energy in a solid where no electron states exist.

An intrinsic semiconductor is a pure semiconductor crystal containing only one element or one compound.

Extrinsic semiconductors doped with donor impurities are called n-type semiconductors because they donate an excess of negative charge carriers.

Quantum Physics & Lasers:

Photoelectric Effect is the emission of electrons from metal by electromagnetic radiation.

Photoelectron is used to indicate that the electron has been emitted when light falls on the surface of a metal.

The work function of a material is defined as the minimum amount of the work necessary to remove a free electron from the surface of the material.

Threshold frequency is the minimum frequency of an incident radiation required to just remove an electron from the surface of a metal.

The scanning tunnelling microsope (STM) is a non-optical microscope which uses the concept of quantum tunnelling by electrons to study surfaces of conductors or semi-conducors at the atomic scale of about 2 Å or 0.2 nm.

Nuclear Physics:

The mass defect of a nucleus is defined as the difference between the mass of the separated nucleons and the combined mass of the nucleus.

Nuclear fission is the disintegration of a heavy nucleus into two lighter nuclei of approximately equal masses.

Nuclear fusion is the combining of the two light nuclei to produce a heavier nucleus.

Radioactive decay is the spontaneous disintegration of the nucleus of an atom which results in the emission of particles.

Background radiation refers to ionizing radiation emitted from a variety of natural and artificial radiation sources.

The activity of a radioactive substance is defined as the average number of atoms disintegrating per unit time.

The decay constant λ of a nucleus is defined as its probability of decay per unit time.

Half-life is defined as the time taken for half the original number of radioactive nuclei to decay.

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17 thoughts on “All Definitions Needed For A Level”

  1. The definitions for waves aren’t correct since a lot of them assume that the waves require a medium. The definitions as a whole are correct but a lot wouldn’t get you the marks in an exam.


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