Module 4 Definitions and Statements Flashcards
Describe what is meant by the term mean drift velocity of the electrons in the wire
the average displacement/distance travelled of the electrons along the wire per second, they move slowly in one direction
through the metal/ lattice (when there is a p.d. across
the wire) because they collide constantly within the lattice
State Kirchhoff’s first law
The sum of the currents entering a point / junction is equal to the sum of the currents leaving (the same point)
State Kirchhoff’s second law.
e.m.f.s = sum /total of p.d.s/sum of voltages (in a loop)
Define the kilowatt hour.
Energy (transformed by a device working) at 1 kW for 1 hour
Define electrical resistivity.
ρ = RA/L
symbols defined: A = cross-sectional area, R = resistance, L = length
Use energy considerations to distinguish between potential difference (p.d.) and electromotive force (e.m.f.).
p. d.: energy transferred per unit charge from electrical form (into other forms, e.g. light/heat)
e. m.f.: energy transferred per unit charge into electrical form (from other forms, e.g. chemical/mechanical)
State Ohm’s law in words.
Current is (directly) proportional to potential difference (for a metal conductor) provided the temperature \ (all) physical condition(s) remains constant
Define resistance.
p.d./voltage (across component) divided by current
State the difference between the directions of conventional current and electron flow.
current moves from + to – (of battery in circuit) and electrons move from – to +
Define potential difference.
energy per unit charge transferred from electrical to other forms
Explain what is meant by internal resistance.
(some) energy is transferred into thermal energy /lost as heat
in (driving charge through) the battery. It behaves as if it has an
(internal) resistance
Explain the term e.m.f
energy transferred from source/changed from some form
to electrical energy per unit charge
Define the term intensity.
intensity is the (incident) energy per unit area per second
Define the terms wavelength, frequency and speed used to describe a progressive wave
λ distance between (neighbouring) identical points/points with same
phase (on the wave)
f number of waves passing a point /cycles/vibrations (at a point) per unit time/second
v distance travelled by the wave (energy) per unit time/second
Explain what is meant by infra-red radiation.
infra red is part of the e-m spectrum
lower f or longer λ than the visible region/light
Explain what is meant by the principle of superposition of two waves
when two waves meet/overlap/interfere/collide/superpose the resultant displacement is the sum of the displacements
State the meaning of amplitude
the maximum displacement from equilibrium or rest position
State the meaning of phase difference
fraction of a cycle between the oscillations at the two points
explain the meaning of interference
when two (or more) waves meet/superpose/overlap (at a point) there is a change in overall displacement
Explain the meaning of coherent
constant phase difference/relationship (between the waves)
State two properties which distinguish electromagnetic waves from other transverse waves
all travel at speed of light through a vacuum
are oscillating E and B fields or are caused by accelerating
charges
Describe what is meant by a plane polarised wave.
oscillations (of particles/e-m fields along the wave) are in one
direction only
perpendicular to the direction of wave propagation/of travel of the
wave/of energy transfer
Describe how a stationary wave is different from a progressive wave.
progressive a wave which transfers energy
stationary a wave which traps/stores energy (in pockets)
OR
progressive : transfers shape/information from one place to
another
stationary where the shape does not move along/which has
nodes and antinodes
Explain what is meant by a line spectrum.
light emitted from (excited isolated) atoms produces a line spectrum a series of (sharp/bright/coloured) lines against a dark background
When used to describe stationary (standing) waves explain the terms antinode and node.
node occurs where the amplitude/displacement is (always) zero
antinode occurs where the amplitude (of the standing wave) takes the maximum (possible) value
State the principle of superposition of waves
when two(or more) waves meet/cross/interact (at a point) the (resultant) displacement is the (vector) sum of the (individual) displacements
Explain what is meant by a progressive wave
a transfer of energy as a result of oscillations (of the source/medium/particles through which energy is travelling)
Explain what is meant by diffraction of a wave.
wavefronts/paths spread out after passing through a gap or around an obstacle
State the condition necessary for electrons to produce observable diffraction when passing through matter, e.g. a thin sheet of graphite in an evacuated chamber.
wavelength of electrons must be comparable/of the order of magnitude of the atomic spacing
Explain what is meant by the work function energy of the metal.
(Minimum ) energy needed to free an electron /an electron to escape (from the metal surface)
Explain what is meant by the de Broglie wavelength of an electron.
electron wavelength depends on its speed/momentum
Define the electronvolt. State its value in joule
an eV is the energy acquired by an electron accelerated/moves through a p.d. of 1 V
1 eV = 1.6 x 10-19 J
Explain what is meant by the de Broglie wavelength of an electron
Electrons are observed to behave as waves/show wavelike properties where the electron wavelength depends on its speed/momentum
Explain what is meant by a photon
a quantum/lump/unit/packet/particle of (e-m) energy/light
Explain what is meant by a continous spectrum
all wavelengths/frequencies are present (in the radiation)
In an experiment it is observed that when blue light is shone on a clean metal surface electrons are emitted, but with red light there is no electron emission. Describe Einstein’s theory to explain these observations.
- Individual photons are absorbed by individual electrons ( in the metal surface)/ one to one interaction
- Only photon with energy above the work function energy will cause photoelectron emission/idea of threshold frequency
- Photon energy is proportional to frequency
- (therefore) blue photons with higher f/shorter wavelength will cause photoemission but red photons will not.
- hf – ϕ = KEmax is the equation resulting from conservation of energy or resulting from the meaning of each term
- A wave model does not explain instantaneous emission
In 1905 Einstein presented a theory to explain the photoelectric effect using the concept of quantisation of radiation proposed by Planck in 1900.
Show, with the aid of a suitably labelled diagram, the arrangement of apparatus that could be used to demonstrate the photoelectric effect. Describe how you would use the apparatus and what would be observed.
A (clean) zinc plate mounted on the cap of a gold-leaf electroscope.
Plate initially charged negatively
A u-v lamp shining on plate
The gold leaf collapses as the charge leaks away from the plate (when ultra-violet light is incident on the zinc plate) so experiment indicates the emission of negative charge/electrons
A simple photocell, eg two plates in a vacuum envelope
A (12 V) dc supply is connected to the photocell and (nano)ammeter.
A suitable frequency/u-v lamp shining on one plate
The presence of u-v /blue light causes a current in the circuit. so experiment indicates the emission of negative charge/electrons
A (potassium) photocell connected across a (high impedance) voltmeter.
Incident light of different frequencies;
produced either by white light source and colour filters of known spectral range or by using a diffraction grating or prism to produce a first order spectrum.
Different p.d.s are set up across the electrodes of the photocell (when the
photocathode is illuminated with light of different frequencies). so experiment indicates the emission of negative charge
A physical quantity is also conserved in the photoelectric effect. Describe and explain the photoelectric effect.
a photon is absorbed by an electron (in a metal surface); causing electron to be emitted (from surface).
Energy is conserved (in the interaction).
Only photons with energy/frequency above the work function energy/threshold frequency will cause emission
(energy of photon) = (work function of metal) + (maximum possible kinetic energy of emitted electron)
work function energy is the minimum energy to release an electron from the surface
Number of electrons emitted also depends on light intensity
Emission is instantaneous
Explain what is meant by a line spectrum
light emitted from (excited isolated) atoms produces a line spectrum
a series of (sharp/bright/coloured) lines against a dark background
Describe how an absorption line spectrum differs from an emission line spectrum
in an absorption spectrum a series of dark lines (appears against a bright background/within a continuous spectrum)
State what is meant by the photoelectric effect.
emission of electron(s) from a metal (surface) when photon(s)/ light/uv/em radiation are incident (on surface)
