Lab Quiz Concepts

phy-1109 PHY-1109 Physics This is a compact, precise, and non-tabular cheat sheet summarizing the experimental physics concepts.

I. Experiment M1: Instrumental Errors

Least Count (Screw Gauge): Distance moved by the screw for one circular scale division; calculated as $Pitch / Divisions$ .
Pitch (Screw Gauge): Distance the screw moves forward/backward during one complete cap rotation.
Slide Calipers Errors: Positive Zero Error occurs if the Vernier zero is in advance of the main scale zero; Negative Zero Error occurs if the Vernier zero is behind the main scale zero.

II. Experiment E12 (L3): Refractive Index (Spectrometer)

Spectrometer Use: Measures refractive index by finding the angle of the prism and the angle of minimum deviation.
Setup: The instrument must be leveled to prevent the image position from changing with the telescope position. The collimator produces a parallel beam of light.
Parallel Rays: If incident rays are parallel, the telescope remains focused for every position of the prism.
Prism Placement: The refracting edge should be placed over/at the center of the prism table, which minimizes error for thick prisms.
Deviation: Deviation is greater for violet light (shorter wavelengths) than for red light. Deviation becomes minimum at a particular angle of incidence. The condition for minimum deviation is when the angle of incidence equals the angle of emergence.
Light: Sodium light is used because it yields a single image and a single value of minimum deviation; white light gives a spectrum. Sodium light is considered monochromatic but contains two wavelengths ( $589 0^{\circ} A$ and $589 6^{\circ} A$ ).
Readings: Both Verniers are read to avoid error due to the non-coincidence of the circular scale center with the axis of rotation.
Telescope: Produces a virtual image at infinity.

III. Experiment E18 (E2): Comparing E.M.F.’s (Potentiometer)

E.M.F. vs. P.D.: E.M.F. is potential difference in an open circuit (the cause); potential difference is present only in a closed circuit (the effect).
High Resistance: Protects the galvanometer from damage, but does not interfere with the null point position.
Driver Cell ( $E$ ): Function is to send current to create a drop of potential between terminals A and B.
Potential Drop ( $ρ$ ): Can be found using the relation $ρ = E_{1} / l_{1}$ .
Sensitiveness: Increased by maximizing the balancing length of the potentiometer.
Wire Type: High-resistance materials like Eureka or Manganin are used so the fall of potential is appreciable over the whole wire length.

IV. Experiment E13 (L4): Newton’s Rings

Purpose: The traveling microscope is used to precisely measure the diameter of the rings.
Cause: Interference between light rays reflected from the lower surface of the lens and the upper surface of the plane glass plate due to the air film of varying thickness.
Shape: Rings are circular because the air film thickness is constant for all points lying on a circle around the contact point.
Center: Appears dark in reflected light because reflection from the lower (denser) surface causes a $π$ phase change, resulting in destructive interference at zero film thickness ( $t = 0$ ).
Light: Monochromatic light is necessary because white light fringes would overlap after the first few rings.

V. Experiment E6 (H1): Specific Heat by Cooling

Newton’s Law: Rate of heat loss ( $\frac{d Q}{d t}$ ) is directly proportional to the temperature difference ( $θ - θ_{0}$ ), holding strictly for small $Δ θ$ ( $\leq 3 0^{\circ} C$ ). It is a quick, but not strictly accurate, method.
Heat Loss: Radiation is the principal process of heat loss from the calorimeter.
Procedure: The same volume of the two liquids must be used to ensure the surface area for radiation is equal. The calorimeter should have thin walls and outer surfaces painted black.
Scope & Error: The law holds only for liquids, not for solids. The chief source of error is evaporation, which is avoided by using a lidded calorimeter with holes for a thermometer and stirrer.

VI. Experiment E20 (MD1): Photoelectric Effect

Effect: Phenomenon where a photon hitting a metal surface causes an electron to be emitted; shows light behaves as a stream of energy packets.
Work Function: The minimum photon energy required to liberate an electron.
Energy/Frequency: Shorter waves (like blue light, $3.1 eV$ ) have higher frequency/energy than longer waves (like red light, $1.8 eV$ ).
Intensity: The number of electrons ejected (current) is proportional to the intensity, independent of frequency above the threshold value ( $ν_{0}$ ).
Threshold Frequency ( $ν_{0}$ ): The minimum frequency required for photo-electron emission.
Stopping Potential: The potential necessary to stop electron flow; measures the maximum kinetic energy of the emitted electrons.
Photoconductivity: An increase in the electrical conductivity of a nonmetallic solid upon exposure to radiation.

VII. Experiment E14/E15 (L5/L6): Diffraction Grating

Diffraction: The bending of light from the edge of a slit.
Grating: A device with a large number of closely spaced parallel slits separated by equal opaque spacing.
Grating Element/Constant: Grating element is $(a + b)$ (slit width + opaque width); grating constant is $1/ (a + b)$ .
Rulings ( $N$ ): Increasing $N$ results in fewer orders but wider angular separation.
Absent Spectrum: The $n^{t h}$ order is absent if $(a + b) < nλ$ . The 2nd order is absent if the slit width ( $a$ ) equals the opaque spacing ( $b$ ).
Purity: Grating spectrum is purer than prismatic spectrum because the angle of diffraction ( $θ$ ) is proportional to $λ$ (for small $θ$ ).
Resolving Power: The ability of an instrument to show nearby spectral lines as separate ones.

VIII. Experiment E17 (E1): Post Office Box (P.O. Box)

Principle: Works on the Wheatstone’s bridge principle: $S / R = Q / P$ .
Procedure: Battery key must be pressed before the galvanometer key to avoid the effect of self-induction.
Range: Cannot measure neither very high nor very low resistance.
Stability: The null point remains constant even if the cell E.M.F. changes because the potential distribution adjusts equally.
Temperature: Calibrated resistance values change with temperature, as resistance increases with rising temperature.
Sensitivity: Increases when the resistance of the galvanometer and battery are lower.

IX. Experiment E7 (H2): Thermal Conductivity (Lees and Charlton’s)

Coefficient ( $k$ ): Quantity of heat flowing per second through opposite $1 cm^{2}$ faces, $1 cm$ apart, at $1^{\circ} C$ temperature difference.
States: Steady state means no absorption of heat and no temperature increase in any part of the bar.
Dependence: Thermal conductivity decreases with a rise of temperature. It depends only on the material, not its dimension.
Procedure: A thin disc of poor conductor must be used to minimize heat loss due to radiation.
Temperature Gradient: Change of temperature per unit distance ( $\frac{d θ}{d x}$ ).

X. Experiment E8 (S1): Melde’s Experiment

Stationary Wave: Formed by the superposition of two identical progressive waves traveling in opposite directions. In Melde’s, this is due to direct waves and waves reflected from the pulley.
Modes: Longitudinal (fork and string vibrate in the same direction) vs. Transverse (fork and string vibrate perpendicular).
Tension: Longitudinal arrangement tension is one-fourth of that in the transverse arrangement.
Nodes ( $N$ ) and Antinodes ( $A$ ): Nodes are points permanently at rest (zero displacement, maximum density change); Antinodes swing with maximum extent.
Resonance: Occurs when the amplitude of vibration of the antinodes is greatest.
Measurement: Count the loops in the central part of the thread; ignore nodes at the prong tip and pulley due to their motion.

Based on this summary, which experiment or concept would you like to practice applying next?

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