Physics Practical/Lab Report St Philips College Stage 2 Physics 2019 Completion Practical Using a Photo-electric apparatus (see below for extensive notes on this) to measure the maximum kinetic energy of emitted electrons from a material which is irradiated with light of different frequencies

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Physics Practical/Lab Report

St Philips College

Stage 2 Physics 2019 Completion Practical

Using a Photo-electric apparatus (see below for extensive notes on this) to measure the maximum kinetic energy of emitted electrons from a material which is irradiated with light of different frequencies. Use these results to determine Plank’s constant and the work function of the irradiated material.

NOTE: Practical will be assessed according to performance standards at end of this task sheet.

Part A

1. Research the photoelectric effect (see below).
    
2. Familiarize yourself with the operation of the photo-electric apparatus.

Part B

Completion Experiment

The Photo-electric effect and determining Plank’s Constant and Work Function of a material

Some Theory:

A photon of light has energy ‘hf’, as proposed by Einstein in 1905, where ‘h’ is a constant and ‘f’ is the frequency in Hertz of the radiated emission.

In the Photo-Electric effect, a photon gives up all its energy to an electron in the surface of the illuminated material. The energy is used for three purposes:

1. 1. Bringing the electron to the surface of the metal
   2. Freeing the electron from the metal’s electrostatic attraction. The energy required to do all this is called the ‘Work Function’ ‘W’ of the metal (in this case Caesium & Antimony alloy, see below) in the phototube. (chemical symbols Cs & Sb).
   3. Providing Kinetic energy to the free electron.

 

It follows then that for a given frequency, the most energetic electrons at the surface of the metal have kinetic energy ‘T’ where:                            T (max) = hf - W                                                          Where ‘W’ is the ‘Work Function’, or the minimum amount of energy required for an electron to be released from the surface of the metal.

The constant ‘h’ was first determined by Maxwell Planck so it is known as Planck’s Constant. In the experiment, we must know the UPPER frequency of the light striking the metal surface in the phototube. The long wave pass coloured glass filters provided (see below) have data information on them that advises this wavelength. Remember: short wavelength means high frequency.

Each Photon will lose all its energy to an electron in the surface of the metal and the maximum kinetic energy of the electrons can be determined by applying a reverse voltage to the tube so that a retarding electric field JUST completely stops the most energetic electrons from reaching the anode. This reverse voltage is called the ‘Backing Voltage’.

If this ‘Backing Voltage’ has a value ‘V’, then the energy supplied by the electric field in stopping the emitted electrons from reaching the anode is ‘eV’, where ‘e’ is the charge on the electron and ‘V’ is the backing voltage. This energy equals the kinetic energy of the electrons, so since  T (max) = eV                                      then it follows that                                                           eV = hf - W

For different wavelengths of light, a graph of ‘T’ as a function of ‘f’ can be plotted. Its

gradient will be Plank’s constant and the y-intercept the work function.

 

FREQUENCY   Hz x1014

Notes on sample graph.

An example of experimental readings. The results indicate a good set of figures that will give a very accurate result. These may not be typical and note that the backing volts have not been converted to energy and thus the slope is not Plank’s constant (but is related).

Typically the slope of the graph will be flatter than this example and therefore the result will be less accurate.

 

BUT: The purpose of these experiments is to teach the principles and the relationships between wavelengths and energy levels. Achieving the precise value calculated for Planck’s Constant using a Photo Tube is not so important.

 

A TYPICAL RESULT:   Due to differences from tube to tube, internally reflected light from the cathode on to the anode causing electrons to be emitted and other factors beyond our control (eg the tube does not perform perfectly; there are errors; electron collisions inside the tube; anode emission; the long wave pass filters are not perfect etc etc), the gradient of the line of best fit will not reproduce the accepted value of h.

Part of this practical is to identify the possible sources of error and analyze what effect they have on the results.

 

Apparatus and Method:

 

THE  ‘IEC’ PHOTO-ELECTRIC UNIT

A bench mounting instrument with a digital meter to indicate either the current through the internal photo-cell or the backing voltage applied to the cell (selected via switch).

 

The photo tube in this model is illuminated by a 12V light source which is filtered through a set of coloured glass long wave pass filters. There are two sets of filters, one with a grey boarder and one with a white boarder.

 

 

When 12V. AC or DC is applied to the instrument it is automatically ON. There is no ON/OFF switch. When ON, the digital displays will show digits.

Controls:

• ‘BACKING VOLTS”– COARSE / FINE ROTARY CONTROLS: Adjusts the DC volts applied to the anode and cathode of the photo cell attempting to completely stop the flow of electrons from the cathode to the anode. The value of this voltage is a measurement of the energy level of the electrons that relate to the selected wavelength of the light.                   The “coarse” control adjusts the voltage rapidly.

The “fine” control adjusts the voltage more slowly to accurately determine exactly zero current flow (zero electrons reaching the anode).

1 nA (nanoamp) is 1x 10^-9 amps.     (this is 1/1000^th of 1 microamp)

The digital ammeter reads to the very small current of 0.1nA.

• ‘VOLTS” AND “NANOAMPS” METERS (selected via switch):                             The VOLT meter displays the value of the backing voltage required to bring the photo-cell current to exactly zero. This voltage value is used in the graph against the frequency of the light to determine Planck’s Constant.

The NANOAMPS meter displays the small current passing through the photo-cell in either milliamps or nanoamps (as selected) down to 0.1 nanoamps (amps x10^-10).

• ‘EXPERIMENT SELECTION: 1 OR 2:      Exp.1 should be selected.

 

  

 

THE PHOTOTUBE:

The phototube is the essential part of the instrument. It is an evacuated glass tube containing an electrode shaped like half of a cylinder. At the open mouth of this electrode, another electrode, usually in the form of a straight rod, is positioned at approximately the focal point of the curved surface.                                            Some anodes are in the shape of a rectangular frame.

 

 

 

Light Source and filter
Shield on the glass to shade the anode

 

 

 

 

Phototube glass envelope

 

 

Cathode coated with caesium & antimony on silver oxide.

 

Metal anode

 

Light beam

 

 

THE ANODE: Light enters the tube to illuminate the cathode. The anode is usually a metal rod to which the electrons flow and it too is illuminated because it stands in front of the cathode. This bombardment of the anode with photons causes the metal surface of the anode rod to release some electrons (an unwanted Photo Electric Effect). Light is reflected also from the glass envelope and from the curved cathode surface itself back on to the anode rod. This anode emission is reduced because it can spoil the migration of electrons from the cathode surface to the anode and therefore create an error when measuring the exact backing voltage required to stop the electron flow from cathode to the anode.

To reduce this error, a shield is usually fitted to the glass envelope of the tube to shield the anode rod from direct light. The reflections inside the tube cannot easily be avoided.

 

 

THE CATHODE:      The curved surface of the cylinder is called the Cathode and is coated with a special compound that easily releases electrons when light (or photons) strike the curved surface. This coating is usually a ceasium (cesium in the USA) and antimony alloy on silver oxide.

When light strikes the metallic surface, the energy contained in a light Photon is passed to an electron which must first rise to the surface of the cathode material, then overcome the tendency to remain on the surface and finally burst off the surface to travel through the vacuum towards the anode rod.

While light is falling on the cathode, this is occurring billions of times per second and thus an extremely small current is constantly flowing between the cathode and the anode.

This current can be several millionths of an amp (microamps).

 

ELECTRONS INSIDE THE PHOTOTUBE:

 

 

 

Light beam                                                                                              After the Photons have

bombarded the metallic

surface of the cathode, electrons are driven off and pass from the surface of the cathode to the anode

Shield on the glass to shade the anode

 

 

 

 

THE PURPOSE OF THE PHOTON ENERGY EXPERIMENT:

To measure the energy of Photons, we need to detect but do not need to measure the current through the phototube. Photons of different energy excite electrons to different energies. The experiment is to determine the highest energy level to which the electrons have been excited by the Photons. There will be only a small percentage of electrons flowing from cathode to the anode with these highest energies and we apply a small reverse voltage (anode negative and cathode positive) JUST high enough to completely stop the flow of electrons. This reverse voltage is called the Backing Voltage and we need to know the voltage that just stops the last few electrons (those with the highest energy of all).

When ALL the electrons stop flowing (absolutely zero current) the voltage has repelled the electrons, including the ones with the highest energy acquired from the photons. It is the energy level of the highest energy electrons that interest us for the following reason:

Each long wave pass filter allows only light LONGER than the wavelength marked on it to pass through.  We are finding the energy of the photons of the shortest wavelengths (highest frequency) light for each filter. The lower frequencies of light produce photons of lower energy and these are stopped as the backing voltage rises. The highest energy electrons are stopped JUST at the moment that the current is absolutely zero.

It is important to find EXACTLY the backing voltage that is just high enough to stop the last microscopic flow of current. With zero backing voltage, the current flow through the tube may be several microamps. As the backing voltage is increased, the current through the tube reduces to less than a microamp and when it falls to 000.0nA, if the backing voltage is increased more, the current will begin to flow backwards because electrons begin to flow from the anode to the cathode.

NOTE:    1 microamp is 1x10^-6 Amp.      1 nanoamp is 1x10^-9 Amp.

 

 

 

Method:

With one of the filters in place we observe the current flowing and we find the backing volts that JUST stops the phototube current. We repeat with each filter then plot the backing volts against the highest frequency of light that comes from each filter.

• See the tube current on the NANOAMPS meter and increase the backing voltage to obtain exactly zero current flow. We use the COARSE and then the FINE controls to get the exact point when the current is at zero.
• When the NANOAMPS is at exactly zero, observe the BACKING VOLTS meter and note the value.
• Note the wavelength of the filter selected and the backing voltage required for that particular wavelength.
• Select the next filter and repeat the exercise until all filters have been used.
• Then convert each wavelength to frequency in Hz and plot a graph of frequency (in Hz) on the X axis against the energy (in Joules) on the Y axis.
• It will be discovered that the line drawn through the points plotted on the graph is a straight line and that a value for Plank’s constant and the work function of the material in the phototube can be obtained.

 

 

 

Notes on running the experiments: step by step:

1. Run the light source at 12 V from a standard laboratory power supply.

2. At the rear of the instrument, slide the light source into position. A short cover extends to the instrument to ensure that unwanted light cannot reach the Photo Cell. An aperture (supplied) may also be useful here.

3. See the NANOAMPS flowing through the tube.                                                              Set the FINE control about half way around its travel. Turn the COARSE control clockwise to increase the backing voltage to reduce the cell current to something close to zero. Then use the FINE control to carefully adjust the backing voltage to achieve exactly zero NANOAMPS. If the display shows a negative sign, you have gone past zero and current if flowing backwards. Turn the control anticlockwise to reduce backing volts. If the display reads 000.0 nA, it is OK if the negative sign appears and disappears repeatedly.

4. Take note of both the wavelength of the filter selected and the backing voltage reading. NOTE:: The wavelength of each filter is indicated on it. Also note if it has a grey or white boarder.

5. Select the next filter and repeat the experiment from 3) to 5) .

Good Luck!

Part C

Write up your experiment using “HA HA MR DC”.

 
The task will be assessed using the performance standards at the end of this task sheet.
 

SACE School:   761

Performance Standards for Stage 2 Physics Photo-Electrical Practical