Flashcards · 7408 · Topic 12

Turning points in physics flashcards

20 cards on Topic 12, subtopics 3.12.1 to 3.12.3. Pick a subtopic or study them all.

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Thermionic emission: Heating a metal gives conduction electrons enough energy to leave the surface. An anode in a vacuum accelerates them: ½mv² = eV.
Why does a discharge tube glow?: Positive ions hit the cathode and free electrons, which accelerate to the anode, ionising and exciting gas atoms. The atoms de-excite and emit light.
Specific charge of the electron from a fine-beam tube: Combine Bev = mv²/r with ½mv² = eV: e/m = 2V/(B²r²).
Significance of Thomson's measurement of e/m: The same value for any gas or cathode, so electrons are in all matter. It is about 1800 times the specific charge of the hydrogen ion, so the electron is much less massive than any atom.
Millikan: condition for a stationary oil drop, and the significance of his results: QV/d = mg. All charges were whole-number multiples of e, so charge is quantised.
Millikan: how is the drop's radius found?: With the field off, the drop falls at terminal speed: mg = 6πηrv with m = (4/3)πr³ρ, giving r = √(9ηv/(2ρg)).

Newton's corpuscular theory vs Huygens' wave theory: prediction for light in water: Newton: light travels faster in water. Huygens: light travels slower in water.
Why was Newton's theory preferred for so long?: His reputation; it explained reflection, refraction and sharp shadows; and the speed of light in water could not then be measured.
Speed of EM waves from Maxwell's theory, and its significance: c = 1/√(μ₀ε₀). Fizeau's measured speed of light matched it, so light is an electromagnetic wave.
Ultraviolet catastrophe and Planck's explanation: Classical theory predicted black-body intensity rising without limit as λ → 0; the observed curve peaks and falls to zero. Planck proposed energy is emitted in quanta, E = hf.
Three features of photoelectricity that wave theory cannot explain: A threshold frequency; instant emission; Ek(max) depends on frequency, not intensity. Einstein: one photon (energy hf) gives its energy to one electron.
Wavelength of an electron accelerated through pd V: λ = h/√(2meV).
TEM vs STM: TEM: electrons are transmitted through a thin sample and focused by magnetic lenses. STM: electrons tunnel between a conducting tip and surface about 1 nm apart; the current is very sensitive to the gap.

Michelson-Morley experiment: what was expected, what was found, and the significance: Rotating the interferometer through 90° was expected to shift the fringes if the Earth moved through the ether. No shift: absolute motion cannot be detected; the speed of light is invariant.
The two postulates of special relativity: 1. Physical laws have the same form in all inertial frames. 2. The speed of light in free space is invariant.
Time dilation and proper time: t = t₀/√(1 − v²/c²). Proper time t₀ is measured in the frame where the clock is at rest; a moving observer measures a longer time.
How do muons provide evidence for time dilation?: Muons made high in the atmosphere reach the ground in far greater numbers than their proper half-life would allow without time dilation.
Length contraction: l = l₀√(1 − v²/c²). Proper length l₀ is measured at rest relative to the object. Only lengths along the direction of motion contract.
Relativistic mass and energy: E = mc², with m = m₀/√(1 − v²/c²). Kinetic energy = mc² − m₀c². The mass rises towards infinity as v → c.
Bertozzi's experiment: Electrons of known energy were timed over a known distance, with the energy checked by the heating of the target. Their speed approached but never exceeded c.