In Millikan's experiment, in which direction does the electric force act on a negatively charged oil droplet held stationary between the plates?

The electric force acts upward on the negatively charged droplet.

When an oil droplet is held stationary in Millikan's apparatus, in which direction does the gravitational force act?

The gravitational force acts downward.

What is the condition for the forces acting on a charged oil droplet when it is held stationary between the parallel plates?

The electric force and gravitational force are balanced (equal in magnitude and opposite in direction).

State the equation for the equilibrium condition of a stationary charged oil droplet between parallel plates, and define each symbol.

QV/d = mg, where Q is the droplet charge, V is the potential difference between plates, d is the plate separation, m is the droplet mass, and g is gravitational field strength.

Calculate the charge on the droplet when mass of droplet = 5.44 × 10⁻¹⁵ kg, gravitational field strength = 9.81 m s⁻², plate separation = 0.006 m, potential difference = 1000 V.

Charge on the droplet = (5.44 × 10⁻¹⁵ × 9.81 × 0.006) ÷ 1000 = 3.20 × 10⁻¹⁹ C.

AQA A-Level Physics 7408

Measurements and Their Errors

Topic overview Use of SI units and their prefixes Limitation of physical measurements Estimation of physical quantities

Particles and Radiation

Topic overview Constituents of the atom Stable and unstable nuclei Particles, antiparticles and photons Particle interactions Classification of particles Quarks and antiquarks Applications of conservation laws The photoelectric effect Collisions of electrons with atoms Energy levels and photon emission Wave-particle duality

Waves

Topic overview Progressive waves Longitudinal and transverse waves Principle of superposition of waves and formation of stationary waves Interference Diffraction Refraction at a plane surface

Mechanics and Materials

Topic overview Scalars and vectors Moments Motion along a straight line Projectile motion Newton's laws of motion Momentum Work, energy and power Conservation of energy Bulk properties of solids The Young modulus

Electricity

Topic overview Basics of electricity Current–voltage characteristics Resistivity Circuits Potential divider Electromotive force and internal resistance

Further Mechanics and Thermal Physics

Topic overview Circular motion Simple harmonic motion (SHM)Simple harmonic systems Forced vibrations and resonance Thermal energy transfer Ideal gases Molecular kinetic theory model

Fields and Their Consequences

Topic overview Fields Newton's law Gravitational field strength Gravitational potential Orbits of planets and satellites Coulomb's law Electric field strength Electric potential Capacitance Parallel plate capacitor Energy stored by a capacitor Capacitor charge and discharge Magnetic flux density Moving charges in a magnetic field Magnetic flux and flux linkage Electromagnetic induction Alternating currents Transformers

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Topic overview Astronomical telescope consisting of two converging lenses Reflecting telescopes Single dish radio telescopes, I-R, U-V and X-ray telescopes Advantages of large diameter telescopes Classification by luminosity Absolute magnitude, M Classification by temperature, black-body radiation Principles of the use of stellar spectral classes The Hertzsprung-Russell (HR) diagram Supernovae, neutron stars and black holes Doppler effect Hubble's law Quasars Detection of exoplanets

Medical PhysicsOptional topic

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Topic overview Concept of moment of inertia Rotational kinetic energy Rotational motion Torque and angular acceleration Angular momentum Work and power First law of thermodynamics Non-flow processes The p–V diagram Engine cycles Second Law and engines Reversed heat engines

Turning Points in PhysicsOptional topic

Topic overview Cathode rays Thermionic emission of electrons Specific charge of the electron Principle of Millikan's determination of the electronic charge, e Newton's corpuscular theory of light Significance of Young's double slits experiment Electromagnetic waves The discovery of photoelectricity Wave–particle duality Electron microscopes The Michelson-Morley experiment Einstein's theory of special relativity Time dilation Length contraction Relativistic mass and energy

ElectronicsOptional topic

Topic overview MOSFET (metal-oxide semiconducting field-effect transistor)Zener diode Photodiode Hall effect sensor Difference between analogue and digital signals LC resonance filters The ideal operational amplifier Inverting amplifier configuration Non-inverting amplifier configuration Summing amplifier configuration Real operational amplifiers Combinational logic Sequential logic Astables Principles of communication systems Transmission media Time-division multiplexing Amplitude (AM) and frequency modulation (FM) techniques

Turning Points in Physics

Principle of Millikan's determination of the electronic charge, e

4 things to know
Paper 3, section B

Overview

The oil drop experiment, Stokes' law, terminal speed, and the quantisation of charge.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What holds at terminal speed for a falling droplet, and how is Stokes' law then used?

Numbers

How is the mass of a droplet found, and what pattern did Millikan see in the charges?

Process

What happens when a field is applied to a falling droplet, and how does drag change with speed?

Key term

What does quantisation of electric charge mean?

Questions from this subtopic

Each one with the model answer underneath.

1
In Millikan's experiment, in which direction does the electric force act on a negatively charged oil droplet held stationary between the plates?
Model answer
The electric force acts upward on the negatively charged droplet.
2
When an oil droplet is held stationary in Millikan's apparatus, in which direction does the gravitational force act?
Model answer
The gravitational force acts downward.
3
What is the condition for the forces acting on a charged oil droplet when it is held stationary between the parallel plates?
Model answer
The electric force and gravitational force are balanced (equal in magnitude and opposite in direction).
4
State the equation for the equilibrium condition of a stationary charged oil droplet between parallel plates, and define each symbol.
Model answer
QV/d = mg, where Q is the droplet charge, V is the potential difference between plates, d is the plate separation, m is the droplet mass, and g is gravitational field strength.
5
Write the formula used to calculate the charge on a stationary oil droplet from the equilibrium condition.
Model answer
Q = mgd/V
6
Calculate the charge on the droplet when mass of droplet = 5.44 × 10⁻¹⁵ kg, gravitational field strength = 9.81 m s⁻², plate separation = 0.006 m, potential difference = 1000 V.
Model answer
Charge on the droplet = (5.44 × 10⁻¹⁵ × 9.81 × 0.006) ÷ 1000 = 3.20 × 10⁻¹⁹ C.

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