Millikan's oil drop experiment explained

Summary

In this educational collaboration between Cricket Science and the University of Sydney, Dr. Sean Crook and Tom Gordon explain Millikan's famous oil drop experiment. The video covers both a simplified version for high school students and a deeper dive into the actual methods used by Millikan to measure the charge of the electron. Through demonstrations and clear explanations, viewers learn about the physics behind balancing gravitational and electric forces to calculate electron charge. Despite complications, Millikan's results closely match the accepted value, though some controversy arose later regarding data selection. This video offers both practical insights and historical context into a key scientific experiment.

Takeaways

• 😀 Millikan's oil drop experiment, conducted in 1909, was pivotal in determining the actual charge of the electron and proving that charge is quantized.
• 😀 The video collaborates between Cricket Science and the University of Sydney to demonstrate and explain Millikan's experiment at both high school and deeper academic levels.
• 😀 The experimental setup involves silicon spheres, a light box, telescope, webcam, and an electric field to suspend the charged particles for analysis.
• 😀 In high school physics, simplifications are made by assuming the mass of the particles is known, which allows students to calculate the charge directly using the formula Q = (M * G * D) / V.
• 😀 Millikan’s original experiment didn't know the mass of the oil drops, and he used methods like determining the terminal velocity of particles to calculate their mass and charge.
• 😀 Millikan’s experiment required measuring the velocity of the droplets both going up and down under different conditions to determine the forces acting on them, including gravity, buoyancy, drag, and electric forces.
• 😀 The electric field in Millikan’s setup counteracted gravity, and by balancing these forces, Millikan could calculate the charge of the droplets.
• 😀 Millikan's results showed that the charge on an electron is quantized, meaning it can be expressed as a whole number multiple of a fundamental charge.
• 😀 Millikan’s experimental charge value was 1.59 x 10^-19 Coulombs, close to the modern accepted value of 1.602 x 10^-19 Coulombs.
• 😀 There was controversy surrounding Millikan’s methods, especially regarding his estimation of air viscosity and his selective data usage, which sparked debates on scientific integrity.
• 😀 Despite criticisms, Millikan’s work remains a cornerstone in physics, and the video highlights the importance of transparency and accuracy in scientific data collection and analysis.

Q & A

What is the main focus of Millikan's oil dropping experiment?

The main focus of Millikan's oil dropping experiment is to determine the charge of an electron by observing oil droplets suspended in an electric field and measuring their behavior in the presence of both gravitational and electric forces.

How did Millikan determine the charge of the electron in his experiment?

Millikan determined the charge of the electron by balancing the forces on oil droplets. The force due to the electric field counteracts the force of gravity, allowing him to calculate the charge on the droplets by measuring their velocity and applying the appropriate formulas.

What role does the electric field play in Millikan's experiment?

The electric field in Millikan's experiment applies a force on the charged oil droplets, which either lifts them upwards or causes them to fall, depending on the direction of the field. By adjusting the strength and polarity of the electric field, Millikan could suspend droplets in equilibrium, helping to determine their charge.

What assumption is made about the mass of the oil droplets in high school physics versions of the experiment?

In high school physics versions of the experiment, it is assumed that the mass of the oil droplets is known, which simplifies the calculations and allows students to focus on determining the charge without needing to measure the mass directly.

Why is the viscosity of air important in Millikan's oil dropping experiment?

The viscosity of air is important because it affects the drag force experienced by the oil droplets as they move through the air. This force must be taken into account when calculating the velocity of the droplets and, consequently, the charge on the droplets.

What was the controversy surrounding Millikan's experiment and his results?

The controversy surrounding Millikan's experiment stemmed from accusations that he selectively used only 'valid' data, potentially excluding results that deviated from the expected value. Some scientists criticized this practice, suggesting that it led to a more accurate but less objective result.

How did Robert Millikan's results compare to the modern accepted value for the charge of an electron?

Millikan's results were surprisingly close to the modern accepted value for the charge of an electron, which is approximately 1.602 × 10⁻¹⁹ coulombs. His measured value was 1.59 × 10⁻¹⁹ coulombs, which was an impressive result for the time.

What is the significance of the quantization of charge observed by Millikan?

The quantization of charge observed by Millikan meant that the charge on the oil droplets was always an integer multiple of a fundamental charge, the charge of a single electron. This was crucial in proving that charge is quantized and paved the way for further advancements in understanding atomic and subatomic particles.

Why was the charge on an electron difficult to measure before Millikan's experiment?

Before Millikan's experiment, the charge on an electron was difficult to measure because it was extremely small and could not be directly observed. Millikan's method of suspending charged droplets in an electric field allowed him to isolate and measure this tiny charge more accurately.

How did Millikan measure the velocity of the oil droplets in his experiment?

Millikan measured the velocity of the oil droplets by tracking their displacement over time as they moved upward or downward in the electric field. He then used this velocity, along with other known factors like the size of the droplets and the forces acting on them, to calculate the charge.