Physics Experiments

Robert Millikan's oil drop experiment (1909-1913) stands as one of physics' most elegant and precise measurements, determining the fundamental unit of electric charge (e = 1.602×10^-19 coulombs) with remarkable accuracy. This ingenious experiment balanced microscopic oil droplets between charged plates, measuring their terminal velocities under gravity alone versus with applied electric field to calculate the charge on individual droplets. Millikan's work, for which he received the 1923 Nobel Prize, provided the first direct evidence that electric charge exists in discrete quanta, confirming the atomic nature of electricity and enabling calculation of numerous fundamental constants. The experimental apparatus consisted of: an atomizer producing fine oil droplets (typically 0.5-1.5 μm diameter); a chamber with two horizontal brass plates separated by insulating material and connected to high voltage (up to 10 kV); an illumination system (initially arc lamp, later focused light) making droplets visible as bright points against dark background; a telescope with graduated eyepiece for measuring droplet motion; and a timing device (initially stopwatch, later electrical timing). The chamber contained air at atmospheric pressure, though Millikan later used lower pressures to reduce air resistance effects. Millikan's method involved: spraying non-volatile oil (initially clock oil, later more refined mineral oil) to create charged droplets through friction; allowing droplets to fall between plates with no field to measure terminal velocity v_g under gravity; applying electric field to balance or reverse droplet motion; measuring terminal velocity v_e under electric influence; and using Stokes' law (relating drag force to velocity for small spheres in viscous medium) to calculate charge. The key insight: by observing many droplets with different charges, the calculated charges were always integer multiples of a smallest value—the elementary charge. The mathematical analysis combined several physical principles: gravitational force F_g = mg = (4/3)πr³ρg; buoyant force F_b = (4/3)πr³σg (σ=air density); viscous drag F_d = 6πηrv (Stokes' law, valid for small Reynolds numbers); and electric force F_e = qE = qV/d. At terminal velocity, forces balance: with no field, mg' = 6πηrv_g where m' = m - buoyant mass; with field, qE = mg' ± 6πηrv_e (sign depending on field direction). Combining equations eliminates radius r, yielding q = (mg'd/V)(v_g ± v_e)/v_g. Millikan faced numerous experimental challenges: droplet evaporation (solved by using non-volatile oils); convection currents (minimized by temperature control and chamber isolation); Brownian motion (addressed by statistical analysis of many measurements); and inaccurate viscosity values (corrected through careful calibration). His most controversial adjustment—the "Stokes' law correction factor" accounting for non-continuum effects at droplet size comparable to air mean free path—was later validated by more sophisticated fluid dynamics. The experiment's precision improved dramatically through refinements: switching to higher viscosity oils; using X-rays or radioactive sources to change droplet charge during observation; implementing automatic timing; and applying statistical analysis to thousands of measurements. Millikan's final 1917 value, e = (1.5924 ± 0.0017)×10^-19 C, was within 0.6% of today's accepted value—extraordinary accuracy for early 20th century experimental physics. Beyond determining e, the experiment enabled calculation of: Avogadro's number N_A = F/e (F=Faraday constant); electron mass m_e = e/(e/m) using Thomson's e/m; Planck's constant via photoelectric effect; and numerous atomic properties. It provided crucial evidence for quantization decades before quantum mechanics was fully developed, demonstrating that electricity, like matter, comes in discrete packets. Modern versions of the experiment use: laser illumination for better visibility; CCD cameras and video analysis for automated tracking; computer-controlled voltage switching; and improved chamber design minimizing convection. Despite technological advances, the basic principle remains unchanged: balancing forces on charged droplets reveals nature's fundamental charge unit. The experiment continues as essential undergraduate physics laboratory, teaching statistical analysis, error estimation, and precision measurement techniques. Millikan's experiment exemplifies how simple apparatus can answer profound questions. Using equipment costing perhaps $1000 (equivalent to $25,000 today), he measured a fundamental constant with parts-per-thousand accuracy—a feat requiring extraordinary experimental skill and theoretical insight. The experiment's legacy extends beyond its numerical result: it established standards for careful measurement, statistical rigor, and experimental design that influenced generations of physicists. Educational versions simplify the apparatus while preserving core concepts: using smaller voltages (200-500 V); larger droplets (5-10 μm) for easier observation; manual timing with stopwatches; and simplified calculations assuming Stokes' law without correction. Students learn: terminal velocity concepts; viscous drag relationships; electric force calculations; statistical analysis of discrete data; and historical context of quantum discovery. Despite simplifications, student results typically yield e within 5-10% of accepted value—testament to the experiment's robustness. The oil drop experiment remains relevant in modern physics education because it connects macroscopic observations (falling droplets) to microscopic reality (quantized charge). Students directly witness quantization: plotting calculated charges reveals peaks at integer multiples of fundamental unit. This tangible demonstration of quantum principle—normally hidden in atomic-scale phenomena—makes abstract quantization concrete, bridging classical and quantum physics in single elegant experiment.

The polarity of water represents one of chemistry and physics' most fundamental and consequential molecular properties, responsible for water's unique solvent capabilities, surface tension, capillary action, and essential role in biological systems. This experiment visually demonstrates water's polar nature through simple yet revealing interactions with charged objects, providing tangible evidence for molecular asymmetry and dipole moment. Understanding water polarity is crucial for explaining phenomena from cloud formation to protein folding, making this experiment both accessible introduction to molecular physics and gateway to sophisticated concepts. Water's polarity arises from its bent molecular geometry (104.5° bond angle) and oxygen's higher electronegativity (3.44 vs. hydrogen's 2.20), creating permanent dipole moment of 1.85 Debye. The oxygen atom bears partial negative charge (δ-), while hydrogens carry partial positive charges (δ+), making water molecules electrically asymmetric despite overall neutrality. This polarity enables hydrogen bonding—intermolecular attraction between δ+ hydrogen and δ- oxygen of adjacent molecules—responsible for water's anomalously high boiling point, surface tension, and solvent properties. Experimental demonstrations of water polarity typically involve: observing water stream deflection by charged objects (static electricity); measuring angle of contact with different surfaces (hydrophilicity vs. hydrophobicity); observing capillary rise in narrow tubes; studying electrolysis of water (separation into H₂ and O₂); and using molecular models to visualize charge distribution. The classic charged rod deflection experiment uses a statically charged rod (glass rubbed with silk becomes positive; plastic rubbed with wool becomes negative) brought near thin stream of water from burette or faucet. The deflection mechanism involves polarization: water molecules, though neutral overall, rotate to align their dipoles with external electric field. Positive rod attracts negative oxygen ends, pulling water stream toward rod; negative rod attracts positive hydrogen ends, producing similar attraction. The force depends on: dipole moment magnitude; field strength (inversely proportional to distance squared); and degree of molecular alignment. For pure water at room temperature, deflection angles of 10-30° are typical with moderately charged rods at 1-2 cm distance. More quantitative experiments measure: deflection angle vs. distance to calibrate field strength; compare different liquids' polarities (water vs. ethanol, acetone, hexane); study temperature effects (polarity decreases with temperature as thermal motion disrupts alignment); and investigate ionic solutions (added electrolytes screen dipole interactions). Advanced versions use: electrometers to measure induced charges; field mills to quantify electric fields; and laser deflection for precise angle measurement. The experiment connects to broader physics concepts: dipole moments in molecular physics; dielectric properties of materials; electrostatics and Coulomb's law; polarization and permittivity; and intermolecular forces. It demonstrates how microscopic asymmetry (atomic-scale charge separation) produces macroscopic effects (visible stream bending)—bridging quantum chemistry with everyday observation. This makes it particularly valuable for teaching the concept of emergent phenomena: simple molecular properties leading to complex bulk behavior. Historical context reveals centuries of investigation into water's peculiar properties: Benjamin Franklin's 18th century observations of oil calming water surfaces; early 19th century studies of capillary action by Laplace and Young; 20th century X-ray diffraction revealing molecular structure; and modern computational chemistry calculating electron distributions. The simple deflection experiment captures essence of this sophisticated understanding in immediately observable form. Educational significance extends beyond physics to chemistry and biology: water as "universal solvent" for ionic compounds (due to hydration shells); role in cell membrane structure (hydrophilic heads vs. hydrophobic tails); importance in protein folding (hydrophobic effect); and function in enzyme catalysis (polar active sites). The experiment naturally leads to discussions of: hydrogen bonding networks in ice structure; anomalous expansion upon freezing; high specific heat capacity moderating climate; and surface tension enabling insect locomotion. Modern extensions include: using charged comb after brushing hair (common classroom demonstration); observing effects with deionized vs. tap water (ions affect polarization); comparing with non-polar liquids like hexane (no deflection); and studying frequency dependence with AC fields (molecules cannot follow rapid reversals above ~10 GHz). Computational simulations visualize electron density maps and dipole alignment animations, complementing physical experiments. Practical applications of water polarity knowledge include: designing surfactants and detergents; developing water-repellent coatings; understanding chromatography separation mechanisms; optimizing electrochemical processes; and modeling atmospheric electricity (lightning, cloud charging). The pharmaceutical industry relies on understanding water-solute interactions for drug design, while materials science engineers superhydrophobic surfaces inspired by lotus leaves. The experiment's simplicity belies deep physical principles: it demonstrates that matter at molecular scale is electrically active, with charge distributions affecting bulk properties. Students observing water bending toward a charged rod witness direct evidence that molecules have internal structure—a conceptual leap from viewing matter as continuous substance to understanding its particulate, electrically complex nature. This foundational understanding underpins much of modern physical science. Safety considerations are minimal but important: using low voltages for any electrical components; ensuring water doesn't contact electrical connections; avoiding slippery surfaces from spills; and using appropriate eye protection if working with glass apparatus. The experiment is particularly suitable for diverse educational settings due to low cost, minimal equipment requirements, and immediate visual impact. Beyond laboratory, water polarity manifests everywhere: raindrops beading on waxed cars; water climbing plant xylem vessels; detergent removing grease; and DNA's double helix stability. The charged rod experiment makes this ubiquitous but invisible property suddenly visible, transforming abstract textbook concept into memorable hands-on experience. This ability to reveal hidden molecular reality through simple observation exemplifies the power of well-designed physics experiments.