# 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 E

_{k}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 E

_{k}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

**(b)**

(i) A reasonable line extrapolated
to 6.8×10

^{14}Hz (allow ± 0.4×10^{14}Hz)
(ii)

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

**(b)**

(i)

There are10 waves in 200Î¼s.

{10 waves in 200Î¼s.
Period T = 200 / 10 = 20Î¼s.

Frequency f = 1 / T = 1 /
(20×10

^{-6}) = 50000 Hz}
Frequency f = 50 kHz

(ii)

{Period of the envelope (modulating
waveform) is 200Î¼s. 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.

6/M/J/02 Q.5(a),Q.15(a)

ReplyDeleteQuestion 5 is on the Physics of Fluids, which is no longer in the syllabus.

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

http://physics-ref.blogspot.com/2015/07/physics-9702-doubts-help-page-181.html

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

ReplyDeleteit's a mistake. from the explanation, it's clear that it should be 'increase' instead of 'decrease'.

DeleteIt has been corrected. Thanks

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?

Deleteno, 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.

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

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

Deletethe reason is explained above.