Molly Ayala
Electricity is the flow of electric charges through a conductor, and the movement of these charges follows well-established physical principles. These principles have been systematically formulated into a set of fundamental electrical laws that govern the behaviour of electric fields, currents, and circuits. Understanding these laws is crucial for designing and analysing electrical systems, from simple circuits to complex power networks. Some of the most important laws related to electrical energy include Coulomb's law, Ohm's law, Kirchhoff's current law, Faraday's law of electromagnetic induction, and Fleming's rule. These laws provide insights into the behaviour of electric charges, currents, voltages, and magnetic fields, helping engineers and scientists design and optimise electrical systems.
| Characteristics | Values |
|---|---|
| Current | The movement of charge over time, expressed in amperes (A) |
| Voltage | Expressed in ohms (Ω) |
| Resistance | The ability of an element to resist the flow of electric current; measured in ohms (Ω) |
| Conductance | The ability of an element to conduct electric current, measured in mhos (℧) or siemens (S) |
| Ohm's Law | The voltage across a resistor is directly proportional to the current flowing through it |
| Coulomb's Law | The electric force between two charged objects is inversely proportional to the square of the distance between them and directly proportional to the product of their charges |
| Faraday's Law | A changing magnetic field induces an electric current in a conductor |
| Kirchhoff's Current Law | The sum of the currents entering a node equals the sum of the currents leaving it |
| Fleming's Rule | Used for motors and generators |
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What You'll Learn
- Current: The movement of charge over time, measured in amperes
- Coulomb's Law: Electric force between two objects is proportional to their charges and inversely proportional to the distance between them
- Ohm's Law: Voltage across a resistor is proportional to the current flowing through it
- Kirchhoff's Current Law: The sum of currents entering a node equals the sum of currents leaving it
- Faraday's Law: A changing magnetic field induces an electric current in a conductor
Current: The movement of charge over time, measured in amperes
Current is the movement of electric charge over time. It is expressed in amperes (A), where 1 ampere is equivalent to 1 coulomb per second. In other words, current measures how much charge is flowing and in which direction it is moving.
In electrical circuits, current is driven by the potential difference between two points, creating a flow of electrons. Electrons move from the negative terminal of a battery to the positive terminal, but the direction of the current is from positive to negative. This is because current is treated as the movement of positive charge, reflecting the original understanding of current before the discovery of the electron.
Ohm's law, which describes the behaviour of electrically conductive materials, states that the voltage across a resistor is directly proportional to the current flowing through it. The law can be expressed mathematically as v=iR, where v is the voltage, i is the current, and R is the resistance. The resistance of a material denotes its ability to resist the flow of electric current and is measured in ohms (Ω), named after German physicist Georg Ohm.
Kirchhoff's current law, also known as the node law, is based on the conservation of electric charge. It states that the sum of the currents entering a node in an electrical circuit must equal the sum of the currents leaving that node, meaning there can be no charge accumulation. This law is particularly important in understanding the behaviour of electric currents in complex circuits with multiple interconnected components.
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Coulomb's Law: Electric force between two objects is proportional to their charges and inversely proportional to the distance between them
Coulomb's Law, or Coulomb's inverse-square law, is an experimental law in physics that calculates the amount of force between two electrically charged particles at rest. It was first published in 1785 by French physicist Charles-Augustin de Coulomb, who used a torsion balance to study the repulsion and attraction forces of charged particles. Coulomb's law states that the magnitude, or absolute value, of the attractive or repulsive electrostatic force between two point charges is directly proportional to the product of the magnitudes of their charges and inversely proportional to the square of the distance between them. In other words, the electric force between two objects is proportional to their charges and inversely proportional to the distance between them.
The electrostatic force between two point charges always acts along the line joining the two charges. This force is conventionally called the Coulomb force or electrostatic force. Coulomb's law is not universal, as it depends on the properties of the intervening medium. It also does not apply to moving charges due to the breaking of symmetry by the specification of the direction of velocity in the problem.
Coulomb's law is expressed by the following equation:
$\displaystyle \mathbf {F} _{1}={\frac {q_{1}q_{2}}{4\pi \varepsilon _{0}}}{{\hat {\mathbf {r} }}_{12} \over {|\mathbf {r} _{12}|}^{2}}F1 is the force exerted by charge q1 on charge q2
- Q1 and q2 are the quantities of each charge
- Ε0 is the dielectric constant of the medium in which the two charges are placed
- R12 is the distance between the charges
The force is along the straight line joining the two charges. If the charges have the same sign, they repel each other; if they have different signs, they attract each other. Coulomb's law is similar to Isaac Newton's inverse-square law of universal gravitation, but gravitational forces always attract, while electrostatic forces can attract or repel.
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Ohm's Law: Voltage across a resistor is proportional to the current flowing through it
Ohm's Law is one of the most basic and important laws of electric circuits. It is named after German physicist Georg Simon Ohm, whose work in electrical research resulted in this famous law.
Ohm's Law states that the voltage across a resistor is directly proportional to the current flowing through it. In other words, the current is directly proportional to the voltage and inversely proportional to the resistance. This relationship can be expressed mathematically as v ∝ i, where v is the voltage and i is the current. The constant of proportionality, R, is called Resistance and has units of ohms, denoted by the symbol Ω. Therefore, the equation can also be written as v=iR.
Ohm's Law can be used to calculate the current and voltage in a circuit. For example, if the voltage across a resistor is known, and the resistance of the resistor is also known, then Ohm's Law can be used to calculate the current flowing through the resistor. Similarly, if the current and resistance are known, the voltage can be calculated.
It is important to note that Ohm's Law assumes that all physical conditions and temperatures remain constant. Additionally, not all resistors obey Ohm's Law. A resistor that obeys Ohm's Law is known as a linear resistor, with constant resistance. Ohm's Law is also not applicable to unilateral electrical elements like diodes and transistors, as they only allow current to flow in one direction.
A hydraulic analogy is often used to describe Ohm's Law. In this analogy, water pressure is the equivalent of voltage, water volume flow rate is the equivalent of current, and flow restrictors such as apertures placed in pipes are the equivalent of resistors.
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Kirchhoff's Current Law: The sum of currents entering a node equals the sum of currents leaving it
Electricity is the movement of charge over time, expressed in amperes (A), where 1 ampere is equivalent to 1 coulomb per second.
Kirchhoff's Current Law, often abbreviated as KCL, is one of two equalities that deal with the current and potential difference (commonly known as voltage) in the lumped element model of electrical circuits. This law, also known as Kirchhoff's first law or Kirchhoff's junction rule, states that the sum of currents entering a node equals the sum of currents leaving it. In other words, the algebraic sum of currents in a network of conductors meeting at a point is zero.
This principle can be expressed mathematically as:
${\displaystyle \sum _{i=1}^{n}I_{i}=0}$
Where n is the total number of branches with currents flowing towards or away from the node. This law is based on the conservation of charge, as there is no loss of current around the junction.
Kirchhoff's Current Law can be applied to any node, regardless of the number of currents entering or exiting. For example, in a simple single junction, the current IT leaving the junction is the sum of the two currents, I1 and I2, entering the same junction (IT = I1 + I2). This can also be written as IT – (I1 + I2) = 0, so if I1 = 3 amperes and I2 = 2 amperes, then the total current IT leaving the junction will be 5 amperes.
Kirchhoff's laws are widely used in electrical engineering and form the basis for network analysis. They were first described in 1845 by German physicist Gustav Kirchhoff, generalizing the work of Georg Ohm, who discovered the relationship between voltage and current in a resistor (Ohm's Law).
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Faraday's Law: A changing magnetic field induces an electric current in a conductor
The basic laws of electricity govern the behaviour of electric currents and magnetic fields. One such law is Ohm's law, which states that the voltage across a resistor is directly proportional to the current flowing through it.
Faraday's law of induction, discovered by Michael Faraday in 1831, describes how a changing magnetic field can induce an electric current in a conductor. This phenomenon, known as electromagnetic induction, is fundamental to the operation of transformers, inductors, and many types of electric motors, generators, and solenoids.
Faraday's experiments showed that a changing current in one coil of wire created a changing magnetic field in an iron ring, which induced a current in a second coil. This led to the development of Faraday's disk or homopolar generator, which produces a steady (DC) current by rotating a copper disk in the presence of a stationary magnet.
Faraday's law is expressed in two closely related but physically distinct statements. The first is the Maxwell-Faraday equation, one of Maxwell's equations, which states that a time-varying magnetic field is accompanied by a circulating electric field. This law applies to the fields themselves and does not require a physical circuit. The second is Faraday's flux rule, or the Faraday-Lenz law, which relates the electromotive force (emf) around a closed conducting loop to the time rate of change of magnetic flux through the loop.
The magnitude and direction of the induced current in a loop can be determined by Faraday's law and Lenz's law, respectively. The induced current produces a magnetic field that opposes the changing flux, and the direction of the current is such that it counteracts the change.
Faraday's law demonstrates the deep connection between electricity and magnetism, showing that a changing magnetic field can induce an electric current, and vice versa. This discovery has had a profound impact on modern technology, forming the basis of our electric power grid and many devices we use today.
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Frequently asked questions
There are several basic laws of electricity, including:
- Coulomb's law
- Ohm's law
- Kirchhoff's current law
- Faraday's law of electromagnetic induction
Coulomb's law states that the electric force between two charged objects is inversely proportional to the square of the distance between them and directly proportional to the product of their charges.
Ohm's law states that the voltage across a resistor is directly proportional to the current flowing through it.
Kirchhoff's current law, also known as the node law, is based on the conservation of electric charge. It states that the sum of the currents entering a node must equal the sum of the currents leaving that node.
Faraday's law of electromagnetic induction explains how a changing magnetic field induces an electric current in a conductor.
