pHYSICS

BSPY205

Experiment 2

Aim of the Experiment

To determine work function from the stopping voltages measured at different frequencies (wavelengths) of light using photoelectric effect experimental setup.

Principle

When a photon with frequency f strikes the surface of a metal, it imparts all of its energy to a conduction electron near the surface of the metal. If the energy of the photon (hf) is greater than the work function (φ), the electron may be ejected from the metal. If the energy is less than the work function, the electron will simply acquire some kinetic energy that will dissipate almost immediately in subsequent collisions with other particles in the metal. By conservation of energy, the maximum kinetic energy with which the electron could be emitted from the metal surface, is related to the energy of the absorbed photon hf, and the work function φ, by the following relation:

Forward Polarity: When electrons are emitted by a target material, they are slowed down as they approach the anode (collector plate), and at least some of their kinetic energy is converted into potential energy.

If the potential energy at the anode is less than the kinetic energy of the electrons, there is a current through the tube. This, however, is possible only when photons are energetic enough to eject electrons from the target material, and the corresponding value of minimum frequency at which emission occurs is called threshold frequency. If we increase the potential at the anode, the current then increases, as stray electrons that could not get into the anode will now be pulled into it.

However, when all electrons emitted from the cathode are captured by the anode at a certain potential, increasing the anode potential further does not cause any further increase in the current. This is called Saturation potential.

Reversed Polarity: We can make the collector plate negative, so that it repels electrons.

Now, in this case, if the potential energy at the anode is less than the the kinetic energy of the electrons, some of the electrons are driven back to the cathode, and hence, there is reduction in current as potential on plate increases in reverse direction.

However, if the potential energy at the collector plate is made is much larger than the kinetic energy of the electrons, all of the electrons are driven back to the cathode, and hence, there is no current. Minimum value of potential of anode at which this happens is called Stopping potential.

Materials required

Photoelectric effect setup consisting of a tungsten-halogen light source with five different colour filters, a Cesium-type vacuum phototube, a built-in power supply and a current multiplier. The base of the phototube is built into a dark room and in front of it a receptor (pipe) is installed to mount filters.

Planck's Constant - 1.mp4

Experimental setup, and Safety instructions

Planck's Constant - 2.mp4

Procedure of Data Acquisition

Procedure

  1. Plug in and switch on the apparatus using the red button at the bottom right corner of the set up.

2. Before the lamp is switched on, put the toggle switch in current mode and check that the dark current is zero.

3. Turn on the lamp source (it may take 5-10 mins. to warm up). Set the light intensity near to maximum. Note that the intensity should be such that the value of current should not exceed the display range. In case it happens, you need to reduce the intensity. You should not change intensity while taking data.

4. Insert one of the five specified filters into the drawtube of the receptor.

5. Now, set the voltage direction switch to “-ve” polarity. Adjust the voltage knob at minimum and current multiplier at X 0.001. Vary the voltage and record the current till the value of current becomes 0. Use the display mode switch to record the values of voltage and the corresponding current each time.

6. Fill up the observation tables and draw necessary plots. Determine the values of work function of the metal used in the phototube.

Observation Table 1 (Wavelength of optical filters)

Observation Table 2

Calculation


  1. Plot Vo v/s f curve, and calculate intercept (which will be the work function of the target material), as per the equation for maximum kinetic energy of the electrons mentioned above.

Result

  1. Work function is __________ eV.

Questions

  1. How does intensity of light vary with distance from the source?

  2. Discuss experimental errors in this experimental technique.

  3. Which materials exhibit Photoelectric effect? Why?


Developed by

Dr. Poonam Sharma

Assistant Professor, Physics

poonam.sharma1@gsfcuniversity.ac.in


Mr. Dhairya Patel

Teaching Assistant , Physics

dhairya.patel@gsfcuniversity.ac.in