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Form 4 Physics Photoelectric Effect Questions and Answers
State one application of thermionic emission.
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1.
An electron in an excited atom falls from energy levels #E_2# to energy level #E_1#. Write an equation relating the energy change to the frequency f, of the radiation emitted. Explain the symbols used.
2.
In an experiment on photo-electricity using metal X, the graph shown in fig. 9 was obtained. Use the graph to answer questions that follow. Sketch on the same axis, a graph for a metal Y, whose work function is higher than metal X.
3.
Electrons emitted from a metal when light of a certain frequency is shone on the metal are found to have a maximum energy of 8.0 x #10^-19#J. If the work function of the metal is 3.2 x #10^-19#J, determine the wavelength of the light used.
4.
Fig. 4 shows a photocell. (i) Label the cathode and anode. (ii) How are electrons produced in the cell? (iii) Draw a simple circuit including the photocell to show the direction of flow of current. (iv) Calculate the photon energy in ultraviolet radiation whose frequency is 8.60 x #10^14# HZ. (Plank’s constant, h=6.63 x #10^-34#J).
5.
It is observed that when ultra violet light is shone onto a clean zinc plate connected to the cap of negatively charged leaf electroscope, the leaf collapses. Explain this observation.
6.
In figure 18, ultra-violet (u.v) light falls on a zinc plate placed on a charged leaf electroscope. It is observed that the leaf collapses. Explain how this observation may be used to determine the type of charge on the electroscope.
7.
Light of a certain wavelength strikes the surface of a metal. State what determines the maximum kinetic energy of the electron emitted.
8.
(a) Figure 8 shows ultra-violet light striking a polished zinc plate placed on a negatively charged gold-leaf electroscope. Explain the following observations: (i) The leaf of the electroscope falls. (ii) When the same experiment was repeated with positively charged electroscope, the leaf did not fall. (b) State two factors which determine the speed of photoelectrons emitted by a metal surface.
9.
The minimum frequency of radiation necessary to cause photoelectric effect on a certain metal surface in 9.06 x #10^14# Hz. Determine the work function of the metal. (Planks constant h=6.63 x #10^-34# Js).
10.
The work function of a certain material is 3.2 eV. Determine the threshold frequency for the material. (1 electron Volt (eV) =1.6 x #10^-19# J and Planks Constant h=6.62x#10^-34#Js).
11.
(a) (i) What is photoelectric effect? A photocell, a source of uv light, a rheostat, a source of e.m.f, a milliammeter, a voltmeter and connecting wires. Draw a circuit diagram to show how photoelectric effect may be demonstrated in the laboratory.
12.
Explain how an ‘excited’ hydrogen atom is able to emit radiations of different wavelengths.
13.
What is meant by the term work function?
14.
Use the information given below to answer question that follow. The kinetic energy (K.E) of an electron, ejected from the surface of a metal illuminated by radiation of frequency, f is given by K.E =hf-Ø, where h is Planck’s constant and Ø is the work function of the surface. If the frequency of the illuminating radiation is just equal to the threshold frequency of the surface, explain why no phot
15.
The graph in figure 8 shows the variation of photoelectric current with applied voltage when a surface was illuminated with light of a certain frequency. Use the information in the figure to answer questions that follow. a) On the same axis, sketch the graph when light of higher intensity but same frequency is used to illuminate the surface. b) Explain your answer in question (a) above.
16.
(a) It is observed that when ultra-violet radiation is directed onto a clean zinc plate connected to the cap of a negatively charged leaf electroscope, the leaf falls. (i) Explain this observation. (ii) State why this observation does not occur if the electroscope is positively charged. (iii) Explain why the leaf of the electroscope does not fall when infra-red radiation is directed onto the zinc
17.
State the meaning of the term “threshold frequency” as used in photo electric emission.
18.
A photon of ultraviolet light having energy E falls on a photo emissive surface whose work function is T. write an expression for the maximum kinetic energy of the resulting photoelectron in terms of E and T.
19.
It is observed that when the cap of an uncharged electroscope is irradiated with light of high frequency, the leaf of electroscope rises. Explain this observation.
20.
State one application of thermionic emission.
21.
(a) State the meaning of the following terms: (i) Photoelectric effect. (ii) Threshold frequency. (b) Figure 5 shows some light incident on the cathode of a photocell. The photocell is connected in series with a battery, variable resistor and micro ammeter. (i) Explain how the micro ammeter reading is affected when the intensity of the incident light increased. (ii) State the reason why the tube
22.
(i) State two factors that determine the speed of the photoelectrons emitted from a metal surface.(ii) The energy of a photon of light is 2.21 eV. (Electronic charge is 1.6 x#10^-19#C and planks constant h is 6.63 x #10^-34# Js). (I) Express this energy in joules. (II) Determine the frequency of the light that produces the photon.
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