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Chapter 1: Basics of Electric Circuits Back To Chapter

1.1: Charge and Current

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Charge and Current

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Charge, an inherent property of matter, is responsible for a wide range of electric phenomena. The SI unit of charge is the coulomb.

All matter comprises atoms, which consist of three subatomic particles: protons, neutrons, and electrons. When the number of protons and electrons are the same, the atom is electrically neutral.

Charges can be transferred from one body to another, and the net charge within the system remains the same.

Charges are quantized, meaning the charge on a body is always the integral multiple of the electronic charge.

When a conducting wire is connected to a battery, the movement of charges within the wire creates current. Current is the time rate of flow of electric charge, measured in amperes.

Conventionally, its direction is considered opposite to the flow of negative charges.

The charge transferred over a specific time interval can be obtained by integrating the current with respect to time.

There are two most common types of currents. A direct current remains constant with time, while the alternating current varies sinusoidally with time.

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.

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