Materials Science and Applied Physics Questions and Answers

Posted on Oct 18, 2026 in Electromechanical Engineering

Electrical Conductivity: Metals vs. Plastics

Why is the electrical conductivity of metals very high while that of plastics is very low?

Metals: Metallic bonding leaves valence electrons delocalised as an “electron sea” (about 1028–1029 electrons/m3). In band theory, the conduction band is partly filled or overlaps the valence band, so there is no energy gap. Electrons move freely under a small field. Conductivity is approximately 106–108 S/m.

Plastics: Atoms are held by strong covalent bonds, so all electrons are localised. The valence band is full and separated from the conduction band by a large band gap (above approximately 4 eV). There are practically no free carriers, so conductivity is about 10−10–10−18 S/m.

Temperature effect: Metal resistance increases with temperature, while insulators become slightly more conductive.

Working Principle of Bimetallic Strips

Q5. Explain the working of a bimetallic strip used in electric irons and refrigerator thermostats. Why are brass and Invar paired?

A bimetallic strip consists of two metals with different thermal expansion coefficients bonded along their length. When heated, the metal with the larger coefficient expands more, which forces the strip to bend towards the less-expanding metal. Deflection is proportional to the temperature change.

Switching action: The strip carries a contact. When the iron or fridge reaches the set temperature, the strip bends enough to break the contact and cut the current. After cooling, it straightens and closes the contact again. An adjusting screw sets the temperature at which this switching occurs.

Why brass and Invar are paired:

  • Brass: α ≈ 19 × 10−6 /K (high expansion)
  • Invar (Fe-36% Ni): α ≈ 1.2 × 10−6 /K (almost zero expansion)
  • This large differential gives maximum bending per degree of temperature change. Both materials are economical, bond well, and are mechanically robust.

Thermocouples and the Seebeck Effect

Q6. Explain how a thermocouple works (Seebeck effect). Why are Pt-Rh thermocouples used above 1200 °C, and how is cold-junction compensation done?

A6 (5 marks)

Seebeck effect: When two dissimilar metals are joined at two junctions maintained at different temperatures, a small electromotive force (EMF) develops: E = αΔT, where α is the Seebeck coefficient. Measuring E gives the temperature.

Why Pt-Rh thermocouples are used above 1200 °C: Base-metal thermocouples (such as Type K and Type J) oxidise, drift, and fail near or above 1200 °C, with their wires approaching melting or mechanical creep. Platinum-Rhodium alloys (Types R, S, and B) have exceptionally high melting points (pure Pt melts at 1768 °C), are chemically inert, and resist oxidation. They remain stable and accurate up to approximately 1600–1700 °C.

Cold-junction compensation: The EMF depends on the difference between the hot and reference junctions.

  • Traditional method: The reference junction is immersed in an ice bath maintained at 0 °C.
  • Modern method: A thermistor or resistance temperature detector (RTD) measures the temperature of the instrument’s terminal block, and internal electronics calculate and add the equivalent voltage automatically. Compensating or extension cables convey the signal accurately to the instrument.

Z

High-Performance Contact Materials

Q. Why are spark plug electrodes in automotive engines frequently tipped with Platinum or Iridium alloys? Why are circuit breaker contact tips commonly made from tungsten or molybdenum alloys instead of pure copper, despite tungsten’s higher electrical resistivity?

Part (a): Why Spark Plugs Use Pt or Ir (2.5 marks)

A spark plug electrode sits inside the combustion chamber, where it faces temperatures of several hundred °C, corrosive combustion gases, and approximately 108 spark events over its operating life. Each spark erodes a small amount of metal. Platinum and iridium are chosen for these key reasons:

  1. Very high melting point: Pt melts at 1768 °C and Ir at 2446 °C. Conventional nickel or copper electrodes melt or soften at significantly lower temperatures.
  2. Spark erosion resistance: Both metals are exceptionally hard and dense; consequently, each spark removes very little material, maintaining a consistent spark gap over long durations.
  3. Oxidation and corrosion resistance: As noble metals, they do not oxidise or react with sulphur and other aggressive combustion by-products at elevated temperatures.
  4. Service intervals reach up to 100,000–160,000 km, compared to only 30,000 km for standard copper-nickel plugs.

Part (b): Circuit Breaker Contacts (2.5 marks)

When a circuit breaker opens or closes under electrical load, an electric arc forms between the contacts. The arc reaches temperatures of several thousand degrees Celsius, leading to melting, vaporisation, pitting, and potential welding of the contacts.

Tungsten (melting point 3422 °C) and molybdenum (melting point 2623 °C) are selected because:

  • Extremely high melting and boiling points: They withstand extreme arc temperatures without melting.
  • High arc erosion resistance and hardness: They exhibit minimal pitting and degradation over thousands of operating cycles.
  • Anti-welding behaviour: The contact surfaces do not fuse or adhere to each other upon interruption.

Diodes, Rectification, and Voltage Regulation

a) Which semiconductor device is used for rectification?
The p-n junction diode (silicon rectifier diode). Power adapters and chargers commonly employ four diodes configured in a bridge rectifier.

b) Why does a diode conduct mainly in one direction?

  • A p-n junction features a depletion region that creates an internal potential barrier (approximately 0.7 V for silicon).
  • Forward bias: Applying an external forward voltage lowers this barrier, enabling majority carriers to cross the junction and allowing a large current to flow.
  • Reverse bias: Reverse polarity widens the potential barrier, leaving only a negligible leakage current of minority carriers.
  • Thus, the diode behaves as a one-way electrical valve, converting alternating current (AC) into direct current (DC).

c) Why is voltage regulation required?

  • Mains supply voltage fluctuates, and rectified outputs contain residual AC ripple.
  • Variations in load current induce fluctuations in output voltage.
  • Sensitive integrated circuits (ICs) and lithium batteries require a stable voltage (e.g., 5 V); over-voltage can permanently damage electronic components or cause battery overcharging.
  • A voltage regulator maintains a steady output level regardless of supply and load variations.

Material Selection: Wood for Furniture and Doors

Q1. Why are tables and indoor doors made of wood rather than steel or other materials?

Wood is chosen because its physical and mechanical properties align closely with the functional requirements of furniture and indoor doors, whereas steel presents practical disadvantages:

  • Light weight: Wood has a relatively low density (about 0.5–0.9 g/cm3) compared with steel (about 7.85 g/cm3). As a result, doors swing smoothly on simple hinges, and tables can be repositioned by one person.
  • Easy to shape and join: Wood can be readily cut, planed, carved, nailed, screwed, and glued using standard hand or power tools. Fabricating steel requires welding, specialized cutting equipment, and complex surface finishing.
  • Low cost and availability: Solid timber and engineered boards (such as plywood and MDF) offer an economical solution for constructing broad, flat surfaces.
  • Good insulator: Wood has low thermal and electrical conductivity, making it warm to the touch and safe for indoor spaces. A steel table feels cold, and an uninsulated steel door readily conducts heat and poses electrical conduction hazards.

Thermal Retention: Ceramic vs. Steel Plates

Q1. Why do ceramic plates take longer to cool down than steel plates?

  • Lower thermal conductivity: Ceramic materials conduct heat poorly (approximately 1–3 W/(m·K)), whereas stainless steel exhibits significantly higher conductivity (about 15–50 W/(m·K); over 50 W/(m·K) for carbon steel). Heat stored within the core of a ceramic plate migrates slowly outward, supplying sustained heat to the plate surface and the food.
  • Higher heat capacity: Ceramic exhibits a specific heat capacity of roughly 800–900 J/(kg·K), compared with about 500 J/(kg·K) for steel. Furthermore, ceramic plates are typically thicker and heavier, granting them greater total thermal mass and allowing them to store more thermal energy per unit of service area.

Material Failure in the Sinking of the Titanic

Q2. Why was the Titanic destroyed?

On 14–15 April 1912, the RMS Titanic collided with an iceberg in the North Atlantic and sank in approximately 2 hours and 40 minutes. The catastrophe resulted from a combination of navigational circumstances and severe metallurgical vulnerabilities:

  • Iceberg collision: The vessel was navigating at high speed (around 22 knots) through a known ice field. The iceberg was spotted too late to prevent a glancing side impact along the starboard hull.
  • Brittle steel: The hull plates contained high concentrations of sulphur and phosphorus. In freezing ocean water (around −2 °C), the steel operated below its ductile-to-brittle transition temperature, causing it to undergo brittle fracture rather than ductile deformation, unzipping the seams on impact.
  • Weak rivets: The forward section utilized wrought-iron rivets containing excessive slag inclusions, which made them prone to brittle shear. Under impact pressure, rivet heads sheared off and hull seams opened, resulting in massive flooding. Modern analyses highlight rivet failure as a decisive factor.

Conductive Elements in the Periodic Table

How many elements in the periodic table are conductive in nature?

The periodic table contains 118 confirmed elements. When classified by electrical conductivity:

  • Metals (conductors): approximately 90–92 elements. This group encompasses alkali metals, alkaline earth metals, transition metals, post-transition metals, lanthanides, and actinides. Silver is the most conductive element, followed by copper, gold, and aluminium.
  • Metalloids (semiconductors): approximately 6–7 elements. These include boron, silicon, germanium, arsenic, antimony, tellurium (and occasionally polonium or astatine). They exhibit intermediate conductivity that increases with temperature or intentional chemical doping.
  • Non-metals (insulators): approximately 17–18 elements. Examples include oxygen, nitrogen, sulphur, phosphorus, halogens, and noble gases. A notable exception is carbon in the allotropic form of graphite (and graphene), which conducts electricity effectively.