Definitions (AQA A-Level Physics): Revision Notes

Definitions

1. Anode

• Definition: A positively charged electrode in a circuit.
• Explanation: In devices like discharge tubes, electrons are attracted to the anode, allowing current to flow.

2. Bertozzi's Experiment

• Definition: An experiment investigating the speed of electrons as their kinetic energy increases, demonstrating that they approach but never exceed the speed of light.
• Explanation: Bertozzi's experiment supported special relativity, showing that even as energy increases, particles cannot surpass the speed of light.

3. Black-Body

• Definition: An idealised object that emits radiation across all wavelengths, with intensity dependent on its temperature.

4. Cathode Rays

• Definition: Beams of electrons emitted from a cathode through thermionic emission in a vacuum tube.
• Explanation: Cathode rays were central to the discovery of the electron and the development of atomic models.

5. Cathode

• Definition: A negatively charged electrode in a circuit.
• Explanation: Electrons are emitted from the cathode in devices like electron guns.

6. Condenser Lens

• Definition: A lens in a Transmission Electron Microscope (TEM) that spreads electron beams before they hit a sample.
• Application: The condenser lens helps achieve clear, focused images in electron microscopy.

7. Constant-Current STM

• Definition: A type of Scanning Tunnelling Microscope (STM) where current remains constant as the probe scans the sample, creating an image based on height variations.
• Explanation: By keeping current steady, it accurately maps surface topography at the atomic level.

8. Dipole Detector

• Definition: A device with two aligned metal rods used to detect radio waves, often in experiments demonstrating wave properties.
• Application: Used in early radio wave detection and experiments on electromagnetic waves.

9. Discharge Tube

• Definition: A sealed glass tube with low pressure and two electrodes (anode and cathode), where current flow causes the gas to glow.
• Explanation: Discharge tubes helped scientists study atomic structure and led to the discovery of subatomic particles.

10. Drag Force

• Definition: A resistive force acting against motion through a fluid, as experienced by particles in Millikan's Oil Drop Experiment.

11. Electron Gun

• Definition: A device that produces a beam of electrons by heating the cathode to emit electrons, then accelerating them towards an anode.
• Explanation: Electron guns are used in cathode ray tubes and particle accelerators to control electron motion.

12. Ether

• Definition: A hypothetical medium once believed to fill all space and serve as a carrier for light waves.
• Historical Significance: The Michelson-Morley experiment disproved the ether theory, leading to the development of modern theories of light propagation.

13. Ether Drift

• Definition: The assumed movement of ether relative to Earth, expected to affect the speed of light.
• Outcome: Experiments like Michelson-Morley showed no evidence of ether drift, supporting the constancy of the speed of light.

14. Fizeau's Determination of the Speed of Light

• Definition: A method for measuring light speed by timing light's journey to a mirror and back using a rotating cogwheel.
• Explanation: This early experiment was one of the first accurate measurements of light speed.

15. Hertz's Discovery of Radio Waves

• Definition: An experiment showing that radio waves are generated by sparks and detected by a dipole antenna or loop.
• Significance: Hertz's work verified Maxwell's predictions about electromagnetic waves.

16. Huygens' Wave Theory

• Definition: A theory stating that light waves consist of wavefronts emitted from secondary points, explaining phenomena like diffraction.
• Explanation: Huygens' theory laid the groundwork for understanding wave behaviour in light.

17. Inertial Frame of Reference

• Definition: A frame of reference that is not accelerating.
• Explanation: In inertial frames, Newton's laws of motion hold true without correction for fictitious forces.

18. Invariance of Speed of Light

• Definition: The principle that the speed of light is constant for all observers, regardless of their motion or the source's motion.
• Significance: This principle is central to Einstein's theory of special relativity.

19. Length Contraction

• Definition: A phenomenon where length appears shorter for objects moving near the speed of light, observed from a stationary frame.
• Formula: $L = L_0 \sqrt{1 - \frac{v^2}{c^2}}$

20. Maxwell's Electromagnetic Theory

• Definition: A theory predicting that light is an electromagnetic wave with perpendicular electric and magnetic fields.
• Explanation: Maxwell's theory unified electric and magnetic fields, leading to the prediction of radio waves and electromagnetic wave propagation.

21. Michelson-Morley Interferometer

• Definition: An experiment using interferometry to test for ether drift, ultimately showing that light's speed is invariant.
• Impact: Its findings refuted the ether theory and influenced the development of relativity.

22. Negative Glow

• Definition: A glow near the cathode in a cathode ray tube, caused by recombination of ions and electrons.
• Explanation: Negative glow helped scientists understand ionisation and recombination processes.

23. Newton's Corpuscular Theory

• Definition: Newton's theory suggesting that light is made of particles (corpuscles) that travel in straight lines.
• Significance: Although eventually replaced by wave and quantum theories, it was an early model for understanding light behaviour.

24. Objective Lens

• Definition: The lens in a TEM that magnifies and focuses the sample image.
• Application: The objective lens is key in resolving fine details in electron microscopy.

25. Paddle Wheel

• Definition: A wheel placed in a discharge tube to demonstrate the presence of cathode rays by rotating when struck by electrons.
• Explanation: The paddle wheel experiment showed that cathode rays have momentum, supporting the particle theory of electrons.

26. Photon

• Definition: A packet of light energy, representing the quantum of electromagnetic radiation.
• Explanation: Photons form the basis of quantum theory, explaining phenomena like the photoelectric effect.

27. Projector Lens

• Definition: The final lens in a TEM that focuses the electron image onto a screen.
• Application: Projector lenses provide the final magnification and clarity needed for viewing electron microscope images.

28. Quantisation of Electric Charge

• Definition: The principle that electric charge exists in discrete packets (multiples of the electron charge).
• Explanation: Quantisation explains why charges are not continuous and supports atomic theory.

29. Quantum Tunnelling

• Definition: A quantum phenomenon where particles can pass through energy barriers, even if they lack the energy to overcome them directly.

30. Relativistic Energy

• Definition: Energy that increases with an object's speed as it approaches the speed of light, due to relativistic mass increase.
• Explanation: At relativistic speeds, energy grows disproportionately, supporting Einstein's relativity.

31. Scanning Tunnelling Microscope (STM)

• Definition: A microscope using quantum tunnelling to image surfaces at the atomic level, with resolutions down to 0.001 nm.
• Application: STMs allow precise mapping of atomic structures.

32. Specific Charge

• Definition: The ratio of charge to mass for a particle or ion.
• Formula: Specific Charge = $\frac{\text{charge}}{\text{mass}}$
• Application: Used to characterise ions and particles in fields like mass spectrometry.

33. Stokes' Law

• Definition: A law describing the drag force on a spherical object moving through a fluid.
• Formula: $F = 6 \pi \eta r v$

34. Stopping Potential

• Definition: The minimum voltage needed to stop photoelectrons emitted from a metal in the photoelectric effect.
• Explanation: Stopping potential provides evidence for quantised light energy in the form of photons.

35. Thermionic Emission

• Definition: The release of electrons from a heated metal surface.

36. Threshold Frequency

• Definition: The minimum frequency of light required to emit photoelectrons from a metal surface.
• Formula: $f = \frac{W}{h}$, where $W$ is the work function.
• Significance: Demonstrates the quantised nature of light and supports the photoelectric effect.

37. Time Dilation

• Definition: A relativistic effect where time appears to pass more slowly for an object moving near the speed of light.
• Explanation: Time dilation is a core concept of relativity, observed in high-speed travel scenarios.

38. Transmission Electron Microscope (TEM)

• Definition: A microscope that passes electrons through a sample to create high-resolution images.
• Resolution: TEMs can achieve resolutions around 0.1 nm, making them essential for studying fine structures at the atomic level.

39. Ultraviolet Catastrophe

• Definition: A discrepancy between classical predictions and observed intensities of black-body radiation at short wavelengths.
• Resolution: The ultraviolet catastrophe was resolved by Planck's quantum hypothesis, leading to quantum mechanics.

40. Work Function

• Definition: The minimum energy required to eject an electron from a metal's surface.
• Application: Work function is essential for understanding the photoelectric effect and material properties.