7/30/2020
PHOTOELECTRIC EFFECT
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1. Explain the term “work function” (1mk)
Minimum amount of energy
... [Show More] needed to dislodge an electron from the surface of a
metal.
2. Define the term monochromatic light. (1mk)
Light or radiation of a single wavelength or frequency.
3. What do you understand by the term photoelectric effect? (1mk)
The process by which electrons are emitted from metal surface when illuminated
with electromagnetic radiation of sufficient frequency.
4. Distinguish between thermionic emission and photoelectric effect. (1mk)
Thermionic emission is the process of emitting electrons from the metal surface due to
heat energy while photoelectric effect is the process of emitting electrons from metal
surface by electromagnetic radiation of sufficient frequency.
5. Give one application of photoelectric effect(1mk)
Applied in: i) Photocells i.e. solar panels
ii) Emissive cells used in conveyor belts.
6. State two factors that affect photoelectric emission from a given metal surface.
i) Intensity of radiation
ii) Energy of radiation
iii) Type of metal
7. Name one factor that determines the velocity of photoelectrons produce on a metal
surface when light shine on it. (1mk)
Frequency of the incident radiation.
Material of the surface
8. Photoelectrons emitted by illuminating a given metallic surface constitute a
“photocurrent”. What is the effect of increasing the intensity of the illumination of the
magnitude of the photocurrent?
Increase in magnitude of photocurrent.
9. A monochromatic beam of radiation is directed on a clean metal surface so as to
produce photoelectrons. Give a reason why some of the ejected photoelectrons have
more kinetic energy than others.
An electron is bounded with nucleus of the metal atom with coulombic interaction.
When a photon is incident on the metal surface, some of the energy of the photon is
used up to overcome coulombic interaction and the remaining energy becomes the
kinetic energy of the emitted photo electrons.
10. Name a device used to convert light energy directly into electric energy.(1mk)
Photocell
11. State two uses of photocell (2mk)
Used in solar panels
Used in burglar alarm
12. What energy conversion occurs in a photocell? (1mk)
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13. What is meant by photo-voltaic effect? (1mk)
Photo-voltaic effect is the process that generates voltage or electric current in
a photo-voltaic cell when it exposed to sunlight.
14. You are provided with a clean Zinc plate, a leaf electroscope, a charging rod and a
source of ultra-violet (U.V.). Describe briefly with the aid of a diagram how photoelectric effect may be demonstrated. (5mks)
15. Sketch on the same axes the graphs of photo-electric current (y-axis) against
the voltage for two different intensities A1 and A2 of ultra-violet where A1> A2
(3mk)
16. Describe with the aid of a labeled diagram an experimental set-up for observing the
photoelectric effect.
Negatively charged Ultra violet light
zinc plate
Gold leaf falls immediately the zinc
plate is illuminated with ultra violet
light
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17. The figure below shows ultra violet light striking a polished zinc plate placed on a
negatively charged gold leaf electroscope.
Explain the following observation
i) The leaf of the electroscope falls.
Photoelectrons emitted from negatively charged zinc plate are repelled and
electroscope becomes discharged causing the lf divergence to fall.
ii) When the same experiment was repeated with a positively charged, the leaf did not fall.
Photoelectrons emitrted from positively charged zinc plate are attracted back
i.e they do not escape. Thus leaf divergence remains.
18. A candle is used to heat the cap of a charged electroscope as shown in the figure below.
Explain the following observation
i) It is observed that the electroscope leaf falls after sometime. Explain why the leaf falls
after some time.
Heat from the candle causes emission of electrons from the cap of charged
electroscope. Emitted electrons repelled by negative charges discharges the
electroscope causing the electroscope leaf to fall.
ii) State with a reason the likely charge of the electroscope.
Negatively charged. Negative charges repel emitted electrons discharging the
electroscope.
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1. The figure below show a circuit diagram for a photocell.
a) Name the parts A and C
A – Anode
C - Cathode
b) Why is the micro ammeter showing a deflection when ultraviolet light is shown on the
photocell?
When UV is illuminated on photocell, photoelectrons are emitted. Emitted
photoelectrons are attracted toward plate A, completing the circuit,
photocurrent flows hence deflection on the micrometer.
c) Explain how the millimeter reading is affected when the intensity of light is increased.
Micrometer reading increases/Deflection on micrometer increases
d) State the factors that determine the velocity of the emitted electrons (2mk)
Energy of radiation
Work function of the metal.
2. The diagram shows a photocell in action
i) The photocell is either evacuated or filled with an inert gas at low pressure. Give one
reason for this.
To ensure that all the electrons emitted by the cathode reach the anode to
maximize the photocurrent.
ii) What is the function of the resistor R in the circuit?
To lower/control the current in the circuit.
iii) State the reason for using a particular radiation such as ultraviolet for a given photocell. [Show Less]