1.1 : Charge and Current

Electric charge is the most fundamental quantity in an electric circuit. The effects of electric charge are encountered daily, such as when a wool sweater sticks to the human body or when a person receives a shock while walking on a carpet.

Charge is an inherent property of the atomic particles that make up matter and is measured in units called coulombs (C). Matter is composed of atoms, each consisting of electrons, protons, and neutrons. Electrons have a negative charge (-e), while protons carry a positive charge (+e), with both charges having the same magnitude. An atom is electrically neutral when it has an equal number of protons and electrons.

Key points about electric charge include:

• The coulomb (C) is a relatively large unit for measuring charges, and practical charge values are often in picocoulombs (pC), nanocoulombs (nC), or microcoulombs (μC).
• Observations reveal that natural charges exist only as integral multiples of the elementary charge (e), the charge carried by an electron or proton.
• The law of conservation of charge asserts that electric charge cannot be created nor destroyed; it can only be transferred. As a result, the total electric charge in a system remains constant. Charge can be moved from one place to another and converted into different forms of energy.

The movement of positive charges in one direction and negative charges in the opposite direction gives rise to electric current. Typically, a conventional current flow is defined as the movement of positive charges, and it is measured in units known as amperes (A). When the current remains constant over time, it is called direct current (DC). In cases where the current varies with time, it is denoted as a time-varying current, often referred to as alternating current (AC). AC is commonly used in households to power appliances like air conditioners, refrigerators, and washing machines.