Conducting Materials: Low and High Resistivity Materials

Conducting Materials: Low and High Resistivity Materials

Every electrical device needs a path for current. The copper wire in your house, the heating coil of an electric iron and the filament of a bulb are all conducting materials. But they are not the same. The wire must waste almost no energy, while the heating coil must turn electrical energy into heat.

This difference comes from resistivity. In this note you will learn how conducting materials are divided into low resistivity and high resistivity materials, what a good conductor needs, and what superconductivity is.

1. What Are Conducting Materials?

Definition: Conducting materials (conductors) are materials that allow electric current to flow easily through them.

They contain a large number of free electrons. When a voltage is applied, these free electrons move and form the current.

Examples: silver, copper, gold, aluminium, steel, brass, bronze, carbon, tungsten, nichrome.

Important point: Conductors are not only metals. Carbon (graphite) also conducts, and so do some liquids such as mercury and salt solutions.

Exam tip: "Explain conducting materials. Give examples" has been asked as a 4-mark short question.

2. Why Do Conductors Conduct?

In a conductor, the valence band and the conduction band overlap. There is no forbidden gap, so valence electrons move freely as free electrons. This is the energy band idea covered in Classification of Electrical Engineering Materials.

3. Resistivity and Conductivity

Resistance of a wire depends on its length, its cross-section area and the material:

R = ρ l / A

Here R is resistance (Ω), ρ is resistivity (Ω·m), l is length (m) and A is cross-section area (m²).

Resistivity (ρ) is the resistance of a conductor of unit length and unit cross-section area. It is a property of the material only. A longer or thinner wire has more resistance, but its resistivity stays the same.

Conductivity (σ) is the reciprocal of resistivity: σ = 1 / ρ. Low resistivity means high conductivity.

Analogy: Think of water flowing in a pipe. A long, narrow pipe full of stones resists the water more. The stones are like the material's resistivity, and the length and width of the pipe are like l and A.

4. Desirable Properties of a Good Conductor

A good conductor for general electrical use should have:

1. Low resistivity (high conductivity), so that power loss is small.
2. High tensile strength, so that it does not break under its own weight and tension.
3. Ductility, so that it can be drawn into thin wires. (Ductility is the property of being drawn into wires.)
4. Malleability and flexibility, so that it can be bent and shaped easily.
5. Good resistance to corrosion, so that it lasts long in air and moisture.
6. Easy soldering and welding, so that joints are simple and reliable.
7. Low cost and easy availability.

Exam tip: "List the desirable properties of a good conductor" has been asked as a 4-mark short question. Write 5 to 6 points with a one-line reason each.

5. Types of Conducting Materials

The syllabus divides conducting materials into two groups on the basis of resistivity.

5.1 Low Resistivity Materials

These materials offer very little opposition to current. They are used where we want current to pass with minimum loss.

Examples: silver, gold, copper, aluminium, steel, ACSR, and alloys such as brass and bronze.

Applications:

• Transmission and distribution lines
• House wiring and cables
• Windings of motors, generators and transformers
• Bus bars and electrical contacts

5.2 High Resistivity Materials

These materials offer large opposition to current. When current flows, they produce heat. They are used where we want heat, or a fixed resistance.

Examples: carbon, tungsten, platinum, mercury, lead, and alloys such as constantan (eureka), nichrome, manganin, phosphor bronze and tin-lead alloy.

Applications:

• Heating elements (heater, electric iron, toaster)
• Rheostats, resistance boxes and starter resistances
• Filaments of bulbs
• Fuse wire and solder

Important point: Tungsten is placed in this group by the syllabus. Its resistivity is higher than copper, but its main use as a filament comes from its very high melting point.

6. Comparison: Low and High Resistivity Materials

Basis	Low resistivity materials	High resistivity materials
Resistivity	Very low (about 10−8 Ω·m)	High (about 10−7 Ω·m and above)
Conductivity	High	Low
Heat produced by current	Very little	More
Main aim of use	To carry current with minimum loss	To produce heat or to give a fixed resistance
Examples	Silver, copper, aluminium, ACSR, brass, bronze	Nichrome, constantan, manganin, tungsten, carbon
Applications	Cables, windings, bus bars, transmission lines	Heaters, rheostats, resistance boxes, filaments

Note: The resistivity values above are approximate textbook values and are not given in the syllabus. For example, copper is about 1.7 × 10⁻⁸ Ω·m and nichrome is about 1.1 × 10⁻⁶ Ω·m.

Exam tip: "Compare low resistivity and high resistivity materials" has been asked as a 4-mark short question. A table like the one above is the best way to answer it.

7. Temperature Coefficient of Resistance

Definition: The temperature coefficient of resistance (α) is the change in resistance per ohm of original resistance for a one-degree rise in temperature.

The resistance at a higher temperature is given by R_t = R₀ (1 + α ΔT), where ΔT is the rise in temperature.

Type of coefficient	What happens on heating	Examples
Positive	Resistance increases	Copper, aluminium, steel, mercury
Negative	Resistance decreases	Carbon, semiconductors, insulators
Almost zero	Resistance stays nearly constant	Manganin, constantan

Important point: Because manganin and constantan hardly change their resistance with temperature, they are used in resistance boxes and measuring instruments.

Exam tip: Previous papers have asked which material has a negative coefficient (carbon) and which has zero coefficient (manganin).

8. Superconductivity

Definition: Superconductivity is the property of some materials by which their electrical resistance becomes exactly zero when they are cooled below a certain temperature, called the critical temperature.

A material in this state is called a superconductor. Current once started in a closed superconducting loop can flow for a very long time without any voltage source.

Examples: mercury (about 4.2 K), lead (about 7.2 K), and the alloy niobium-tin. These temperatures are approximate and are not from the syllabus.

Properties:

• Zero electrical resistance below the critical temperature
• Relative permeability is zero, so a superconductor pushes out a magnetic field
• Superconductivity is lost if the temperature, the magnetic field or the current becomes too high

Applications:

• Powerful electromagnets, for example in MRI machines
• Loss-free power cables and generators (still mostly in the research and trial stage)
• Magnetic levitation trains

Important point: Superconductors work only at very low temperatures, so they need costly cooling. This limits their everyday use.

Exam tip: "Define super conductivity with two examples and mention its applications" has been asked as a 4-mark short question. The syllabus text for Unit II does not name this topic, but the papers have used it, so learn it.

Everyday Examples to Remember

• House wiring is copper or aluminium: low resistivity, so no heat loss in the wire.
• The heating coil of a room heater is nichrome: high resistivity, so the coil gets red hot.
• The wire connecting your heater to the plug stays cool, while the coil glows. Same current, different resistivity.
• An MRI machine in a hospital uses superconducting magnets cooled to very low temperature.

MCQ and True/False Facts

• Materials which allow the flow of current through them are called conductors.
• Ductility is the property of being drawn into wires.
• Copper and aluminium are the most commonly used conducting materials. Copper is the most used.
• Carbon has a negative temperature coefficient of resistance.
• Manganin has a zero (almost zero) temperature coefficient of resistance.
• The relative permeability of a superconducting material is zero.
• The electrons in the outermost orbit of an atom are called valence electrons, not free electrons. So the statement "the electrons in the outermost orbit are known as free electrons" is False.

Important Questions for Examinations

Short Answer Questions

1. What are conducting materials? Give examples.
2. List the desirable properties of a good conductor.
3. Compare low resistivity and high resistivity materials.
4. Define temperature coefficient of resistance. Name one material each with positive, negative and almost zero coefficient.
5. Define superconductivity with two examples and mention its applications.

Long Answer Question

1. Explain conducting materials and their two types (low resistivity and high resistivity). State the desirable properties of a good conductor and compare the two types with examples and applications.

Quick Revision

• Conductors have many free electrons and no forbidden gap.
• R = ρ l / A. Resistivity depends only on the material. σ = 1 / ρ.
• Low resistivity: carry current with low loss (silver, copper, aluminium, ACSR, brass, bronze).
• High resistivity: produce heat or give fixed resistance (nichrome, constantan, manganin, tungsten, carbon).
• Good conductor: low resistivity, strong, ductile, corrosion resistant, easy to solder, low cost.
• Positive coefficient: metals. Negative: carbon. Almost zero: manganin, constantan.
• Superconductor: zero resistance below the critical temperature. Relative permeability is zero.

Related Topics to Study Next

Before this note: Classification of Electrical Engineering Materials. Next: Low Resistivity Materials; High Resistivity Materials; Germanium and Silicon.

Conclusion

Conducting materials are divided by resistivity. Low resistivity materials carry current with little loss, so they are used for wires, cables and windings. High resistivity materials resist current and produce heat, so they are used for heaters, resistors and filaments. A good conductor is also strong, ductile and corrosion resistant. Superconductors are a special case in which resistance vanishes completely at very low temperatures.