Unit 2: Statistical & Plasma Physics (Lab: PHYDSC353P)
Table of Contents
1. Laboratory Objectives
The objective of this unit is to bridge theoretical concepts of Statistical Mechanics and Plasma Physics with experimental observations. You will focus on measuring fundamental constants and understanding the behavior of ionized gases.
2. Estimation of Boltzmann Constant
The Boltzmann constant (k) can be estimated using the V-I characteristics of a P-N junction diode.
By plotting ln(I) vs. V in the forward bias region, the slope of the resulting straight line allows for the calculation of k.
3. Verification of Stefan's Law
Stefan's Law states that the total power (P) radiated by a blackbody is proportional to the fourth power of its absolute temperature (T).
In the lab, this is often verified using an electric bulb (incandescent lamp). By measuring the resistance of the filament at different voltages, the temperature can be determined, and the relation P \propto T4 can be checked.
4. Determination of Planck's Constant
This is typically performed using the Photoelectric Effect. By using filters of different colors (wavelengths) and measuring the Stopping Potential (V0), Planck's constant (h) is determined from the slope of the V0 vs. frequency (f) graph.
5. Langmuir Probe Characteristics
A Langmuir Probe is a small electrode inserted into a plasma. By varying the potential applied to the probe and measuring the current collected, one can determine the local properties of the plasma.
The I-V Curve:
- Ion Saturation Region: Probe is highly negative; collects only ions.
- Electron Transition Region: Probe potential increases; electrons begin to overcome the repulsion.
- Electron Saturation Region: Probe is positive; collects primarily electrons.
6. Determination of Plasma Parameters
From the Langmuir probe data, key Plasma Parameters can be calculated:
- Electron Temperature (Te): Derived from the slope of the transition region in the ln(Ie) vs. V plot.
- Electron Density (ne): Calculated from the electron saturation current.
- Plasma Potential (Vp): The potential at which the probe no longer affects the plasma.
7. Study of Glow Discharge
A Glow Discharge is a plasma formed by the passage of electric current through a low-pressure gas. In this experiment, you observe the distinct luminous regions:
Key Observation: As pressure decreases, the "dark spaces" expand, and the "positive column" may exhibit striations (alternating bright and dark bands).
Lab Exam Focus Corner
Frequently Asked Questions
- Why is a vacuum pump necessary for plasma experiments? Plasma is easily quenched by collisions with neutral molecules at atmospheric pressure; a vacuum allows the mean free path to be long enough for ionization to occur.
- In Stefan's law, why is the resistance of the bulb used? The resistance of a metal filament increases with temperature. By knowing the resistance at room temperature (R0), we can use the temperature coefficient of resistance to find T.
Common Mistakes
- Temperature Conversion: Forgetting to add 273 to Celsius values when verifying Stefan's Law. The law only works with Kelvin.
- Probe Contamination: In Langmuir probe experiments, if the probe is dirty, the work function changes, leading to errors in the I-V curve.
Practical Tips
Tip: When plotting the Langmuir probe data, use a semi-log plot for the electron current. The linear part of this plot is the most reliable way to find the Electron Temperature (Te).