Modern Physics - Definitions - Level 3

Question 1

Ionisation

Answer 1

Ionisation is the ability of nuclear radiation to take an electron off an atom, making it an ion.

Question 2

Rest mass

Answer 2

The mass of a particle is a variable which increases with speed. The rest mass is the mass of the particle when it is stationary.

Question 3

Radioactivity

Answer 3

Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Radiation is emitted by nuclear isotopes which are unstable.

Question 4

Alpha emission

Answer 4

The release of an alpha particle (2+ helium ion or helium nucleus) from an unstable nucleus

Question 5

Electron volt

Answer 5

An electron volt is the amount of energy required to move one electron (charge in joules) between two points with the voltage difference of 1 volt.

(1 eV = 1.6 x 10^-19 J)

Question 6

Cutoff voltage (Vo)

Answer 6

The cutoff voltage is the voltage applied against the voltage in the plate to stop electrons with maximum velocity from ejecting from the metal. The photocurrent becomes zero with the cut-off voltage.

(Maximum opposing voltage that pushes the electrons back to the cathode with maximum kinetic energy and velocity).

Question 7

Beta emission

Answer 7

The release of high-energy electrons as ionizing radiation (causes a neutron to convert into a proton) from an unstable nucleus

Question 8

Nuclear fusion reaction

Answer 8

Nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high speed, and join to form a new type of atomic nucleus.

• During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy).

Question 9

Half-life

Answer 9

The half-life of a radioactive substance is the time after which exactly one-half of the originally active nuclei remain, being a measure of how quickly radioactive decay (disintegration) takes place.

Question 10

Binding (Ionisation) energy

Answer 10

Binding energy is the energy required to completely remove an electron from an atom.

Question 11

Nuclear binding energy

Answer 11

Nuclear binding energy is the energy required to split the nucleus of an atom into its constituent parts (nucleons - protons and neutrons).

Question 12

Mass defect

Answer 12

The mass defect is the difference between the mass of a nucleus and the sum of the masses of its constituent particles (nucleons).

Question 13

Binding energy per nucleon

Answer 13

The binding energy per nucleon is the total work done pulling the nucleons from a nucleus divided by the number of nucleons. The more work required to do this, the greater the binding energy per nucleon, and the more stable the nucleus of an atom.

Question 14

Absorber

Answer 14

An absorber is a material that prevents radioactive emissions from passing through it.

Question 15

Background radiation

Answer 15

Background radiation is the nuclear radiation that arises naturally from cosmic rays and radioactive isotopes in the soil and air.

Question 16

Cathode rays

Answer 16

Negatively charged particles (electrons) are emitted from a negative terminal in an evacuated glass tube.

Question 17

Continuous spectra

Answer 17

The continuous spectrum is the separation of the electromagnetic spectrum into its discrete frequencies with no gaps (missing frequencies).

Question 18

Controlled nuclear fission

Answer 18

Involves the slow and useful release of energy in a nuclear reactor.

Question 19

Critical mass

Answer 19

Mass of fissionable material needed to sustain a chain reaction.

Question 20

Electromagnetic spectrum

Answer 20

A family of waves (eg. light), that do not require a medium, are transverse, and at travel at 3 x 10^8 ms^-1 in a vacuum.

Question 21

Energy absorption

Answer 21

The amount of energy taken in by an electron when excited.

Question 22

Isotopes

Answer 22

Atoms of an element with the same chemical properties (same number of protons), but with different masses (number of neutrons).

Question 23

Nuclear force

Answer 23

The nuclear force is the force of attraction between subatomic particles which bind the nucleus together.

Question 24

Nuclear reactor

Answer 24

A steel vessel in which a controlled chain reaction of fissionable materials releases energy.

Question 25

Quanta

Answer 25

The fixed amounts of energy absorbed or emitted by matter.

Question 26

Spectra

Answer 26

The separation of the electromagnetic spectrum into its discrete frequencies (eg. ROYGBIV for visible light).

Question 27

Threshold frequency (fo)

Answer 27

The threshold frequency is the minimum frequency of photons (incident radiation) that would cause electrons to be emitted from a metal surface.

(Just free the electrons from their atoms and hold them on the surface of the metal. Any frequency above the threshold will cause electrons to be emitted out from the metal).

Question 28

Work function of a metal

Answer 28

The work function is the minimum energy required to just eject electrons from the surface of a metal (during the photoelectric effect). The energy of the work function is converted to heat which increases the temperature of the metal.

Question 29

Photoelectric Effect

Answer 29

The photoelectric effect is the emitting of electrons from a metal when it is struck by a light beam (radiation), and the frequency of the incident radiation is larger than the threshold frequency (fo) of the metal. Meaning, that the energy of the radiation is greater than the work function of the metal, and there is sufficient energy in each photon for electrons to escape the attraction of their atoms.

Question 30

Photon

Answer 30

A photon is a packet of electromagnetic radiation; that is a monochromatic (single frequency or wavelength) quantum of electromagnetic energy.

Question 31

Nuclear fission reaction (chain reaction)

Answer 31

Nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei).

• The fission process often produces free neutrons and photons in the form of gamma rays, and releases a very large amount of energy.

Question 32

Wave-particle duality

Answer 32

Wave-particle duality is the concept that every particle may be described as either a particle or a wave. Different models of light explain the behaviour of light in different situations.

• When light passes through a double slit an interference pattern is produced, showing wave behaviour.
• Whereas, when light interacts with matter it behaves like a particle.

Question 33

Excitation energy

Answer 33

Excitation energy is the energy required to
excite an atom from ground state (or lower
state) to a higher state.