Understanding Electric Charges and Material Conductivity
Understanding Electric Charges
Matter contains two types of electric charges of opposite signs: positive and negative. Usually, bodies are electrically neutral. However, through certain physical processes, they can gain or lose electric charges, thus becoming electrically charged. These processes include:
- Charging by Friction
- Charging by Induction
- Charging by Contact
Charging by Friction
In everyday life, electrical phenomena often occur through friction. You can observe that bodies with like charges repel, and bodies with unlike charges attract. When two bodies are rubbed together, some electrons from one body are transferred to the other. The body that loses electrons becomes positively charged, while the body that gains electrons becomes negatively charged. Protons do not participate in this transfer because they are tightly bound within the atomic nucleus.
Charging by Induction
Consider an example: if you rub a pen and then bring it near small pieces of paper, the paper is attracted to the pen. When the charged pen is brought near the paper, charges of the same sign on the paper are repelled, while charges of the opposite sign are attracted, causing the paper to stick to the pen.
Material Conductivity: Conductors, Insulators, & Semiconductors
Metals that allow electric charge to move freely are called conductors, while materials that do not allow electric charges to pass through are insulators. Semiconductors, such as silicon and germanium, fall into an intermediate category. These materials are fundamental to building diodes, transistors, and integrated circuits (chips).
Why Are Some Materials Good Conductors?
Some substances naturally possess more free electrons in their conduction band than others. The behavior of a material can also change under varying external conditions. The property of certain substances having free electrons in the conduction band is called conductivity. These materials are capable of conducting electricity under the action of external forces.
Metals like copper have electrons in their outermost shell that are weakly bound to the nucleus, making them excellent conductors. While gold is an even better conductor, its high cost limits its widespread use.
Why Are Some Materials Poor Conductors?
Poor conductors, or insulators, are materials where electrons are tightly bound to their nuclei, preventing them from moving freely within the material. Consequently, they do not conduct electricity well. Common insulators include air, porcelain, and glass.
What Differentiates Conductors from Insulators?
The distinction between a conductor and an insulator is one of degree rather than kind, as all substances conduct electricity to some extent. In conductive substances, atoms possess outer electrons that are weakly bound to the nucleus, existing in a state of semi-freedom. This grants them greater mobility, as seen in metals. In contrast, insulating substances have nuclei that retain all their electrons tightly, severely limiting their mobility.
For instance, a good conductor like silver or copper can have a conductivity a billion times greater than that of a good insulator such as glass or mica.
Electron Energy Levels and Bands
In an atom, electrons orbit the nucleus in distinct energy levels or shells. The energy possessed by electrons varies depending on their shell. Electrons in shells very close to the nucleus experience a strong attractive force, making them tightly bound. Conversely, electrons in outer shells are more loosely bound, facilitating electron exchanges in these outermost layers.
This variation in electron energy levels allows us to classify them into energy bands. To better understand atomic behavior, particularly in conductivity, we focus on two key bands: the valence band and the conduction band.
The Valence Band
The valence band is an energy level where electrons participate in chemical bonding. Electrons in this band can be transferred between atoms, forming ions that attract due to their opposite charges, or they can be shared by multiple atoms, forming molecules.
For example, a sodium (Na) atom has 11 electrons (2 in the first layer, 8 in the second, and 1 in the third). A chlorine (Cl) atom has 17 electrons (2 in the first, 8 in the second, and 7 in the third). According to the octet rule, atoms tend to achieve 8 electrons in their valence shell. Thus, sodium will donate its single valence electron to chlorine. Sodium will then have 8 electrons in its new outermost shell, while chlorine will accept that electron, completing its valence shell to 8 electrons.
The Conduction Band
The conduction band is an energy level above the valence band. Electrons in the conduction band are free to move throughout the material and are responsible for electrical conductivity. In conductors, the valence and conduction bands overlap or are very close, allowing electrons to easily move into the conduction band. In insulators, there is a large energy gap between the valence and conduction bands, making it difficult for electrons to move into the conduction band.