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 swe…
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.
View the full transcript and gain access to JoVE Core videos
Q1: What is electric charge and how is it measured?
Electric charge is an inherent property of matter responsible for electric phenomena, measured in coulombs (C). Since the coulomb is relatively large, practical values are often expressed in picocoulombs (pC), nanocoulombs (nC), or microcoulombs (μC). Electrons carry negative charge (-e) and protons carry positive charge (+e), with equal magnitudes.
Q2: Why is charge quantized and what does that mean?
Charge is quantized, meaning any charge on a body is always an integral multiple of the elementary charge (e), the charge carried by an electron or proton. Natural charges cannot exist as arbitrary values; they must be whole-number multiples of this fundamental unit, reflecting the discrete nature of atomic structure.
Q3: How does the law of conservation of charge apply to electric systems?
The law of conservation of charge states that electric charge cannot be created or destroyed; it can only be transferred between bodies. When charges move from one location to another, the total electric charge in the system remains constant, though charge distribution may change.
Q4: What is electric current and how does it relate to charge movement?
Electric current is the time rate of flow of electric charge, measured in amperes (A). It arises when charges move through a conducting wire connected to a battery. Conventionally, current direction is defined as the flow of positive charges, opposite to negative charge movement.
Q5: What is the difference between direct current and alternating current?
Direct current (DC) remains constant over time, while alternating current (AC) varies sinusoidally with time. AC is commonly used in households to power appliances like air conditioners, refrigerators, and washing machines, whereas DC is used in batteries and electronic devices.
Q6: How do you calculate the total charge transferred in a circuit over time?
The charge transferred over a specific time interval is obtained by integrating the current with respect to time. This relationship allows you to determine total charge flow when current varies, essential for understanding how electric circuit elements function in practical applications.
Q7: What determines whether an atom is electrically neutral?
An atom is electrically neutral when the number of protons equals the number of electrons. Since protons carry positive charge and electrons carry negative charge with equal magnitudes, equal quantities result in zero net charge on the atom. This balance is fundamental to understanding charge behavior.