PHYSICS UNIT 2

Question 1

Electric current, I

Answer 1

This is the rate of flow of electric charge. I = ∆Q/∆t
Unit: A

Question 2

Efficiency of a system

Answer 2

% Efficiency = 100× useful work (or energy) out/
work (or energy) put in

Question 3

Potential difference (pd), V

Answer 3

The pd between two points is the energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other.

Unit: V [=JC-1]

Question 4

Ohm’s law

Answer 4

The current across a component at constant temperature is directly proportional to the pd across it.

Question 5

Electrical resistance, R

Answer 5

The resistance of a conductor is the pd (V) placed across it divided by the resulting current (I) through it.
R = V/I

Unit: Ω [= VA-1]

Question 6

Resistivity, ρ

Answer 6

The resistance, R, of a metal wire of length L and cross sectional area A is given by R = ρL/A , in which ρ the resistivity, is a constant (at constant temperature) for the material of the wire.

Unit: Ω m

Question 7

Superconducting transition temperature, Tc

Answer 7

The temperature at which a material, when cooled, loses all its electrical resistance, and becomes superconducting.

Question 8

The law of conservation of charge

Answer 8

Electric charge cannot be created or destroyed, (though
positive and negative charges can neutralise each other).
Charge cannot pile up at a point in a circuit.

Question 9

Emf, E

Answer 9

The emf of a source is the energy converted from some other form (e.g. chemical) to electrical potential energy per coulomb of charge flowing through the source.

Unit: V

Question 10

Progressive wave

Answer 10

A pattern of disturbances travelling through a medium and
carrying energy with it, involving the particles of the medium oscillating about their equilibrium positions.

Question 11

Transverse wave

Answer 11

A transverse wave is one where the particle oscillations are at right angles to the direction of travel (or propagation) of the wave.

Question 12

Longitudinal wave

Answer 12

A longitudinal wave is one where the particle oscillations are parallel to the direction of travel (or propagation) of the wave

Question 13

Polarised wave

Answer 13

A polarised wave is a transverse wave in which particle
oscillations occur in only one of the directions at right angles to the direction of wave propagation.

Question 14

Waves in phase

Answer 14

Waves arriving at a point are said to be in phase if they have the same frequency and are at the same point in their cycles at the same time

Question 15

Wave sources in phase

Answer 15

Wave sources are in phase if the waves have the same
frequency and are at the same point in their cycles at the
same time, as they leave the sources.

Question 16

Wavelength of a progressive wave

Answer 16

The wavelength of a progressive wave is the minimum
distance (measured along the direction of propagation)
between two points on the wave oscillating in phase.

Question 17

Frequency of a wave

Answer 17

The frequency of a wave is the number of cycles of a wave
that pass a given point in one second. [or equivalently the
number of cycles of oscillation per second performed by any particle in the medium through which the wave is passing.]

Question 18

Speed of a wave

Answer 18

The speed of a wave is the distance that the wave profile
moves per unit time.

Question 19

Diffraction

Answer 19

Diffraction is the spreading out of waves when they meet
obstacles, such as the edges of a slit. Some of the wave’s
energy travels into the geometrical shadows of the obstacles.

Question 20

The principle of superposition

Answer 20

The principle of superposition states that if waves from two sources occupy the same region then the total displacement at any one point is the vector sum of their individual displacements at that point.

Question 21

Phase difference

Answer 21

Phase difference is the difference in position of 2 points within a cycle of oscillation. It is given as a fraction of the cycle or as an angle, where one whole cycle is 2π or 360°, together with a statement of which point is ahead in the cycle.

Question 22

Coherence

Answer 22

Waves or wave sources, which have a constant phase
difference between them (and therefore must have the same frequency) are said to be coherent.

Question 23

Stationary (or standing) wave

Answer 23

A stationary wave is a pattern of disturbances in a medium, in which energy is not propagated. The amplitude of particle oscillations is zero at equally-spaced nodes, rising to maxima at antinodes, midway between the nodes.

Question 24

Refractive index, n

Answer 24

For light, Snell’s law may be written: n1 sin θ1 = n2 sin θ2
in which θ1 and θ2 are angles to the normal for light passing between medium 1 and medium 2; n1 and n2 are called the refractive indices of medium 1 and medium 2 respectively.

Question 25

Snell’s law

Answer 25

At the boundary between any two given materials, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

Question 26

Critical angle, C

Answer 26

When light approaches the boundary between two media from the 'slower' medium, the critical angle is the largest angle of incidence for which refraction can occur. The refracted wave is then travelling at 90° to the normal.

Question 27

Photoelectric effect

Answer 27

When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.

Question 28

Work function, φ

Answer 28

The work function of a surface is the minimum energy needed to remove an electron from the surface. Unit: J or eV

Question 29

Electron volt (eV)

Answer 29

This is the energy transferred when an electron moves between two points with a potential difference of 1 V between them. 1 eV = 1.60 × 10-19 J

Question 30

Ionisation

Answer 30

The removal of one or more electrons from an atom.

Question 31

Ionisation energy

Answer 31

The ionization energy of an atom is the minimum energy needed to remove an electron from the atom in its ground state. Unit: J

Question 32

Stimulated emission

Answer 32

This is the emission of a photon from an excited atom, triggered by a passing photon of energy equal to the energy gap between the excited state and a state of lower energy in the atom. The emitted photon has the same frequency, phase, direction of travel and polarisation direction as the passing photon.

Question 33

Population inversion

Answer 33

A population inversion is a situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state.

Question 34

Pumping

Answer 34

Pumping is feeding energy into the amplifying medium of a laser to produce a population inversion.