A-level Waves and Ting G482

Question 1

Define the Coulomb

Answer 1

The SI unit of electrical charge. 1 Coulomb of electrical charge is the amount of charge transferred by a current of 1Amp in 1 Second

Question 2

Define potential difference (p.d.);

Answer 2

Electrical energy transfered per unit charge when electrical energy is converted to another form.

Question 3

define the volt

Answer 3

1 Volt is equal to 1 Joule per Coulomb

Question 4

define electromotive force (e.m.f.) of a source such as a cell or a power supply

Answer 4

The amount of energy per unit charge converted to electrical from other forms, usually in a cell or power supply.

(N.B. This is almost the exact opposite of P.D.)

Question 5

Define resistance

Answer 5

Resistance, R, in any circuit (series, parallel, or any combination) is the opposition created to the flow of current, I, when a potential difference, V, is applied to the circuit.

This opposition to the flow of current is expressed in Ohm’s Law as R = V/I, where R is the resistance in ohms, V is the applied potential in volts, and I is the resulting current in amperes.

Question 6

Define the ohm

Answer 6

1 Ohm is the resistance of a component when a potential difference of 1 volt is produced per ampere of current.

Question 7

define resistivity of a material

Answer 7

The resistivity of a wire of length “l” resistance “R” and area of cross section “A” is given by p = RA/l

Question 8

# define the kilowatt-hour (kW h) as a unit of
 energy

Answer 8

A unit of energy equal to 36 MJ or 1kW for 1h

Question 9

define and use the terms displacement,

amplitude, wavelength, period, phase
difference, frequency and speed of a wave

Answer 9

Displacement-Distance from the mean position expressed as a vector
Amplitude- Maximum displacement
Wavelength-Distance between neighbouring identical points
Period-Time taking for one complete oscillation of a particle
Phase Difference- The fraction of a cycle between the oscillations of two particles
Frequency-Number of waves passing a point per unit time
Speed-Distance travelled by the wave per unit time

Question 10

define the terms nodes and antinodes

Answer 10

Node-Fixed point on a standing wave where the amplitude is always zero

Antinode-Fixed point along a standing wave where the amplitude of the oscillations will periodically be at a maximum.

Question 11

define and use the terms fundamental mode of vibration and harmonics

Answer 11

Simplest pattern of movement and has the lowest possible frequency band and the longest wavelength
Harmonics are different modes of vibration of a wave with increasing frequency and decreasing wavelength

Question 12

define and use the electronvolt (eV) as a unit of energy

Answer 12

1 eV is gained or lost when an electron moves through a potential difference of 1V

Energy acquired by an electron accelerated through a p.d of 1V.

1eV=1.6×10^(-19)J

Question 13

define and use the terms work function and threshold frequency

Answer 13

Work function- the minimum energy required to release an electron from the surface of a material

Threshold frequency- Frequency of a photon that will just emit an electron from a substance without any kinetic energy

Question 14

Define the term intensity

Answer 14

Intensity is the (incident) energy per unit area per second

Question 15

state what is meant by the term mean drift
velocity of charge carriers

Answer 15

The average distance travelled by the charge carriers along the wire per second

Question 16

state and use Ohm’s law

Answer 16

For a metallic conductor at constant temperature, the current in the conductor is directly proportional to the potential difference across it

Question 17

state and use Ohm’s law

Answer 17

For a metallic conductor at constant temperature, the current in the conductor is directly proportional to the potential difference across it

Question 18

state Kirchhoff’s second law and appreciate that this is a consequence of conservation of energy

Answer 18

Energy is conserved
Sum of e.m.f’s=sum of voltages (total of p.d’s in a loop)

Question 19

state typical values for the wavelengths of the different regions of the electromagnetic
spectrum from radio waves to γ-rays

Answer 19

Visible 600-400nm (5 x 10-7)
UV-A 400-315nm
UV-B 315-260nm
UV-C 260-100nm

Question 20

state three things that electromagnetic waves have in common

Answer 20

Travel at the same speed “c” in a vacuum

All transfer both Energy and Information

All are transverse (travel at 90 degrees to the direction of oscillation)

Question 21

state why polarisers can be used to see deeper into water.

Answer 21

Light is partially polarised upon reflection.

Question 22

State the principle of superposition of waves

Answer 22

When two waves meet at a point and interfere the sum of their individual dislacement is equal to the sum of the resultant displacement.

Question 23

State what is meant by constructive/destructive interference

Answer 23

interference is when two or more waves interact/superpose and there is a change in overall intensity or displacement.

Constructive interference is where net displacement is larger than the displacement of the two original waves

Destructive interference is where the net displacement is smaller that one ( or all) of the original waves

Question 24

What is a photon?

Answer 24

A photon is a quantum (discrete amount) of energy of electromagnetic radiation

Question 25

This isn’t really a question - just something you should know:

Energy is conserved when a photon interacts with an electron

Answer 25

You should also know that

1) Charge carriers in an electrolyte are ions (silly chemistry)
2) Charge carriers moving through metal wires are electrons

Question 26

State what Malus’ law is

Answer 26

Malus’s law states that the intensity of transmitted light from a polarising filter is:

I = I₀cos²Ø

Question 27

You must also make sure you know all the circuit symbols for

1) LED
2) Thermister
3) LDR
4) Potentiometer
5) variable resistor
6) plus all the others

Answer 27

Look them up now, go on, it’ll be good for you

Question 28

Apply graphical methods to illustrate the principles of superposition

Answer 28

The following diagram explains the effect of superposing two waves that are 1) in phase (top diagram) and 2) in antiphase.

Question 29

Describe how an ammeter may be used to measure current in a circuit

Answer 29

Must be put in series to measure the current

Question 30

describe Kirchhoff’s first law and appreciate that this is a consequence of conservation of charge

Answer 30

(sum of/total) current into a junction equals the (sum of/total) current out. Charge is conserved

Question 31

describe the difference between conductors, semiconductors and insulators in terms of the number density n

Answer 31

Conductors have lots of charge carriers (large number density) and therefore can easily let current flow, semi conductors have fewer free delocalised charge carriers (smaller number density) so less current flows, it is important to note that semi conductors conduct better at higher temperatures as more electrons break free from the atoms. Insulators have very few/no charge carriers (number density is closer to 0) and therefore don’t let any current flow.

Question 32

describe how a voltmeter may be used to determine the p.d. across a component

Answer 32

The voltmeter must be put in parallel

Question 33

describe the difference between e.m.f. and
p.d. in terms of energy transfer

Answer 33

Potential difference is transferring electrical energy into other forms (heat, light, sound). EMF is transferring other forms of energy into electrical (chemical energy stored in a battery)

Question 34

describe the I–V characteristics of a resistor at constant temperature, filament lamp and light-emitting diode (LED)

Answer 34

Resistor follows ohms law so straight line through origin. Filament Lamp is non-ohmic as its temperature varies so the line should go up and then curve so the gradient= 0. LED is non-ohmic as well, so no current should flow until a certain threshold voltage, so flat line to indicate no current, then a upwards line to show current is now flowing.

Question 35

describe an experiment to obtain the I–V
characteristics of a resistor at constant
temperature, filament lamp and light-emitting
diode (LED)

Answer 35

Ammeter, Resistor/Filament Lamp/LED and Potentiometer in series. Place voltmeter in parallel with the component being tested. Limit the current flowing by varying the potentiometer accordingly, taking current and potential difference readings respectively

Question 36

describe the uses and benefits of using light emitting diodes (LEDs).

Answer 36

Advantages or LED over a filament lamp in a torch-Draws lower current, light lasts longer, LEDs more efficient at converting electrical energy into light, more robust and a longer working life.

Question 37

describe how the resistivities of metals and semiconductors are affected by temperature

Answer 37

Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased.

Question 38

describe how the resistance of a pure metal wire (light bulb filament) and of a negative temperature coefficient (NTC) thermistor is affected by temperature

Answer 38

Thermistors are temperature sensitive resistors. However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature.
In a pure metal a greater resistance slows the flow of electrons so a smaller current flows as temperature increases.

Question 39

Define Power

Answer 39

power is the rate at which energy is transferred, used, or transformed

Question 40

describe how the resistance of a lightdependent resistor (LDR) depends on the intensity of light

Answer 40

Resistance decreases with increase in light intensity
LDR must be shielded or be at some distance from the lamp when it switches on as the light shining will cause it to switch the illumination off causing an on/off oscillation

Question 41

describe and explain the use of thermistors and light-dependent resistors in potential divider circuits

Answer 41

Thermistor/LDR can be used to provide an output voltage, which depends on temperature/light intensity.

The Thermistor/LDR (with a parralel branch across it) can be put in series with a second resistor.

When the temp/light intensity increases, the voltage across the thermistor/LDR will fall…

Question 42

describe the advantages of using dataloggers to monitor physical changes

Answer 42

Continuous record for a very long time scale of observations
Can record very short timescale signals (at intervals)
Automatic recording/remote sensing
Data can be fed directly to a computer (for analysis)

Question 43

describe and distinguish between progressive longitudinal and transverse waves

Answer 43

Longitudinal = oscillations/vibration of particles/medium in direction of travel of the wave e.g. sound
Transverse = oscillations/vibration of particles/medium (in the plane) at right angles to the direction of travel of the wave e.g. surface water, string, electromagnetic

Question 44

describe differences and similarities between different regions of the electromagnetic spectrum

Answer 44

Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each travelling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons, their wavelength and frequency ranges. Radio waves have photons with low energies, and gamma-rays have photons with the greatest energies.

The similarities are that they are all transverse, travel at the speed of light, and carry energy and information.

Question 45

describe some of the practical uses of
electromagnetic waves

Answer 45

Radio stations. Radio waves are emitted by stars and gases in space.
Microwaves in space are used by astronomers to learn about the structure of nearby galaxies.
Our skin emits infrared light and In space, IR light maps the dust between stars.
Visible radiation is emitted by everything from fireflies to light bulbs to stars … also by fast-moving particles hitting other particles. It’s the light the human eye can see.
The Sun, Stars and other “hot” objects in space emit UV radiation.
Hot gases in the Universe also emit X-rays. They are used in scanning the bones in the body.
Radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. The biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of+\ASA ways.

Question 46

describe the characteristics and dangers of UV-A, UV-B and UV-C radiations and explain the role of sunscreen

Answer 46

Suncreen filters out (reflects) UV-B protecting skin
UV-A causes tanning or skin ageing, it makes up 99% of UV light. 400-315nm
UV-B causes damage to cells, skin cancer and sunburn. 315-200nm
UV-C is filtered out by the atmosphere. 260-100nm

Question 47

describe experiments that demonstrate two source interference using sound, light and microwaves

Answer 47

Two speakers playing the same note, hear constructive (loud) and destructive (quiet) interference.
Light or microwaves pointed towards two slits constructive and destructive interference can be observed.

Question 48

describe constructive interference and
destructive interference in terms of path
difference and phase difference

Answer 48

If the path difference between two light waves is (n+1/2)wavelengths, then the interference between them will be destructive.
For constructive interference, path difference between two waves is n x wavelength

Question 49

describe the Young double-slit experiment and explain how it is a classical confirmation of the wave-nature of light

Answer 49

Monochromatic source sent through 2 slits which diffracts the source, since they diffract and give an interference pattern it shows wave nature of light.

Question 50

describe an experiment to determine the
wavelength of monochromatic light using a
laser and a double slit

Answer 50

λ=ax/D

a=d on diagram=slit separation

x=fringe separation

D=L on diagram=distance between slit and screen

Question 51

describe the use of a diffraction grating to
determine the wavelength of light (the
structure and use of a spectrometer are not
required

Answer 51

d sinθ=nλ

n=order of diffraction

Light is shone through a diffraction grating and makes a pattern of bright and dark fringes. the angle to the screen, and the seperation of the slits in the diffraction grating, can be used to calculate the wavelength of light.

Question 52

describe the similarities and differences between progressive and stationary waves

Answer 52

Progressive-A wave which transfers energy, it transfers shape/information from one place to another
Stationary-A wave which stores energy, the shape does not move along.

Stationary wave = the incident wave is reflected at the end of the pipe/string. Reflected wave interferes/superposes with the incident wave to produce (a resultant wave with) nodes and/or antinodes

Question 53

Describe the paticulate nature of electromagnetic radiation

Answer 53

A photon is a quantum//packet/particle of (e-m) energy

Question 54

describe an experiment using LEDs to estimate the Planck constant h using the equation eV_min = hf

Answer 54

Using the equipment drawn below, the voltage from a variable voltage source is increased until photons are emitted from the LED. The energy of the electron causing the photoemission is eV, the wavelength/frequency of light emitted can be obtained from the manufacturer. h = eV_min/f

ALTERNATIVELY, eVmin Vs f can be plotted for LOTS of different LEDs with different wavelengths, the gradient of this line will be equal to plancks constant,

Question 55

describe and explain the phenomenon of the photoelectric effect

Answer 55

A photon is absorbed by an electron in a metal surface causing an electron to be emitted. Energy is conserved. Only photons with energy above the work function energy will be emitted. Energy=work function + Max KE of electron. The work function is the minimum energy required to release an electron from the surface.

Number of electrons emitted also depends on light intensity
Emission is instantaneous

Question 56

Describe and explain an experiment (using a gold leaf electroscope) to show the photoelectric emission of electrons.

*I don’t think you need to know this, but better safe than sorry

Answer 56

A clean zinc plate is mounted on the cap of a gold leaf electroscope where the plate is initially charged negatively. Shine a UV light on the plate and watch the gold leaf collapse as charge leaves the place, indicating the emission of electrons
Work function energy is the minimum energy to release an electron from the surface

Question 57

describe the origin of emission and
absorption line spectra

Answer 57

(Emission) Line spectrum = light emitted from (excited isolated) atoms produces a line spectrum a series of (sharp/bright/coloured) lines against a dark background.
Absorption spectrum is a series of dark lines (appears against a bright background/within a continuous spectrum)

Question 58

determine the speed of sound in air from
measurements on stationary waves in a pipe
closed at one end

Answer 58

Sound waves in the tube are in the form of standing waves, and the amplitude of vibrations of air is zero at equally spaced intervals along the tube.. The distance between the nodes is one half wavelength λ/2 of the sound. By measuring the distance between the piles, the wavelength λ of the sound in air can be found. If the frequency f of the sound is known, multiplying it by the wavelength gives the speed of sound c in air:

Question 59

explain that electric current is a net flow of
charged particles

Answer 59

There is a current when charged particles flow past a point in a circuit. Current is the rate of flow of charge

Question 60

explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the
movement of ions

Answer 60

Wires are made from metal. The metal contains a sea of delocalised electrons which move in random directions. When a cell is connected to the wire and electrical force is applied to the electrons making them ‘drift’. They still move in random directions however they have an overall velocity or movement, creating a current. This can happen with ions in an electrolyte too.

Question 61

explain what is meant by conventional current and electron flow

Answer 61

The direction of the current is from the positive terminal to the negative. However electrons are what flow in metals and are negatively charged and therefore flow from negative to positive

Question 62

explain how a fuse works as a safety device

Answer 62

The fuse needs to have a current rating big enough to cover the initial current in the circuit, if there is a surge of current, the thin fuse wire will rapidly heat up and melt, this will stop the current flowing. The fuse wire will protect devices from surges of current, and will also protect the used when used with an earth wire.

Question 63

explain that all sources of e.m.f. have an
internal resistance

Answer 63

All sources of emf have what is known as INTERNAL RESISTANCE (r) to the flow of electric current. The internal resistance of a fresh battery is usually small but increases with use. Thus the voltage across the terminals of a battery is less than the emf of the battery.

Question 64

explain how a potential divider circuit can be used to produce a variable p.d

Answer 64

If you use two fixed resistors in series and connect the ends to a voltage supply then the voltage between both ends and the mid point where they connect will be in proportion their resistor value. It is possible to vary the midpoint voltage by changing one of the resistors. This can be done readily with a variable resistor which than then be altered to get any value between 0-MaxV
By adjusting the value of R1, the potential dropped

Question 65

explain what is meant by reflection, refraction and diffraction of waves such as sound and light

Answer 65

Reflection-Bouncing back of wave from a surface
Refraction-Change in direction of a wave as it crosses and interface between two materials where its speed changes
Diffraction-Spreading of a wave when it passes through a gap or past the edge of an object

Question 66

explain the meaning of the term terminal p.d.;

Answer 66

The terminal p.d. of a source is the potential difference across its terminals (duh)

Question 67

explain what is meant by plane polarised
waves and understand the polarisation of
electromagnetic waves

Answer 67

Transverse waves/vibrations in plane are normal to the direction of energy propagation. Oscillations are in one direction only

Question 68

explain that polarisation is a phenomenon
associated with transverse waves only

Answer 68

Electromagnetic waves are transverse waves so can be polarised whereas sound waves cannot since they are not transverse.

Question 69

explain the terms interference, coherence,
path difference and phase difference

Answer 69

Interference-When two waves meet at a point

Coherence-Constant phase difference between the two wavesor are continuous and have the same f/period/ λ

Path difference-of any point in an interference pattern of waves is the difference between the distance travelled by each wave from their source to that point

Phase difference- difference in position of similar points in two waves expressed as an angle.

Question 70

explain the advantages of using multiple slits in an experiment to find the wavelength of light.

Answer 70

Multiple slits result in a more clear, well defined diffraction pattern with grater brightness/sharpness.

Question 71

explain that the photoelectric effect provides evidence for a particulate nature of
electromagnetic radiation while phenomena
such as interference and diffraction provide
evidence for a wave nature

Answer 71

There is a threshold frequency which suggests particle nature, as the wave theory states that photoelectric emission should happen as long as the light is bright enough. However this is not the case. Solution is to assume that light comes in discrete packets, or “quanta” with energy = hf

Diffraction and interference are wave properties, suggesting that electromagnetic radiation has wave nature.

Question 72

explain the formation of stationary (standing) waves using graphical methods

Answer 72

(open end tube and speaker) Using a tube with one end closed and a loud speaker, the incident wave is reflected at the end of the pipe and it interferes with the incident wave to produce a resultant wave

(string and oscillator) The incident wave is reflected at the fixed end of the wire, the reflected wave interferes with the incident wave to produce a resultant wave with nodes and antinodes

Question 73

explain and use Einstein’s photoelectric equation

Answer 73

Individual photons are absorbed by individual electrons in the metal’s surface. These electrons must absorb sufficient energy to overcome the work function energy of the metal, the remaining energy is expressed as the kinetic energy of the electron. The number of electrons emitted depends on light intensity as emission is instantaneous.

Question 74

explain why the maximum kinetic energy of the electrons is independent of intensity and why the photoelectric current in a photocell circuit is proportional to intensity of the incident radiation

Answer 74

hf=∅+KE_MAX Therefore independent of intensity.
The larger the intensity, the greater the number of photons emitted, therefore releasing more electrons generating a larger current.

Question 75

explain electron diffraction as evidence for the wave nature of particles like electrons

Answer 75

Electron diffraction refers to the wave nature of electrons by firing electrons at a sample and observing the resulting interference pattern. This phenomenon is commonly known as the wave-particle duality, which states that the behaviour of a particle of matter (in this case the incident electron) can be described by a wave.
Diffraction is a property unique to waves. If electrons can be diffracted then they are behaving as waves

Question 76

explain electron diffraction as evidence for the wave nature of particles like electrons
explain that electrons travelling through
polycrystalline graphite will be diffracted by
the atoms and the spacing between the
atoms

Answer 76

Graphite, because of its layered structure, can act as a diffraction grating with very small slit diameter.

Question 77

explain that the diffraction of electrons by
matter can be used to determine the
arrangement of atoms and the size of nuclei.

Answer 77

Slow moving electrons, electrons with wavelengths (E=hf) of the order of magnitude of the structure (or nuclei) can be used to probe the properties of atomic structures. From the distance/angle between the bright circles of the diffraction pattern, an equation can be used to calculate the inter-atomic spacing.

Question 78

explain how spectral lines are evidence for the existence of discrete energy levels in
isolated atoms, ie in a gas discharge lamp

Answer 78

Photon produced by electron moving between levels
Photon energy equal to energy difference between levels

Question 79

Explain how sunscreen protects the human skin

Answer 79

Filters out/blocks/reflects/absorbs UV (-B)

Question 80

Explain why electrons can be emitted from a clean metal surface illuminated with bright UV light but never when IR is used, however intense

Answer 80

Energy of the infra-red photon is less than the work function of the metal surface, so no electrons will be released.

Because E=hf, UV light is comprised of photons with grater individual energy which can overcome the work-function.

Question 81

Explain what is meant by the de Broglie wavelength of an electron

Answer 81

Electrons are observed to behave as waves/show wavelike properties where the electron wavelength depends on its speed/momentum

Question 82

Explain what is meant by a continuous spectrum

Answer 82

All wavelengths/frequencies are present (in the radiation) (there are no gaps)

Question 83

derive from the definitions of speed,
frequency and wavelength, the wave
equation v = fλ

Answer 83

v=x/t and f=1/t then v=x/(1/f) Wavelength λ is the displacement x between subsequent wave peaks therefore v=fλ