How Did Millikan Measure the Charge of an Electron?
The Millikans Oil Drop Experiment is a landmark study in physics that measured the elementary electric charge carried by a single electron. Conducted by Robert A. Millikan in 1909, this experiment offered direct evidence that electric charge is quantized, meaning it occurs in discrete units. Understanding Millikan's oil drop experiment is essential for anyone studying modern physics or learning about the structure of atoms and subatomic particles.
Purpose and Apparatus
The main aim of Millikan's oil drop experiment was to determine the value of the elementary electric charge. The experiment utilized a carefully designed setup, which included:
- A chamber with two horizontally aligned metal plates
- Oil droplets introduced between the plates
- A source of X-rays (to charge oil drops by ionizing air)
- A microscope for observing the droplets' movement
- A variable DC power supply to apply an electric field
Principle and Procedure
Millikan's oil drop experiment explained the quantization of electric charge through careful measurement. The process is as follows:
Observation of Droplets
- Tiny oil drops are sprayed into the chamber.
- In the absence of an electric field, oil droplets fall due to gravity, gradually reaching terminal velocity due to the resistance of air. Terminal velocity, $v_1$, is measured.
- Terminal velocity equation: \( v_1 = \frac{l_1}{t_1} \), where $l_1$ is the distance traveled and $t_1$ is the time taken.
Forces Acting on the Oil Drop
- Weight of the droplet $(F_G)$ acts downward.
- Buoyant force and air drag (as per Stoke’s law) oppose the motion.
- At terminal velocity in the absence of an electric field, the forces balance: $F_{\text{drag}} = F_G$.
To understand more about balanced forces in physics, visit this detailed article.
Application of Electric Field
- An electric field is applied by charging the plates.
- Charged oil drops experience an upward electric force, $F_e$, in addition to buoyancy and drag.
- If $F_e$ exceeds gravity and drag, droplets rise. Adjusting the voltage allows some droplets to become stationary, indicating balanced forces.
- The electric force: \( F_e = qE \), where $q$ is the charge on the drop and $E = V/d$ is the electric field between plates.
$V$ is the potential difference, and $d$ is the distance between the plates. The process enables calculation of the droplet's charge.
For a full understanding of how electric fields work, check this electric field resource.
Millikan's Oil Drop Experiment Formula
- Charge on an oil drop found using: \( q = \frac{mg}{E} \), where $m$ is mass, $g$ is gravity, $E$ is electric field.
- Mass of a droplet calculated using density, radius, and known forces.
- Repeated measurements show $q$ is always a multiple of \( 1.6 \times 10^{-19} \) C, confirming charge quantization.
To learn more about forces in motion and measurement, read about force and motion here.
Significance and Impact
- The value of the elementary charge $e = 1.6 \times 10^{-19}$ C was established.
- Proved that electric charge is quantized and supported atomic theory.
- Millikan’s technique set a new standard for precision in experimental physics.
Relevant Diagrams and Notes
A typical Millikan's oil drop experiment diagram shows the chamber, plates, and path of the droplets with and without the electric field. Detailed Millikan's oil drop experiment figure and class notes can aid in understanding the procedure and analysis in both class 11 and class 12.
Conclusion
The Millikans Oil Drop Experiment revolutionized our understanding of electric charge, revealing its quantized nature. By measuring the charge of an electron directly, Millikan provided a foundation for modern atomic theory and countless advancements in physics. Today, this experiment is a fundamental topic in physics courses. For comprehensive guides on related concepts, visit average velocity and force definitions on Vedantu. Understanding this ground-breaking experiment equips students with crucial insights into how scientific measurements shape our knowledge of the universe.
FAQs on Understanding Millikan’s Oil Drop Experiment
1. What is Millikan’s Oil Drop Experiment?
Millikan’s Oil Drop Experiment was a groundbreaking physics experiment that measured the fundamental electric charge of an electron. It involved balancing the gravitational and electric forces on tiny, charged oil droplets. Key points include:
- Performed by Robert A. Millikan in 1909.
- Helped determine the elementary charge (e) with great precision.
- Confirmed that electric charge is quantized.
- Used a chamber, atomizer, X-rays, and a microscope.
2. What is the principle behind Millikan’s Oil Drop Experiment?
The principle behind Millikan’s Oil Drop Experiment is balancing the weight of oil drops with the upward electrical force to calculate the charge. It operates on:
- Concept of electrostatic equilibrium.
- Gravitational force versus electric field force on a charged droplet.
- Measuring the voltage at which a droplet is suspended.
3. What apparatus was used in Millikan’s Oil Drop Experiment?
The following apparatus were used in the Oil Drop Experiment:
- An atomiser to create fine oil droplets.
- A pair of horizontal metal plates to set up a uniform electric field.
- A microscope to observe droplets.
- X-ray source to ionise air and impart charge to droplets.
4. What was the significance of Millikan’s Oil Drop Experiment?
The experiment’s significance lies in its precise measurement of the elementary charge and confirmation that electric charge is quantized. Key impacts:
- Established charge quantization.
- Provided accurate value for the charge of an electron: 1.602 × 10⁻¹⁹ C.
- Supported atomic theory and quantum physics.
5. How is the charge on an oil drop calculated in Millikan’s experiment?
The charge q on an oil drop is calculated by equating the electric force and gravitational force:
- The droplet is suspended when qE = mg.
- q = mg/E, where m is the droplet’s mass, g is acceleration due to gravity, and E is electric field strength.
- Mass is found using droplet’s measured radius and density.
6. Why was X-ray used in Millikan’s Oil Drop Experiment?
X-rays were used to ionise the air inside the experimental chamber, which allowed oil droplets to pick up extra electrons and become charged. This process enabled measurement and manipulation of droplets with different charges.
7. What is the formula used in Millikan’s Oil Drop Experiment?
The key formula for the Oil Drop Experiment is:
- q = mg/E or q = (4/3)πr³ρg / E
- q is charge, m is mass, g is gravity, E is electric field, r is radius, ρ is density.
8. State the conclusion of Millikan’s Oil Drop Experiment.
The main conclusion is that electric charge is quantized, existing only as integral multiples of the elementary charge e = 1.6 × 10⁻¹⁹ C. No fractional charges were observed, confirming the atomic theory of electricity.
9. What are the limitations of Millikan’s Oil Drop Experiment?
The experiment had certain limitations:
- It used approximations for viscosity and air resistance.
- Only small charges were measurable.
- Errors due to Brownian motion and air currents.
- Precise temperature control required.
10. Who was Robert Millikan and what prize did he receive for his experiment?
Robert Andrews Millikan was an American experimental physicist who accurately measured the charge of the electron using the Oil Drop Experiment. He was awarded the Nobel Prize in Physics in 1923 for this fundamental discovery.
11. Why is Millikan’s Oil Drop Experiment important for students to learn?
Learning about Millikan’s Oil Drop Experiment helps students:
- Understand the quantization of electric charge.
- Connect theoretical physics to experimental proof.
- Learn data analysis in physics experiments.
- Prepare for questions in CBSE and competitive exams.
