Model 6 Mark Questions

Question 1

Terminal Velocity of an oil drop between two charged plates as P.d increases

Answer 1

• At terminal velocity the forces on the drop are balanced OR weight = drag (no resultant force, N1st Law)
• The P.d increases the electrostatic force acting upwards on the drop
• The electrostatic force increases as p.d. increases
• The net upward force causes the drop to have a negative acceleration
• As speed decreases drag decreases
• The drop remains stationary when the forces are balanced OR until the drop remains stationary when weight=electrostatic force

Question 2

Transformer which goes from mains to low voltage battery with a Diode

Answer 2

• The supply creates a changing magnetic field in the iron core
• Rate of change of flux in toothbrush coil is equal to rate of change of flux in charger coil (for an ideal transformer) -The changing flux linkage in the coil of the toothbrush induces an e.m.f. according to Faraday’s law
• E = – N d/dt so to step down the e.m.f. there must be fewer turns in the toothbrush coil
• The e.m.f. in the toothbrush coil must be larger than the toothbrush battery
• Diode is included so battery is not discharged by the alternating e.m.f.

Question 3

Sound source and areas of maxima and minima in a room (superposition/interference)

Answer 3

-Sound waves incident upon surfaces within the concert hall will be reflected.
- Some frequencies will arrive from different directions with a phase difference of (any odd multiple of)
π radians OR path difference is an odd number of half wavelengths
- Destructive superposition/interference will occur, causing
the waves with those frequencies to be quieter than others.
- Other frequencies arrive with a phase difference of zero or (any multiple of) 2π radians (1) OR a whole number of
wavelengths
- Constructive superposition/interference will occur, causing waves with those frequencies to be louder than others.
- Problem arises due to reflections from walls, so use
absorbing material on surfaces to reduce reflections

Question 4

Sound source and areas of maxima and minima in a room (standing waves)

Answer 4

-Sound waves incident upon surfaces within the concert hall will be reflected.
- Reflections from walls set up standing waves (in room)
- Nodes and antinodes are formed for certain frequencies of sound
- Nodes are areas of zero/low amplitude so the frequencies of those sound waves will be quieter than others
- Antinodes are areas of maximum amplitude so the
frequencies of these sound waves will be louder than others
- Problem arises due to reflections from walls, so use
absorbing material on surfaces to reduce reflections

Question 5

Polarized light going through a polarizing filter rotating through 2π

Answer 5

• a polarising filter restricts the (electric field) vibrations of the (transverse) light wave to a single plane
• including the direction of propagation of the light
• the light incident on the filter must be plane polarised
• when the angle of rotation is a multiple of π rad (including zero), the plane of polarisation of the incident light is perpendicular to the transmission axis of the polarising filter hence the intensity of the transmitted light is zero
• when the angle of rotation is an odd multiple of π/2 rad the plane of polarisation of the incident light is the same as that of the transmission axis of the polarising filter hence maximum light is transmitted
• the intensity of the transmitted light varies from a minimum to a maximum as the angle of rotation varies as shown by the graph

OR

-a polarising filter restricts the (electric field) vibrations of the (transverse) light wave to a single direction
-perpendicular to the direction of propagation of the light
-the light incident on the filter is plane polarised
-when the angle of rotation is a multiple of π rad (including zero), the plane of polarisation of the incident light is perpendicular to the transmission axis of the
polarising filter hence the intensity of the transmitted light is zero
-when the angle of rotation is an odd multiple of π/2 rad the plane of polarisation of the incident light is the same as that of the transmission axis of the polarising filter hence maximum light is transmitted
-the intensity of the transmitted light varies from a minimum to a
maximum as the angle of rotation varies as shown by the graph

Question 6

Explain how the cyclotron produces a high‑energy proton beam

Answer 6

-There is an alternating p.d./E-field
-P.d./E-field accelerates protons between dees
-Magnetic field perpendicular to plane of dees
-Proton path curved by magnetic field
-As velocity of protons increases radius of path in dees increases
-The time for which a proton is in a dee remains constant
Or the frequency of p.d./E-field is constant

Question 7

Explain how the LINAC produces a high-energy proton beam

Answer 7

• Consists of a long straight tube containing a series of electrodes
• The charge on each electrode alternates along the tube between positive and negative
• The electrodes are connected to an alternating P.d supply so the charge is continiously changing from +ve to -ve on each electrode
• This means the electric field is continiously changing overtime, between the electrodes
• The alternating P.d means that the particle is always attracted to the next electrode and repelled from the previous one
• Each time a particle passes an electrode it accelerates and speeds up
• To compensate for the increased speed, the length of each electrode is increased so that the particle stays in each one for the same amount of time so that the time period is the same as the alternating P.d

Question 8

Experiments on the photoelectric effect show that the kinetic energy of photoelectrons released depends upon the frequency of the incident light and not on its intensity and light below a certain threshold frequency cannot release photoelectrons. How do these conclusions support a particle but no wave theory of light? [6]

Answer 8

[1] Particle theory: E = hf implied packets/photons
[1] One photon releases one electron giving it k.e.
[1] Increase f ⇒ greater k.e. electrons
[1] Lower f; finally ke = O ie no electrons released Waves
[1] Energy depends on intensity / (amplitude)2
[1] More intense light should give greater k.e–NOT SEEN
[1] More intense light gives more electrons but no change in maximum kinetic energy
[1] Waves continuous ∴ when enough are absorbed electrons should be released–NOT SEEN

Question 9

The diagram shows monochromatic light on a photocell. As the reverse potential difference between the anode and cathode is increased the current measured by the micrometer. When the potential difference reaches V, called the stopping potential, the current is zero. Explain the observation. [5]

Answer 9

[1] Photons release e– at photocathode; e– travel to anode making a current
[1] Photon energy > work function of photocathode
OR All energy of A photon goes to an electron
[1] Electrons released with a range of kinetic energies
[1] So smaller kinetic energy electrons stopped at lower pds
[1] PD opposes kinetic energy of these electrons
[1] Vs supplies enough energy to stop electrons with kinetic energy max

Question 10

What be the effect on the stopping potential of increasing only the intensity of the incident radiation and increasing only the frequency of the incidence radiation? [2]

Answer 10

[1] no effect

[1] increases stopping potential

Question 11

Explain the observation that the emission of photoelectrons from the surface of a metal surface can take instantaneously? [2]

Answer 11

[1] Particle theory: one photon (interacts with) one electron
[1] Wave theory allows energy to ‘build up’, i.e. time delay

Question 12

Explain how the incident light with a frequency below a certain threshold frequency cannot release electrons from a metal frequency [2]

Answer 12

[1] Particle theory: f too low then not enough energy (is released by photon to knock out an electron)
[1] Wave theory: Any frequency beam will produce enough energy (to release an electron, i.e.
should emit whatever the frequency)

Question 13

Explain how the electron gun creates a beam of electrons. [4]

Answer 13

[1] hot filament
[1] thermionic emission / electrons have enough energy to leave
[1] anode and cathode / ± electrodes [identified]
[1] E–field OR force direction OR cause of acceleration
[1] collimation [eg gap in anode identified as causing beam]
[1] need for vacuum

Question 14

Explain what is meant by the term electric field. [1]

Answer 14

[1] Region/area/space in which a positively charge experiences force

Question 15

Explain what is meant by the term line spectra [2]

Answer 15

[1] Specific frequencies or wavelengths
[1] Detail, e.g. absorption/emission
OR within narrow band of wavelengths

Question 16

Explain how line spectra provide evidence for the existence of energy levels in atoms [4]

Answer 16

[1] Photons
[1] Associated with particular energies
[1] Electron transitions
[1] Discrete levels (to provide line spectra)

Question 17

Explain what is meant by the term work function [5]

Answer 17

[1] Electron requires certain amount of energy to escape from surface 
[1] This energy comes from one photon 
[1] Use of E = hf 
[1] (So photon needs) minimum frequency 
[1] Hence maximum wavelength
OR use of E = hc/λ

Question 18

Photons hitting the cathode cause photoelectrons to be released from the surface of the cathode if it is made from a material with a suitable work function.
Describe how an electron escapes from the surface of the cathode. Include the terms in bold from the passage above. [3]

Answer 18

[1] Electron (near surface of cathode) absorbs photon and gains energy
[1] Work function is energy needed for electron to escape from surface
[1] Electrons released in this way are called photoelectrons

Question 19

Why some photoelectrons will have less than k.e. max? [1]

Answer 19

One point from:
• photon energy might be transferred to electron below surface
• so some energy transferred to atoms on the way to surface
• hence electron leaves surface with less energy than max
• max is for electron from the surface
• lower energy photon responsible for emission (1)

Question 20

What is a photon? [2]

Answer 20

[1] Packet/quantum/particle of energy

[1] accept E = hf for energy

Question 21

Explain the term electron excitation [2]

Answer 21

[1] Electrons/atoms gain energy

[1] and electrons move to higher (energy) levels

Question 22

Explain how exited electrons emit radiation [2]

Answer 22

[1] Electrons (lose energy as they) drop to lower levels

[1] Emit photons / electromagnetic radiation (1)

Question 23

Explain why only certain wavelengths of radiation are emitted by exited electrons [3]

Answer 23

[1] Wavelength (of photon) depends one energy
[1] Photon energy depends on difference in energy levels
[1] Levels discrete / only certain differences / photon energies possible (and therefore certain wavelengths)