Module 4 Standard Answers

Question 1

Describe and explain the shape of the I-V characteristic of a filament lamp.

Answer 1

• The filament does not obey Ohm’s Law.
• The current through the filament lamp is not directly proportional to the pd across its ends, as
the graph is not a straight line through the origin.
• As the current (and pd) through the filament lamp increases, the temperature of the filament
lamp increases.
• As the temperature increases, there are increased ion vibrations and more frequent electron-
ion collisions.
• The resistance of the filament lamp increases.

Question 2

Describe and explain the shape of the I-V characteristic of a diode/LED.

Answer 2

• The diode/LED does not obey Ohm’s Law.
• The current through the diode/LED is not directly proportional to the pd across its ends, as the
graph is not a straight line through the origin.
• Below the threshold voltage, the diode/LED has an infinite resistance.
• Above the threshold voltage, the diode/LED’s resistance decreases.
• Approximately 0.5V above the threshold voltage, the current flowing through the diode/LED
increases linearly with pd.

Question 3

Describe the factors which affect the resistance of a metal wire.

Answer 3

• Temperature: As temperature increases, resistance increases.
• Resistivity: A greater resistivity results in a greater resistance.
• Cross-sectional area: As cross-sectional area increases, resistance decreases.
• Length: As length increases, resistance increases

Question 4

graph of resistivity vs temperature for a metallic conductor.

Answer 4

• The resistivity of a metallic conductor increases as the temperature increases, due to more
frequent electron-ion collisions.

Question 5

Describe how the resistivity / resistance of a semiconductor / NTC thermistor varies with
temperature

Answer 5

• The resistivity / resistance decreases as temperature increases.
• The decrease in resistivity / resistance is most rapid at lower temperatures.
• As the temperature increases, more electrons can break free of their atoms to become
conduction electrons, increasing the number density, and decreasing the resistivity / resistance.
• As the temperature increases, there are increased ion vibrations and more frequent electron-
ion collisions, but this effect is small compared to the increase in the number density.

Question 6

Describe how the resistivity / resistance of an LDR varies with light intensity.

Answer 6

• The resistivity / resistance decreases as light intensity increases.
• The decrease in resistivity / resistance is most rapid at lower light intensities.
• As the light intensity increases, more electrons can break free of their atoms to become
conduction electrons, increasing the number density, and decreasing the resistivity / resistance.
• As the light intensity increases, there are more frequent electron-ion collisions, but this effect is
small compared to the increase in the number density

Question 7

Potential Divider Circuit: Describe and explain how the current / pd across the fixed
resistor will change.

Answer 7

• The resistance of an electrical component increases/decreases (BIRD TURD).
• The total resistance in the circuit increases/decreases.
• By I = V/R, the total current in the circuit increases/decreases (as total pd is constant).
• By V = IR, the pd across the fixed resistor increases/decreases (as its resistance is constant

Question 8

Describe the benefits of using a potential divider circuit instead of a variable resistor to
take measurements for the I-V characteristic of a component.

Answer 8

• A potential divider circuit produces a full range of readings for current and pd for the
component, from 0 to a maximum possible value.
• A variable resistor does not have a high enough resistance to give low current and low pd
readings for the component connected in series (only high current and high pd readings). A
variable resistor will not give a full range of readings.

Question 9

graphs associated with the internal resistance of a charged cell.

Answer 9

• A charged cell outputs the maximum power when the resistance of the circuit is equal in
magnitude to the internal resistance of the ce

Question 10

State the typical number density / example for a…

Answer 10

• Conductor: n = 1 x 1029, eg. copper.
• Semiconductor: n = 1 x 1016, eg. silicon.
• Insulator: n = negligible, eg. plastic

Question 11

State the wave phenomena experienced by…

Answer 11

[i] both transverse and longitudinal waves.
• Reflection, refraction, diffraction and interference.
[ii] transverse waves only.
• Polarisation.

Question 12

The graph shows how the intensity of light transmitted through a polaroid filter varies as
the filter is rotated through 360°
. Explain the series of maxima and minima.

Answer 12

• When the axis of transmission of the polaroid is parallel to the light’s plane of polarisation,
there is maximum transmission of light (at 0°, 180°, 360°).
• There is zero transmission of light when the polaroid’s axis of transmission is perpendicular to
the plane of polarisation of the light (at 90°, 270°

Question 13

metal grille is placed between a microwave transmitter and detector. Initially, the
transmitter and grille are vertically aligned. Explain why the detected signal varies from
zero to maximum as the grille is rotated through 90°.

Answer 13

• The emitted microwaves are plane polarised.
• The metal grille will absorb microwave radiation with a plane of polarisation parallel to its
metal bars.
• Initially, the metal grille bars are parallel to the microwave’s plane of polarisation, and so all
microwave radiation is absorbed by the grille and the detected signal is zero.
• At 90°, the metal grille bars are perpendicular to the microwave’s plane of polarisation, and so
there is negligible absorption of microwaves by the grille and the detected signal is maximum

Question 14

Two polarising filters are aligned perpendicular to one another. Describe and explain the
transmission of light through the second polariser.

Answer 14

• The lamp produces unpolarised light. The first polariser only transmits the vertical component
of the light’s oscillations.
• The second polariser’s axis of transmission is perpendicular to the light’s plane of
polarisation, and so no light is transmitted through the second polariser.

Question 15

Two polarising filters are aligned perpendicular to one another. Describe and explain the
transmission of light through the second polariser.

How would this differ when an additional polariser is inserted between the previous two,
with its axis of transmission aligned somewhere between the previous two.

Answer 15

• When inserted, the additional polariser would transmit a component of the vertically polarised
light through as its axis of transmission is not perpendicular to the light’s plane of polarisation.
• Now, the light’s plane of polarisation is not perpendicular to the second polariser’s axis of
transmission, and so a component of this light will be transmitted.

Question 16

State when the effects of diffraction become significant / maximum.

Answer 16

• Maximum diffraction occurs when the wavelength of the wave is a similar size / comparable to
the width of the gap (or obstacle) that it passes.

Question 17

Describe the main features that all electromagnetic waves have in common.

Answer 17

• They are all transverse waves.
• They all have the same speed in a vacuum (c = 3.00x108 ms-1).
• They all are transferred in discrete amounts of energy called photons.
• They can all be reflected, refracted, diffracted and polarised.

Question 18

State some major differences between Gamma rays and X-rays.

Answer 18

• X-rays are emitted from the collision of high-speed electrons with a metal surface.
• Gamma rays are emitted from the radioactive decay of atomic nuclei

Question 19

Consider the Young Double Slit Experiment. Explain…
[i] How the apparatus setup ensure that the two slits act a coherent sources.

Answer 19

• The two slits are illuminated using the same light source.
• The light leaving the two slits has the same wavelength and frequency, so the light leaving the
two slits will have a constant phase difference.

Question 20

Consider the Young Double Slit Experiment. Explain…

ii] Why an interference pattern of bright and dark fringes is formed on the screen.

Answer 20

• At the bright fringes, the light waves meet in phase as the path difference between the two
waves is equal to a whole number of wavelengths, resulting in constructive interference.
• At the dark fringes, the light waves meet in antiphase as the path difference between the two
waves is equal to an odd number of half wavelengths, resulting in destructive interference.

Question 21

Consider the Young Double Slit Experiment. Describe how the interference pattern
would change if…
[i] a white light source was used instead of the monochromatic source.

Answer 21

• There would be a central white fringe. The other bright fringes would be coloured.

Question 22

Consider the Young Double Slit Experiment. Describe how the interference pattern
would change if…

ii] the number of slits was increased.

Answer 22

• The bright and dark fringes become much more defined and separated

Question 23

Describe some differences between a progressive wave and a stationary wave

Answer 23

Progressive waves transfer energy from one point to another. Stationary waves store/trap
energy in pockets.
• The amplitude of a progressive wave is the same at every point. The amplitude of a stationary
wave varies sinusoidally along the wave.
• The phase difference between neighbouring points along a progressive wave varies
sinusoidally. All points located between two nodes on a stationary wave are in phase (two
points located either side of a node are in antiphase)

Question 24

Describe and explain how a stationary wave is formed.

Answer 24

• A progressive wave is transmitted and then reflected (by a barrier / metal plate).
• The reflected wave superposes/interferes with an incident wave.
• This produces a resultant wave with nodes and antinodes, ie. a stationary wave.
• At the points of zero amplitude (nodes), the incident wave and reflected wave destructively
interfere and perfectly cancel.
• At the points of maximum amplitude (antinodes), the incident wave and reflected wave
constructively interfere.
• If a stationary sound wave is formed in a tube, a resonance (loud sound) is heard.

Question 25

The diagram shows an arrangement to produce a stationary wave. A microwave detector (D) is slowly moved along the line TP. Explain how you could use the detector (D) to determine the wavelength and frequency of the microwaves

Answer 25

• The distance between adjacent nodes on a stationary wave is λ/2. • With the detector (D), measure the distance between two adjacent points along TP where the detected signal is zero (nodes). Wavelength λ = 2 × distance between nodes. • By the wave equation v = λf, the frequency of the microwaves f = c/

Question 26

Under what circumstances does a quantity in v = λf remain constant?

Answer 26

• Frequency remains constant during refraction. • Wavelength remains constant during diffraction. • Wave speed remains constant provided the wave is moving along the same medium

Question 27

The diagram shows a zinc plate attached to a charged gold-leaf electroscope. When the zinc plate is exposed to high frequency ultra-violet radiation, it loses electrons from its surface and consequently the gold leaf falls rapidly. Use the photoelectric effect to describe how the ultra-violet radiation interacts with the surface electrons of the zinc plate. Explain why visible light, no matter how intense, does not release electrons from the zinc plate.

Answer 27

An individual photon is absorbed by an individual electron. It is a one-to-one interaction. • Only photons with energy above the workfunction energy of the metal will cause photoelectron emission, and this emission will be instantaneous. • The energy of a photon is proportional to the frequency of the photon. • As a result, UV photons will cause photoelectron emission as UV photons have a higher frequency than visible light photons, which have too low a frequency to cause emission. • Energy is conserved during the photon-electron interaction, where the electron absorbs the photon. • So the maximum kinetic energy a photoelectron can leave the metal surface with is the difference between the energy of the incoming photon and the workfunction of the metal (KEmax = hf – 𝜙𝜙).

Question 28

Suggest how increasing the intensity of the electromagnetic radiation would affect photoelectron emission.

Answer 28

• A larger number of photons would be incident on the surface of the metal in a given time. • As each photon has an energy above the workfunction of the metal, more electrons would be ejected in a given time. • The metal surface would lose charge at a faster rate. • There would be no change to the maximum kinetic energy of the electrons which leave the metal surf

Question 29

Explain why photoelectrons are emitted with a range of kinetic energies, which typically are less than the maximum.

Answer 29

• Maximum kinetic energy corresponds to an electron emitted from the surface of the metal. • Most electrons will be beneath the surface and so energy is required to bring these electrons to the surface. • The ejected electrons have a range of kinetic energies because some electrons collide with the ions in the meta

Question 30

Explain why the existence of a threshold frequency and an instantaneous emission provide evidence for the particulate nature of electromagnetic radiation, as opposed to a wave theory.

Answer 30

• Wave theory predicts that any photon frequency would cause the emission of an electron if the exposure time were sufficiently long. • The energy of a photon is directly proportional to its frequency. • Instantaneous electron emission will only occur if the photon frequency is greater than the threshold frequency, a property of the metal.

Question 31

Describe an instance where electrons demonstrate… [i] wave-like behaviour

Answer 31

• Moving electrons behave like a wave, showing diffraction effects when diffracted by matter (such as graphite). • This occurs when the de Broglie wavelength of the electron is comparable to the separation between neighbouring atoms in the matter. • This is shown by the diffraction pattern produced, which is an interference effect associated with waves.

Question 32

Describe an instance where electrons demonstrate… ii] particular (particle-like) behaviour.

Answer 32

• Electrons are deflected by electric fields/other charges. • Moving electrons are deflected in magnetic fields. • Electrons have mass and charge, similar to other particles.

Question 33

Suggest why using neutrons may be preferable to electrons when investigating matter.

Answer 33

• Neutrons do not have a charge and so are not influenced by any electrical forces which may be present in the matter.

Question 34

Both electrons and photons can be considered as particles. State some differences between their properties.

Answer 34

• Electrons have mass; photons have zero mass. • Electrons have charge; photons are uncharged. • Photons travel at the speed of light in a vacuum, whereas electrons cannot travel at this speed

Question 35

Electrons are accelerated to high speeds using the electron-gun. These high-speed electrons pass through a thin layer of graphite and emerge to produce a pattern of rings on the fluorescent phosphor screen. Explain how this experiment demonstrates the wave-like nature of electrons. Suggest what happens to the pattern of rings when the speed of the electrons is increased.

Answer 35

• Moving electrons behave like a wave: they are diffracted when passing between carbon atoms in the graphite. The pattern of rings forms as a result of interference. • Diffraction occurs when the de Broglie wavelength of the electron is a similar size / comparable to the atomic separation / spacing of the graphite. • The bright rings are regions of constructive interference and the dark rings are a result of destructive interference. • When the speed of the electrons is increased, their de Broglie wavelength decreases. • The de Broglie wavelength of the electrons is now smaller than the atomic separation. Less diffraction occurs and the rings become smaller in radius.

Question 36

Suggest how, within the electron-gun, this experiment provides evidence for the particle-like property of the electrons.

Answer 36

• Electrons have mass / momentum / charge and they can be accelerated

Question 37

Describe an experiment to obtain the I-V characteristic of an electrical component.

Answer 37

Set up circuit as shown. • Vary the pd across the electrical component by adjusting the slider on the rheostat. • Move the slider from one end of the rheostat to the other to ensure the full range of pd from 0.0 to 6.0V. • Measure and record the current through the electrical component and the pd across the electrical component for at least six different pairs of I and V. • Reverse the connections of the power supply to the circuit and repeat the procedure. • Plot graph of I vs V. • (For the fixed resistor only: As gradient is constant, resistance = 1 / gradient.) • (For the diode/LED: Move the slider by small amounts to determine the turn on voltage accurately. A current limiting resistor is placed in series with the diode/LED, preventing it from overheating and breaking.)

Question 38

Describe an experiment to determine the emf and internal resistance of a charged cell.

Answer 38

• Move the slider from one end of the rheostat to the other to vary the terminal pd (V) across the charged cell. • Measure and record the terminal pd and current for at least six different pairs of I and V. • Plot a graph of V vs I. • The y-intercept of the line of best fit = emf. • The gradient of the line of best fit = −r. Hence, internal resistance = −gradient.

Question 39

Describe a ripple tank experiment to demonstrate interference between water waves. State how the speed of the waves could be reduced.

Answer 39

Two dippers are vibrated up and down from the same vibration generator. • This ensures the wave sources are coherent. • Reducing the depth of the water in the tank would reduce the speed of the waves

Question 40

Describe an experiment to determine the resistivity of a metal wire.

Answer 40

• Measure the diameter (d) of the wire using a digital calliper / micrometer. Measure at 6 different positions and angles along the wire and then calculate a mean average. • Measure multiple lengths (L) of the wire using a metre ruler. • For each length, measure and record the pd across the wire and the current flowing through the wire using the voltmeter and ammeter respectively. • For each pair of current and pd values, calculate R = V / I. • Plot a graph of R vs L. Gradient = ρ / A. • Calculate A = πd2/4, then ρ = gradient x A.

Question 41

Describe an experiment to determine the critical angle and hence the refractive index of the semi-circular block.

Answer 41

• Position a raybox so that the incident ray enters through the curved edge and the refracted ray emerges along the boundary of flat edge of the block. • Mark the incident and refracted rays using a pencil and ruler on paper. • Mark the normal line on the flat edge, perpendicular to the block’s boundary. • Connect the incident and emergent rays with a straight line. • Measure the angle of incidence (θ1) to the normal using a protractor. This is the critical angle (θc). • Snell’s Law: n1 sin(θ1) = n2 sin(θ2), where n2 = 1 (in air), θ1 = θc and θ2 = 90°. The refractive index of the block n1 = 1 / sin(θc)

Question 42

Describe an experiment to determine the refractive index of the rectangular block.

Answer 42

• Position a raybox so that the incident and emergent rays are located on the opposing longer edges of the block. • Mark the incident and emergent rays using a pencil and ruler on to the paper. • Mark on the normal line where the incident ray enters the block, perpendicular to the block’s boundary. • Connect the incident and emergent rays with a straight line. • Measure the angle of incidence (θ1) and angle of refraction (θ2) to the normal, using a protractor. • Snell’s Law: n1 sin(θ1) = n2 sin(θ2), where n1 = 1 (in air). So the refractive index of the block n2 = sin(θ1) / sin(θ2

Question 43

Describe an experiment the demonstrate the wave-like nature of electrons. Draw a labelled diagram to support your answer.

Answer 43

• Electrons are emitted from the metal filament cathode within an electron gun, by thermionic emission. • These electrons are accelerated through a high pd. • The electrons are diffracted when passing through a thin graphite sheet. • The electrons are detected upon impact with a phosphor (fluorescent) screen. This produces a visible interference pattern