Elementary Charge

Summary

In this AP Physics essentials video, Mr. Andersen discusses elementary charge, the smallest unit of electric charge, determined by Robert Millikan's oil drop experiment. He explains how Millikan used a microscope and charged plates to observe oil drops influenced by gravity and electromagnetic forces. Through careful manipulation of voltage, Millikan calculated the elementary charge to be approximately 1.6 x 10^-19 Coulombs, demonstrating that charge is quantized and cannot exist in fractional amounts. The video also touches on the composition of atoms, highlighting the charges of protons, electrons, and neutrons, reinforcing the concept of charge quantization.

Takeaways

• 🔬 Elementary charge is the smallest unit of charge that an object can possess, quantified as approximately 1.6 × 10^(-19) Coulombs.
• 🧪 Robert Millikan discovered elementary charge through the oil drop experiment, where he manipulated voltage to balance oil droplets in an electromagnetic field.
• ⚡ Charge is quantized, meaning it can only exist in discrete amounts, such as 0, +1, or -1 elementary charge, and never in fractions.
• 🌧️ In Millikan's experiment, oil droplets were sprayed through an atomizer and fell through an electromagnetic field between two charged plates.
• ⚖️ The forces acting on the droplets included gravitational pull downwards, air resistance upwards, and an adjustable electromagnetic force.
• 🔋 Electrons carry a negative elementary charge, while protons carry a positive elementary charge; neutrons are neutral and have no charge.
• 🔗 Protons are composed of quarks, which have fractional charges (+2/3 for up quarks and -1/3 for down quarks), but they combine to form whole numbers of elementary charge.
• ❗ The existence of an electric charge smaller than the elementary charge has not been isolated, reinforcing the concept of charge quantization.
• 💡 Millikan's oil drop experiment was crucial in confirming the quantization of electric charge, leading to a better understanding of atomic structure.
• 📏 The precision of Millikan's measurements was impressive, being only about 1% off from the accepted value for elementary charge.

Q & A

What is elementary charge?

Elementary charge is the smallest unit of electric charge that an object can have, calculated to be approximately 1.6 x 10^-19 Coulombs.

Who determined the value of elementary charge?

The value of elementary charge was determined by Robert Millikan through the Millikan oil drop experiment.

How did Millikan conduct his experiment?

Millikan used an atomizer to create tiny oil droplets, observed them through a microscope, and adjusted the voltage between charged plates to make the droplets hover.

What does it mean for electric charge to be quantized?

Quantized charge means that electric charge exists in discrete units, meaning it can only take on certain values and cannot be in between, such as fractions of elementary charge.

What types of charges do protons and electrons have?

Protons have a positive elementary charge, while electrons have a negative elementary charge.

What is the role of air resistance in Millikan's experiment?

Air resistance affects the motion of the oil droplets as they fall, allowing Millikan to measure the balance of forces acting on the droplets.

What happens to the oil droplets when they are exposed to x-rays in the experiment?

X-rays knock electrons off the oil droplets or cause certain atoms to gain electrons, affecting their charge.

How did Millikan calculate the elementary charge?

Millikan calculated the elementary charge by balancing the gravitational force on the droplet with the upward electromagnetic force, measuring the voltage required for the droplet to hover.

Why can’t charges exist in fractions of elementary charge?

Charges are quantized, meaning they can only exist in whole multiples of elementary charge and not in fractions, like half a charge.

How do quarks relate to the concept of charge quantization?

Quarks have fractional charges (up quarks have a charge of +2/3 and down quarks have -1/3), but they combine to form protons and neutrons, which have whole number charges.