What did the oil drop experiment show about electric charge?

It showed that charge is not continuous but comes only in whole-number multiples of a single smallest amount, the charge on the electron.

An oil drop of mass 4.1 × 10⁻¹⁵ kg is held stationary between plates 6.0 mm apart with 500 V across them. Calculate the charge on the drop.

q = mgd/V = 4.1 × 10⁻¹⁵ × 9.81 × 6.0 × 10⁻³ ÷ 500 = 4.8 × 10⁻¹⁹ C.

A drop is found to carry a charge of 4.8 × 10⁻¹⁹ C. How many elementary charges does it hold?

Dividing by 1.60 × 10⁻¹⁹ C gives 3, so the drop holds three elementary charges.

How is the mass of an individual oil drop determined in the experiment?

With the field switched off the drop reaches a steady terminal velocity, and that velocity together with the viscous drag law and the density of the oil gives its radius and hence its mass.

OCR A-Level Physics B H557

Planning

Topic overview Choosing apparatus and techniques Identifying and controlling variables Judging whether a method will work

Implementing

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Analysis

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Evaluation

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Our Place in the Universe

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Charge and Field

Topic overview The uniform electric field Radial fields and the inverse square law Electric potential and potential energy Field and potential graphs Field lines and equipotential surfaces The electronvolt and accelerating charges Millikan's experiment and the discreteness of charge Moving charges in magnetic fields Comparing electric and gravitational fields

Probing Deep Into Matter

Topic overview Particle accelerators and high-energy beams Scattering and the nuclear atom Paths of scattered particles and closest approach Discrete energy levels in atoms Electron standing waves and the confined electron Quarks, hadrons and leptons Conservation laws and nuclear equations Charged particles in detectors and magnetic fields Relativistic energy and the relativistic factor

Ionising Radiation and Risk

Topic overview The nature of ionising radiations Effects on living tissue and radiation safety Absorbed dose, effective dose and risk Binding energy and nuclear stability The Nuclear Valley and modes of decay Fission, fusion and nuclear power Activity, half-life and the decay constant Investigating absorption and half-life

Charge and Field

Millikan's experiment and the discreteness of charge

3 things to know
Components 01, 02 and 03

Overview

Recall questions on what the oil drop experiment showed, its apparatus, q = mgd/V for a stationary drop and a worked calculation, counting elementary charges, finding a drop's mass from terminal velocity, why oil is used, how drops are charged, the evidence for quantised charge, the elementary charge, measuring many drops, balancing a drop, and a doubtful result.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Outline the apparatus used in the oil drop experiment.

Key term

Write the relationship used to find the charge on a drop that is held stationary between the plates.

Key term

Why is oil used rather than water in Millikan's experiment?

Questions from this subtopic

Each one with the model answer underneath.

1
What did the oil drop experiment show about electric charge?
Model answer
It showed that charge is not continuous but comes only in whole-number multiples of a single smallest amount, the charge on the electron.
2
Outline the apparatus used in the oil drop experiment.
Model answer
Fine oil drops from an atomiser fall into the space between two horizontal parallel plates connected to a variable high-voltage supply, and individual drops are watched through a microscope.
3
Write the relationship used to find the charge on a drop that is held stationary between the plates.
Model answer
The electric force balances the weight, so qV/d = mg and therefore q = mgd/V.
4
An oil drop of mass 4.1 × 10⁻¹⁵ kg is held stationary between plates 6.0 mm apart with 500 V across them. Calculate the charge on the drop.
Model answer
q = mgd/V = 4.1 × 10⁻¹⁵ × 9.81 × 6.0 × 10⁻³ ÷ 500 = 4.8 × 10⁻¹⁹ C.
5
A drop is found to carry a charge of 4.8 × 10⁻¹⁹ C. How many elementary charges does it hold?
Model answer
Dividing by 1.60 × 10⁻¹⁹ C gives 3, so the drop holds three elementary charges.
6
How is the mass of an individual oil drop determined in the experiment?
Model answer
With the field switched off the drop reaches a steady terminal velocity, and that velocity together with the viscous drag law and the density of the oil gives its radius and hence its mass.

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