Title: InkPhysics / Turning points
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Proper time.
Back: The time between two events measured in the frame in which they happen at the same place: t₀.

Front: DEFINITION
Define: Specific charge of the electron.
Back: The magnitude of the electron's charge divided by its mass: e/m.

Front: RECALL
State Thomson's specific charge result.
Back: Thomson measured e/m at about 1800 times hydrogen's, the first evidence for a particle far less massive than any atom.

Front: RECALL
Explain why the droplet is timed falling with the field off.
Back: Its terminal speed gives the radius through Stokes' law, and the radius gives the otherwise unweighable mass.

Front: SPOT THE ERROR
Say what is wrong with this claim: With a strong enough accelerator you could push an electron past the speed of light.
Back: Nothing can be pushed past c: the energy needed grows without limit as speed approaches c, because in AQA's relativistic-mass convention the effective mass m₀/√(1 − v²/c²) grows without limit while the rest mass stays fixed, so electrons approach but never reach c.

Front: RECALL
State the condition for a droplet falling at terminal speed.
Back: The viscous drag 6πηrv equals the droplet's weight.

Front: RECALL
Explain how Millikan's droplets became charged.
Back: By friction as the spray left the atomiser.

Front: RECALL
State what Hertz's experiments showed.
Back: Electrically generated radio waves travel at c, so light belongs to an electromagnetic family.

Front: DEFINITION
Define: Time dilation.
Back: An observer moving relative to the events measures a longer time than the proper time: t = t₀/√(1 − v^2/c^2).

Front: RECALL
State how to find the speed of electrons leaving an electron gun.
Back: The work done by the accelerating pd becomes kinetic energy: eV = ½mv^2.

Front: DEFINITION
Define: The ultraviolet catastrophe.
Back: Classical wave theory predicts black-body intensity rising without limit at short wavelengths, contrary to measurement.

Front: RECALL
State what crossed fields add to the electron gun.
Back: The undeflected condition gives the beam speed, which combined with the gun energy equation yields e/m.

Front: EQUATION · NOT ON THE DATA SHEET
Kinetic energy at a relativistic speed:
E_k = ?\(E_{k} = \, ?\)E_k = ?
Back: E_k = (m − m_0)c^2\(E_{k} = (m − m_{0})c^{2}\)E_k = (m − m_0)c^2

Front: SPOT THE ERROR
Say what is wrong with this claim: Waves are waves and particles are particles, and nothing is ever both.
Back: Everything shows both behaviours: light diffracts as a wave yet exchanges energy in photons, and electrons diffract through crystals. Which behaviour appears depends on the experiment.

Front: RECALL
State what the Michelson-Morley experiment set out to detect.
Back: The Earth's absolute motion through the ether, as an ether wind across the apparatus.

Front: RECALL
Explain why the interferometer was rotated through 90°.
Back: Rotation swaps the arms, reversing any ether-wind time difference, so the fringes should shift.

Front: DEFINITION
Define: Inertial frame of reference.
Back: A frame moving at constant velocity, in which Newton's first law holds.

Front: DEFINITION
Define: Proper length.
Back: The length of an object measured in its own rest frame: l₀.

Front: DEFINITION
Define: Thermionic emission.
Back: Electrons gain enough kinetic energy from a heated metal to escape its surface.

Front: RECALL
State the nature of an electromagnetic wave.
Back: Oscillating electric and magnetic fields at right angles, needing no medium.

Front: DEFINITION
Define: Rest mass.
Back: The mass of an object measured in the frame where it is at rest: m₀.

Front: SPOT THE ERROR
Say what is wrong with this claim: Light must travel through something; empty space cannot carry a wave.
Back: Light needs no medium: the interferometer could have detected the supposed aether wind but found no significant shift in any orientation or season, undermining the aether picture and supporting the postulate that light's speed is the same in every inertial frame.

Front: RECALL
State Maxwell's formula for the speed of electromagnetic waves.
Back: c = 1/√(μ₀ε₀), built entirely from the two electrical constants.

Front: RECALL
Explain why Young's dark fringes rule out particles.
Back: Two particle streams can only add; only waves arriving out of step can cancel to darkness.

Front: SPOT THE ERROR
Say what is wrong with this claim: A theory backed by a great enough scientist counts as proved.
Back: Authority is not evidence: Newton's particle theory predicted light travels faster in glass, wave theory predicted slower, and Foucault's measurement found it slower, deciding for waves.

Front: DEFINITION
Define: Bertozzi's experiment.
Back: Measured electron speed by time of flight against kinetic energy from the pd, finding speed levelling off just below c.

Front: DEFINITION
Define: First postulate of special relativity.
Back: Physical laws have the same form in all inertial frames.

Front: DEFINITION
Define: Planck's quantum hypothesis.
Back: Energy is emitted and absorbed in discrete quanta of size E = hf.

Front: RECALL
State the evidence for time dilation.
Back: Far more cosmic-ray muons reach the ground than their rest lifetime allows without dilation.

Front: DEFINITION
Define: Length contraction.
Back: A moving object is measured shorter along its direction of motion: l = l₀√(1 − v^2/c^2).

Front: SPOT THE ERROR
Say what is wrong with this claim: The atom is the smallest unit of matter; nothing can be pulled out of one.
Back: Atoms are not indivisible: Thomson showed cathode rays have the same specific charge whatever the cathode metal, about 1800 times hydrogen's, so a particle far lighter than any atom comes from inside atoms.

Front: SPOT THE ERROR
Say what is wrong with this claim: Electric charge comes in any amount you like, like water from a tap.
Back: Charge is quantised: Millikan's droplets only ever carried whole-number multiples of 1.6 × 10⁻¹⁹ C, so charge comes in units of the electron charge, never in arbitrary amounts.

Front: RECALL
State the de Broglie wavelength of an electron accelerated through pd V.
Back: λ = h/√(2meV).