Maeen

Maeen 5 Jul 2018

Question

A lamp is placed 30 cm above a metal surface which contains atoms of diameter 20 x 10-10 m. The lamp can be considered as a point source with power 0.015 W. It may be assumed that each electron can collect energy from a circular area which has a radius equal to that of the atom

a i) Find the intensity of the electromagnetic radiation directed on the atom.

ii) Hence, calculate the power incident on each atom.

iii) Determine, on the basis of wave theory, the time required for an electron to collect sufficient energy to be emitted from the metal if the work function of the metal is 3.2 x 10-19 J

iv) Comment on this calculation with actual experimental data and conclude the validity of wave theory.


Answer

a i) If we treat the lamp as a point source that emits light in all directions (3D),

Maeen-01.jpg

Note that the area used  is the total area which the power of the lamp is spread over.

aii) Now that we know the intensity of the light incident on the atom, we can calculate the power incident on the atom.

Maeen-2-01.jpg

Note that the area used is the area of the atom which absorbs the power.

a iii) Since the electron requires a certain amount of energy to be emitted, we can calculate the time needed.

Maeen-2-02.jpg

a iv) This calculation shows that if light were to be treated as a wave, the electron will required 7.68s to collect enough energy to be emitted. However, the experimental data shows that electrons will be emitted almost immediately, with a time much less than 7.68s.

This means that the wave theory cannot be used to describe the photoelectric effect.

Maeen 20 Apr 2018

Question

A breathing monitor consists of an 8-turn coil attached to a patient’s chest. As a breath is inhaled, the area of the coil varies from 0.120 m2 to 0.124 m2. The magnetic flux density of the Earth is 50 x 10-6 T and makes an angle of 22.5o with the axis of the coil.

If the patient inhales for 1.59s, what is the average emf induced in the coil during the inhalation?

A) 0.116 μV

B) 0.126 μV

C) 0.930 μV

D) 1.01 μV


Answer

To calculate the induced emf, the magnetic flux linkage of the coil has to be calculated.

Before inhalation,

Maeen-01.jpg
Maeen-02.jpg

After inhalation,

Maeen-03.jpg
Maeen-04.jpg

The induced emf can now be calculated.

Maeen-05.jpg

Answer is C

Maeen 16 Apr 2018

Question

A displacement against position graph for a longitudinal wave is shown below. Which points represents the compressions and rarefactions?

Maeen-01.jpg

Answer

For the graph, the position axis represents the location of the equilibrium position of a particle. The displacement axis gives the displacement of the particle from its equilibrium position. For this question, we assume that positive displacement is towards the right.

For example, in the graph below, Particle A has a equilibrium position at 0.80 m and it is displaced 1.5 mm to the right of its equilibrium position.

Maeen-02.jpg

We can show the displacement on the graph:

Maeen-03.jpg

The displacements of B1, B2, B3, C1, C2 and C3 are shown.

Maeen-04.jpg

Hence, B2 is the compression and C2 is the rarefaction.