The properties of materials govern how they interact with the world, allowing scientists to identify substances and predict their behavior. Among these defining characteristics, the ability of a material to allow the flow of electric charge is important. Electrical conductivity is categorized as a physical property of matter.
Defining Electrical Conductivity
Electrical conductivity measures a material’s inherent capacity to transmit an electric current. This describes the ease with which charged particles move through a substance when an electric potential, or voltage, is applied. Materials with high conductivity, such as metals, permit charge to flow readily, while those with low conductivity, like rubber, oppose this movement.
The underlying mechanism for charge transport differs depending on the material type. In solid metals, conductivity is facilitated by the movement of delocalized electrons that are not tightly bound to any single atom. These electrons drift in the direction of the electric field, creating the electric current. Conversely, in liquid solutions, such as saltwater, the current is carried not by electrons but by the migration of positively and negatively charged ions.
The mobility of these charge carriers determines the material’s overall conductivity. When an electric field is applied, the charge carriers accelerate, but they frequently collide with the atoms or ions that form the material’s structure, which restricts their flow. This internal friction gives rise to the material’s electrical resistance. A material’s composition dictates the number of available charge carriers and how easily they can move.
The Criteria for a Physical Property
The classification of any material characteristic as a physical property rests on one core criterion: the observation or measurement must not change the substance’s chemical identity. Physical properties can be assessed without transforming the material into a new substance with a different molecular composition. For example, determining the density of iron involves measuring its mass and volume, but the iron atoms remain chemically unchanged.
This category includes observable characteristics like color, hardness, and state of matter. It also encompasses measurable quantities such as melting point and boiling point, which involve a change in state but not a change in chemical makeup. When ice melts into liquid water, the substance is still \(\text{H}_2\text{O}\). Since the chemical identity is preserved, this confirms that melting point is a physical property.
Distinguishing Physical from Chemical Properties
To understand why electrical conductivity is a physical property, it is helpful to contrast it with chemical properties. Chemical properties are those that can only be observed or quantified when a substance undergoes a chemical reaction that fundamentally changes its composition. Unlike physical properties, measuring a chemical property results in the formation of a new substance.
A common example of a chemical property is flammability, which is a material’s ability to burn or ignite. Determining flammability requires ignition, a process that converts the original substance into different products like ash and carbon dioxide. Similarly, reactivity with acid or susceptibility to corrosion are chemical properties because observing them requires a chemical transformation.
Measuring electrical conductivity involves applying a voltage and observing the resulting current flow, a process that leaves the material’s atomic and molecular structure intact. The electrons or ions carrying the current are merely passing through the material rather than reacting to form new chemical bonds. Because the substance remains chemically the same, conductivity is classified as a physical property.
Quantifying Conductivity
Electrical conductivity is a quantifiable measure, allowing for precise comparison between different materials. The standard unit for conductivity is the Siemens per meter (\(\text{S/m}\)). One Siemens is defined as the reciprocal of one Ohm, the unit of electrical resistance.
Conductivity is often discussed alongside its inverse, electrical resistivity. Resistivity (\(\rho\)) represents a material’s opposition to the flow of charge and is measured in Ohm-meters (\(\Omega\cdot\text{m}\)). A material with high conductivity possesses low resistivity, and the relationship is expressed simply as \(\sigma = 1/\rho\).
This quantification allows for the classification of materials into conductors, semiconductors, and insulators. For example, copper’s conductivity is measured in the millions of \(\text{S/m}\), while an electrical insulator like glass is practically zero. These numerical values are inherent to the material and are independent of its shape or size, making conductivity an intensive physical property.