Beatrice Lang
The laws of electricity describe the behaviour of electrical charges, their interactions, and how they produce electric fields and currents. There are four basic laws of electricity: Ohm's law, Kirchhoff's laws, and the power law. Ohm's law, discovered in the 1920s, explains the relationship between current, resistance, and potential. The first of Kirchhoff's two laws states that the sum of currents at any point in a circuit must equal zero, while the second law states that the sum of voltages in a closed loop must equal zero.
| Characteristics | Values |
|---|---|
| Relationship between current, resistance, and potential | Current (I) measured in amps (A), resistance (R) measured in ohms (Ω), and potential (V) measured in volts (V) |
| First Kirchhoff's Law | Sum of currents at any point in a circuit must equal zero |
| Second Kirchhoff's Law | Sum of voltages in a closed loop must equal zero |
| Ohm's Law | Current is approximately proportional to the electric field for most materials |
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What You'll Learn
Ohm's Law: Relationship between current, resistance, and potential
The three laws of electricity describe the behaviour of electrical charges, their interactions, and how they produce electric fields and currents. One of these laws is Ohm's Law, which describes the relationship between current, resistance, and potential (voltage).
Ohm's Law, discovered by German physicist and mathematician Georg Simon Ohm and published in his 1827 paper "The Galvanic Circuit Investigated Mathematically", established the relationship between resistance, current, and voltage. An electric circuit is formed when a conductive path is created to allow for the continuous movement of electric charge through the conductors of a circuit. This continuous movement of electric charge is referred to as a current, often described in terms of "flow".
The force that motivates charge carriers to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two points. It is the potential difference between two points as it is applied across a wire or an electric component. In other words, voltage is a measurement of how much potential energy exists to move charge carriers from one particular point in a circuit to another. This voltage can be measured in volts, where 1 volt is equal to 1 joule of electric potential energy per (divided by) 1 coulomb of charge.
Ohm's principal discovery was that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage across it, for any given temperature. This relationship can be expressed by the equation: Voltage (V) = Current (I) x Resistance (R), or V = IR. This equation can be rearranged to solve for any of the three parameters, depending on what is unknown. For example, to calculate the current in a circuit, the equation becomes: Current (I) = Voltage (V) / Resistance (R), or I = V/R.
Ohm's Law can be applied to any circuit with a single source of voltage and resistance to determine the unknown values of voltage, current, or resistance. By knowing any two of these three parameters, the third can always be calculated using Ohm's Law.
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Kirchhoff's Laws: Currents and voltages in a closed loop
The three laws of electricity are Kirchhoff's Circuit Laws, which consist of the Current Law (KCL) and the Voltage Law (KVL), and the law of conservation of energy. These laws were first described by German physicist Gustav Kirchhoff in 1845, generalizing the work of Georg Ohm.
Kirchhoff's Circuit Laws allow us to solve complex circuit problems by defining a set of basic network laws and theorems for voltages and currents in a circuit. They can be applied to both time and frequency domains and form the basis for network analysis.
Kirchhoff's Voltage Law (KVL) states that the algebraic sum of all voltage sources and voltage drops within a closed loop must equal zero. In other words, the total voltage around a closed loop network is equal to the sum of all voltage drops within the same loop. This is also known as the Conservation of Energy.
When applying Kirchhoff's Voltage Law to a specific circuit element, it is important to pay attention to the algebraic signs of the voltage drops across elements and the emf's of sources, as well as the direction of the current flowing through the element. The direction of current flow around a closed circuit can be assumed to be either clockwise or anticlockwise.
Kirchhoff's Current Law (KCL) states that the sum of the currents flowing into a node (junction) in an electrical circuit is equal. This law is dependent on the assumption that the net charge in any wire, junction, or lumped component is constant.
By applying Kirchhoff's Circuit Laws, we can calculate and find the currents and voltages around any closed circuit, as long as we know the values of the electrical components within it.
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Power Law: Power lost as energy per unit time
The laws of electricity govern the behaviour of electrical charges, their interaction with one another, and how they produce electric fields and currents. One of the fundamental concepts in understanding electricity is the relationship between power, energy, and time. This relationship is encapsulated in the equation: Energy = Power x Time, or alternatively, Power = Energy/Time. In other words, power is the rate at which energy is transferred or consumed.
In the context of electrical circuits, power is the product of voltage and current. Voltage acts as the source of energy, delivering power, while the connected load, such as a light bulb or heater, absorbs it. This absorbed electrical power is then converted into other forms of energy, such as heat or light. The amount of power consumed or generated is typically measured in watts or kilowatts (1 kilowatt = 1000 watts), and time is usually measured in hours.
When we consider the unit of electrical energy, it is given in kilowatt-hours (kWhr). This is calculated by multiplying the power consumption or generation in kilowatts by the time in hours. For example, a 100-watt light bulb switched on for 10 hours will consume 1 kilowatt-hour of electricity. This is a standard unit of energy used by electricity meters in households to calculate the amount of electrical energy consumed and, consequently, the cost reflected in the bills.
It's important to note that electrical energy can also be measured in joules, which is a more convenient unit when dealing with smaller amounts of energy. One watt is equal to one joule per second, so if one joule of work is absorbed or delivered at a constant rate of one second, the corresponding power is one watt. However, when calculating the energy consumed by certain components, the value in joules can become quite large.
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Hydraulic analogy: Ohm's law applied to fluid flow
The three laws of electricity describe the behaviour of electrical charges, their interactions, and how they produce electric fields and currents. One of these laws is Ohm's law, which can be applied to fluid flow through the hydraulic analogy.
Ohm's law can be understood through a water-and-pipe analogy. In this model, a water pump exerts pressure (voltage) to push water around a "circuit" (current) through a restriction (resistance). The three variables of voltage, current, and resistance are interrelated. For example, if the resistance to water flow remains constant and pump pressure increases, the flow rate will also increase.
The hydraulic analogy can be further explored through the concept of hydraulic ohms, which are the units of hydraulic impedance. Hydraulic impedance is defined as the ratio of pressure to volume flow rate, with pressure and volume flow variables treated as phasors, possessing both phase and magnitude. This is similar to the relationship between acoustic impedance and acoustic pressure in acoustics.
The hydraulic analogy has its limitations, however, as it does not fully align with Kirchhoff's current law. In hydraulic systems, the amount of liquid is typically not constant, and elements such as pistons and open pools can lead to changes in volume. Electric currents, on the other hand, require closed loops. Additionally, the typical velocity of charge carriers within a conductor is relatively low, and the "electrical friction" is extremely high, resulting in slower charges compared to the flow of water in pipes.
Despite these differences, the hydraulic analogy can still provide insights into electrical systems. For example, the inertia of water flowing through a pipe in a hydraulic ram system is analogous to the inductance effect in electrical systems, where inductors "filter out" rapid changes in flow while allowing slow variations in current.
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Modern developments: Ohm's law and electromagnetic theory
Ohm's law, named after German physicist Georg Ohm, is one of the earliest quantitative descriptions of the physics of electricity. In 1827, Ohm described measurements of applied voltage and current through simple electrical circuits with wires of varying lengths. According to Ohm's law, the electric current through a conductor between two points is directly proportional to the voltage across the two points. Ohm's law can be applied to a single component, a group of components, or an entire circuit. It is fundamental to all electronics.
The law can be expressed using three mathematical equations, where 'I' is the current through the conductor, 'V' is the voltage across the conductor, and 'R' is the resistance of the conductor. The 'R' in this relation is constant, independent of the current. If the resistance is not constant, the equation can still be used as a definition of static/DC resistance.
In 1900, Paul Drude proposed the first (classical) model of electrical conduction, known as the Drude model, which provided a scientific explanation for Ohm's law. The Drude model describes a solid conductor as consisting of a stationary lattice of atoms (ions) with conduction electrons moving randomly within it. When a voltage is applied across a conductor, an electric field is created, causing the electrons to accelerate in the direction of the field, resulting in an electric current. However, collisions with atoms cause the electrons to scatter and randomize their motion, converting kinetic energy into heat (thermal energy).
In the modern era, scientists continue to refine the theory of electromagnetism to incorporate the principles of modern physics, including quantum mechanics and relativity. Electromagnetism has been studied for thousands of years, with ancient civilizations like the Greeks and Mayans developing theories to explain lightning, static electricity, and the attraction between magnetized pieces of iron ore. However, it was not until the late 18th century that scientists began to formulate a mathematical framework for understanding electromagnetic interactions. Gian Romagnosi, in 1802, was among the first to discover and publish a link between human-made electric current and magnetism. Ørsted's similar experiment in 1820 gained wider recognition and influenced Ampère's work in electrodynamics, leading to the establishment of a mathematical foundation for the subject.
The work of prominent scientists like Coulomb, Gauss, Faraday, and Maxwell in the 18th and 19th centuries resulted in namesake laws that helped elucidate the formation and interaction of electromagnetic fields. Maxwell's treatise, published in 1873, unified previous developments and proposed that light was an electromagnetic wave. The understanding of the nature of light was further refined in the 19th century, with the realization that light and other electromagnetic waves are quantized, self-propagating oscillatory electromagnetic field disturbances called photons.
Today, electromagnetism plays a crucial role in various aspects of modern technology, including electrical energy production, transformation, and distribution; light, heat, and sound production and detection; fiber optic and wireless communication; sensors; computation; electrolysis; electroplating; and mechanical motors and actuators.
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Frequently asked questions
There are four basic laws of electricity: Ohm's law, Kirchhoff's laws, and the power law.
Ohm's law explains the relationship between current, measured in amps, resistance, measured in ohms, and potential, measured in volts.
Kirchhoff's laws state that the sum of the currents at any point in a circuit must equal zero, and that the sum of the voltages in a closed loop must also equal zero.
When a current passes through a resistor, the temperature of the resistor increases, and power (energy per unit time) is lost.
