PH2 2015

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 x usefelwork / work 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 [= J C-1]

Question 4

Ohm’s law

Answer 4

The current in a metal wire at constant temperature is

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
V= IR
Unit: Ω [= V A-1]

Question 6

Resistivity, ρ

Answer 6

The resistance, R, of a metal wire of length L and crosssectional
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 super-conducting. Some materials (e.g. copper) never become superconducting
however low the temperature becomes

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 in
line with (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

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.
[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 15

Wavelength of a

progressive wave

Answer 15

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 16

Frequency of a

wave

Answer 16

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 17

Speed of a wave

Answer 17

The speed of a wave is the distance that the wave profile

moves per unit time

Question 18

Diffraction

Answer 18

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 19

The principle of

superposition

Answer 19

The principle of superposition states that if waves from two
sources [or travelling by different routes from the same source]
occupy the same region then the total displacement at any
one point is the vector sum of their individual displacements at
that point.

Question 20

Phase difference

Answer 20

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 21

Coherence

Answer 21

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 22

Stationary (or

standing) wave

Answer 22

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 23

Refractive

index, n

Answer 23

For light, Snell’s law may be written: n1 sin theta1 = n2 sin theta2 in which theta1 and theta2 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.
The refractive index of a vacuum is fixed by convention as exactly 1. For air, n = 1.000

Question 24

Snell’s law

Answer 24

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 25

Critical angle, C

Answer 25

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 26

Photoelectric

effect

Answer 26

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

Question 27

Work function, phi

Answer 27

The work function of a surface is the minimum energy needed

to remove an electron from the surface. Unit: J or eV

Question 28

Electron volt (eV)

Answer 28

This is the energy transferred when an electron moves
between two points with a potential difference of 1V between
them. 1 eV = 1.60 x 10-19 J
So for an electron being accelerated it is the kinetic energy
acquired when accelerated through a pd of 1V.

Question 29

Ionisation

Answer 29

The removal of one or more electrons from an atom.

Question 30

Ionisation energy

Answer 30

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 31

Stimulated

emission

Answer 31

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 32

Population

inversion

Answer 32

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 (i.e. a less excited state or the ground
state) of the same system.

Question 33

Pumping

Answer 33

Pumping is feeding energy into the amplifying medium of a

laser to produce a population inversion.