Tuesday, July 21, 2015

Physics 9702 Doubts | Help Page 179

  • Physics 9702 Doubts | Help Page 179

Question 882: [Quantum Physics > Photoelectric effect]
Electrons are emitted from a metal surface when it is illuminated with suitable electromagnetic radiation.
(a) Name the effect described above.

(b) The variation with frequency f of the maximum kinetic energy Ek of the emitted electrons is shown in Fig.1.

Use Fig.1 to determine
(i) threshold frequency of the radiation,
(ii) a value for the Planck constant.

(c) On Fig.1, draw a line to show variation with frequency f of the maximum kinetic energy Ek of the emitted electrons for a second metal which has a lower work function than that in (b).

(d) The kinetic energy of the electrons is described as the maximum. Suggest why emitted electrons are likely to have a range of values of kinetic energy for any one frequency of the electromagnetic radiation.

Reference: Past Exam Paper – June 2002 Paper 4 Q7

Solution 882:
(a) Photoelectric effect

(i) A reasonable line extrapolated to 6.8×1014 Hz (allow ± 0.4×1014 Hz)

An attempt at finding the gradient.
From the gradient, a working should be shown to give 6.6×10-34 Js (allow ± 0.4×10-34 Js)

(c) The line should have the same gradient and be to the left of the line drawn (previously to find the threshold frequency).

(d) The maximum kinetic energy corresponds to electrons emitted from the surface. Other electrons require energy to be brought to the surface.

Question 883: [Work, Energy and Power]
A mass at point X inside a uniform gravitational field experiences a gravitational force of 0.200 N.
It has 1.00 J of gravitational potential energy.

The mass is then moved to point Y.
What is its new gravitational potential energy?
A 0.90 J                       B 0.94 J                       C 1.06 J                       D 1.10 J

Reference: Past Exam Paper – June 2014 Paper 12 Q14

Solution 883:
Answer: C.
Consider an object close to the surface of Earth. The gravitational field is directed downwards, towards the surface of the Earth.
At the surface, the gravitational potential energy (GPE) = 0
As the object is moved upwards, its GPE increases. That is, as the object moves against the (direction of the) gravitational field, its GPE increases.

So, when moved from X to Y, the GPE should increase. [A and B are incorrect]

GPE = mgh
Weight = mg = 0.200N
The distance h to be considered is 30cm, which is (anti-)parallel to the gravitational field.

At point X, the mass already has 1.00J of GPE.
GPE at Y = 1 + (0.2×0.3) = 1.06J

Question 884: [Options > Telecommunications]
Fig.1 shows the signal received at the aerial of a radio.

(a) State the form of modulation illustrated in Fig.1.

(b) Calculate
(i) frequency of the transmission,
(ii) frequency of the modulating waveform.

(c) On Fig.2, draw a graph to show variation with frequency f of the signal shown in Fig.1. Show appropriate values on the frequency axis.

Reference: Past Exam Paper – June 2002 Paper 6 Q14

Solution 884:
(a) Amplitude modulated

There are10 waves in 200μs.
{10 waves in 200μs. Period T = 200 / 10 = 20μs.
Frequency f = 1 / T = 1 / (20×10-6) = 50000 Hz}
Frequency f = 50 kHz

{Period of the envelope (modulating waveform) is 200μs. Frequency = 1 / (200×10-6) = 5000 Hz}
Frequency = 5 kHz

(c) The graph consists of 3 vertical lines. The carrier frequency is longer than the equal sidebands. The frequencies should be shown correctly.


  1. Replies
    1. Question 5 is on the Physics of Fluids, which is no longer in the syllabus.

      For 6/M/J/02 Q.15(a), see solution 890 at

  2. for solution 883 you have mentioned gravitational potential energy should decrease so why is A and B incorrect it is showing values less than 1. Can you please explain it would be a great help. Thanks

    1. it's a mistake. from the explanation, it's clear that it should be 'increase' instead of 'decrease'.
      It has been corrected. Thanks

    2. im still confused i mean its moving towards y so its potential energy should decrease when at x it potential energy should be more ryt?

    3. no, read the example on raising an object on Earth again. At a higher height, the object has more GPE, but the direction of the gravitational force is downwards. SO, against against the gravitational field causes the object to have more potential.

  3. In question 883, why do we consider the distance that is perpendicular (anti-parallel) to the direction of gravitational field?

    1. anti-parallel is not perpendicular. it is the opposite direction to the field.

      the reason is explained above.

    2. I'd the work done in brining the mass from x to y...


    3. no, it's as explained above


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