G482 EXAM Definintions Flashcards
Kirchhoff’s second law
(sum of) e.m.f.s = sum /total of p.d.s/sum of voltages in/around a (closed) loop (in a circuit)
energy is conserved
Kirchhoff’s first law
sum of/total current into a junction equals the sum of/total current out conservation of charge
Progressive wave
is a transfer of energy
as a result of oscillations (of the source/medium/particles through which energy is travelling)
The principle of superposition of waves
When 2 or more waves meet at a point
The (resultant) displacement is equal to the (vector) sum of the displacements of each wave
The photoelectric effect
emission of electron(s) from a metal (surface) when photon(s)/
light/uv/em radiation are incident (on surface)
Electron volt (eV)
energy to accelerate/move an electron through a p.d. of 1 V
Work function of a metal
the minimum energy required to release an electron from the surface of a metal
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/AW
or there is a voltage drop across/decrease in voltage from the battery when a current is drawn from it/AW
Phase difference
relates to the oscillation of two points on the (same) wave
how far ‘out of step’ one oscillation is from the other/AW
Coherent
constant phase difference/relationship (between the waves)
Power
Work done/energy transfer(red) per unit time
Kilowatt-hour
(a unit of) energy equal to 3.6 MJ or 1 kW for 1 h
de Broglie wavelength of an electron
electron wavelength depends on its speed/momentum
Displacement and amplitude
displacement : (any) distance moved from equilibrium of a point/particle on a wave
amplitude: maximum displacement (caused by wave motion)
Frequency
number of wavelengths passing a point /vibrations at a point per unit time/second or produced by the wave source
Phase difference
between two points on the same wave/waves of the same frequency, how far through the cycle one point is compared to the other
Electromagnetic wave properties
travel in a vacuum
same speed (in vacuum)/at c
caused by accelerating charges
are (oscillating) electric and magnetic fields
Word equation for resistivity of a material
resistivity = resistance x area (of cross-section)/length
Ohm’s law
p.d./voltage must be proportional to current
As long as temperature and/or (other) physical conditions
remain constant
Line on I/V graph is straight and through the origin
Transverse waves vs longitudinal waves
displacement/oscillation (of particles) is normal/perpendicular to direction of energy transfer in transverse wave
displacement/oscillation (of particles) is parallel to direction of energy transfer in longitudinal wave
Diffraction of a wave
wavefronts/paths spread out after passing through a gap
or around an obstacle
Potential difference
Energy transferred per unit charge
From electrical to other forms
Intensity
energy per unit area per unit time
Stationary wave vs progressive wave
progressive a wave which transfers energy
(((progressive : transfers shape/information from one place to
another)))
stationary a wave which traps/stores energy (in pockets)
(((stationary where the shape does not move along/which has
nodes and antinodes)))
Plane polarised wave
reference to a transverse wave or to vibrations in plane normal to
the direction of (energy) propagation
oscillations/vibrations in one direction only/confined to single plane (containing the direction of propagation)
Malus’ law
I=I0 cos^2θ
where I0 is the maximum intensity (of the polarised beam)
when θ is zero maximum intensity transmitted/ image bright
when θ is 90o minimum/zero intensity transmitted/image dim/vanished
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).
Einstein’s photoelectric energy equation…
(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
A photon
a quantum/lump/unit/packet/particle of (e-m) energy/light
A continuous spectrum
all wavelengths/frequencies are present (in the radiation)
Mean drift velocity of electrons in wire
the average displacement/distance travelled of the electrons along the
wire per second;
(over time/on average) they move slowly in one direction through the
metal/Cu lattice (when there is a p.d. across the wire)
(because) they collide constantly/in a short distance with the lattice
Wavelength, frequency and speed of 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
Energy levels (how they explain photon emission)
electrons have discrete energies in atom/AW
each photon produced by electron moving between levels
photon energy equal to energy difference between levels
electron loses energy/making transition in correct direction
Node and antinode in a standing wave
node occurs where the amplitude/displacement is (always) zero
antinode occurs where the amplitude (of the standing wave) takes the only maximum (possible) value
Electric current
A net flow of charged particles
I, measured in amps, the amount of electrical charge transferred per unit time (rate of flow of electrical charge)
Conventional current vs electron flow
Conventional-a model used to describe the movement of charged particles in a circuit. positive to negative
Electron flow-the movement of electrons (charged particles) around a circuit. negative to positive
Coulomb
One coulomb is 1A x 1s. Unit of electrical charge 1.6x10^-19 C per electron
Number density difference between conductors, semiconductors and insulators
CONDUCTORS (metals)
what makes them good conductors.
INSULATORS (rubber, plastic)
Have a much lower electron density (n) (≈ 10 9 m-3 = 1 mm-3). This means that there is only 1 electron which is free to move per mm3 and that is why insulators cannot conduct.
SEMICONDUCTORS (silicon, germanium)
Have an electron density (n) (≈ 10 19 m-3 = 10 10 mm-3) which lies between that of a conductor and an insulator. The value of n increases with increasing temperature, which means that it behaves as an insulator when it is cold and as a conductor when it is warm.
Volt
1V=1 joule per coulomb. Unit of potential difference and e.m.f.
Difference between e.m.f. And p.d. In terms of energy transfer
E.m.f. (the voltage across an electrical source) is a voltage where
Electrical energy is being transferred from the source to the charge.
p.d. (the voltage across circuit components) is a Voltage where
Electrical energy of the charge is being transferred to other energy forms in the circuit components.
Resistance
a measure of opposition to the flow of charge (i.e. to electric current) caused by the repeated collisions between the charge carriers (usually electrons) in the material with each other and with the fixed positive ions of the material. It is measured in OHM (Ω)
The ohm
A conductor has a resistance of 1 ohm (Ω) if the current in it is 1 ampere (A) when the pd across it is 1 volt (V).
Terminal p.d.
If an external circuit is connected to the cell or battery, the reading
on the voltmeter drops to a value less than E.
when there is a current through the cell, some of its energy is converted into heat by the cell’s internal resistance.
The decrease in voltage is called the ‘LOST VOLTS’ of the cell and it is proportional to the current.
The reading (V) which is < E indicated by the voltmeter is the TERMINAL PD of the cell and also the pd across the resistor R.
Period of a wave
The time taken for one complete pattern of oscillation in seconds
Reflection
Waves rebound from a barrier, changing direction, remaining in the same medium (e.g. air)
Refraction
When waves change direction, when they travel from one medium to another, due to a difference in the wave speed in each medium
UV-A, UV-B, UV-C and sunscreen
Most of the solar UV incident on Earth is absorbed as it passes through the atmosphere
about 98% of that which reaches ground level is UVA. The remaining 2% is mainly UVB since most of the UVC is absorbed by the ozone layer.
UVA, B and C cause damage to collagen fibres in the skin which results in premature wrinkling and ageing of the skin.
UVB induces the production of vitamin D in the skin, but it can cause sunburn and it can damage DNA in skin cells which may lead to skin cancer. High intensities of UVB can also lead to the formation of cataracts in the eyes.
SUNSCREENS filter out or absorb UVB And so protect against sunburn and the Possibility of skin cancer.
UV TYPE
UVA (Long wave)
UVB (medium wave)
UVC
WAVELENGTH RANGE
A)320 nm to 400 nm
B)280 nm to 320 nm
C)100 nm to 280 nm
Electromotive force (e.m.f.)
energy transferred from source/changed from some form
to electrical energy;
per unit charge (to drive charge round a complete circuit)
