Electric Charge and Electric Field

Introduction to Electric Charge

Electric charge is a fundamental property of matter that gives rise to electric forces and fields. It is quantized and measured in coulombs (C). The concept of charge is essential in understanding interactions at the atomic and molecular level, especially in biological systems.

• Unit of Charge: The SI unit is the coulomb (C).

• Elementary Charge: The charge of a single electron is , where C.

• Quantization: Electric charge exists in discrete units, multiples of the elementary charge.

• Historical Note: The unit is named after Charles-Augustin de Coulomb, a French physicist.

Charge in the Atom

Atoms consist of protons, neutrons, and electrons. The balance of charges determines the atom's net charge.

• Proton: Mass kg, charge .

• Electron: Mass kg, charge .

• Neutron: Electrically neutral, mass .

• Example: A neutral lithium atom has 3 protons, 4 neutrons, and 3 electrons. The number of electrons equals the number of protons, resulting in zero net charge.

Conductors and Insulators

Materials are classified based on how freely their electrons can move, affecting their ability to conduct electric charge.

• Insulators: Electrons are tightly bound to nuclei and cannot move freely. Charging by friction leaves localized patches of charge.

• Conductors: Valence electrons are weakly bound and can move throughout the material, forming a "sea of electrons." Metals are typical conductors.

• Diagram: Insulators show electrons fixed to atoms; metals show mobile electrons.

Estimating Number of Electrons in Water

Calculating the number of electrons in a given volume of water is useful for understanding charge distribution in biological systems.

• Molar Mass of Water: Approximately 18 g/mol.

• Density of Water: .

• Avogadro's Number: molecules/mol.

• Calculation: In 1 L (1 kg) of water, there are mol. Total molecules .

• Electrons per H2O: 10 electrons (2 from H, 8 from O). Total electrons .

• Example: One liter of water contains approximately electrons.

Electric Dipole

An electric dipole consists of two equal and opposite charges separated by a distance. Dipoles are important in molecular biology, as many molecules (e.g., water) have dipole moments.

• Definition: A pair of point charges, and , separated by distance .

• Dipole Moment: , directed from negative to positive charge.

• Example: Water molecule (H2O) forms an electric dipole due to its molecular geometry.

• Applications: Dipoles affect how molecules interact, dissolve, and respond to electric fields.

Additional info: The concept of electric dipoles is fundamental in understanding molecular polarity and interactions in biological systems.