Wiktor Wise
The electromagnetic force is one of the four fundamental forces of nature, and it is responsible for many of the chemical and physical phenomena we observe in our daily lives. Electricity and magnetism, while once considered separate forces, are closely intertwined phenomena that make up electromagnetism. The electric force acts between all charged particles, whether they are moving or not, while the magnetic force acts between moving charged particles. This means that every charged particle gives off an electric field, and when those charged particles are in motion, they give off magnetic fields. The behaviour of the electromagnetic field is described by a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.
| Characteristics | Values |
|---|---|
| Nature of electromagnetism | Interaction between electrically charged particles via electromagnetic fields |
| Types of forces | Electric force, magnetic force |
| Electric force | Acts between all charged particles, whether or not they are moving |
| Magnetic force | Acts between moving charged particles |
| Combination of forces | Electric and magnetic forces combine to form the electromagnetic force |
| Equations | Maxwell's equations describe the behaviour of electromagnetic fields; Lorentz force law describes the electromagnetic force |
| Relativity | As an observer moves with charged particles, magnetic fields transform into electric fields and vice versa |
| Induction | Moving a conducting wire in a magnetic field induces a current in the wire |
| Creation of magnetic fields | Electric charges in motion, such as an electric current in a wire, create magnetic fields |
| Magnetic fields | Can attract or repel other magnets and change the motion of other charged particles |
| Lorentz force | The force acting on a charged particle in a magnetic field depends on the charge, velocity of the particle, and strength of the magnetic field |
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What You'll Learn
Electric and magnetic forces are distinct but intertwined phenomena
Electric and magnetic forces are fundamental aspects of nature, with electricity acting on all charged particles, whether in motion or at rest, and magnetism acting on moving charged particles. These forces are distinct but intertwined phenomena, with electricity and magnetism unified under the concept of electromagnetism.
The electric force causes an attraction between particles with opposite charges and a repulsion between particles with the same charge. On the other hand, magnetic force acts on moving charged particles, attracting or repelling other magnets and influencing the motion of other charged particles. This relationship between electricity and magnetism is described by Faraday's Law of Induction, forming the basis for electromagnets and electric motors.
The concept of electromagnetism was developed in the 19th century by scientists such as Michael Faraday, André-Marie Ampère, James Clerk Maxwell, Oliver Heaviside, Heinrich Hertz, and J.J. Thomson. Maxwell's equations describe the behaviour of the electromagnetic field, and the electromagnetic force is given by the Lorentz force law. The Lorentz force is exerted on particles in a magnetic field and has the unique property of causing particles to move at right angles to their original motion.
The interplay between electric and magnetic forces is observed in various contexts. For example, an electric current inside a wire creates a corresponding circumferential magnetic field outside the wire, and a magnetic field can induce a current in a conducting wire. Additionally, the electromagnetic force plays a crucial role in modern technology, including electrical energy production, light and sound production, wireless communication, and computation.
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Magnetic force acts between moving charged particles
The relationship between electricity and magnetism is a fundamental aspect of physics, with the electromagnetic force being one of the four fundamental forces of nature. The interplay of these forces is observed in various natural phenomena and modern technologies.
The magnetic force specifically acts on moving charged particles, such as those in electric currents. This is a fundamental principle, with every charged particle emitting an electric field, regardless of its motion, and moving charged particles generating magnetic fields.
The behaviour of these electric and magnetic fields is described by Maxwell's equations in classical electromagnetism. One peculiar aspect is the challenge of reconciling classical electromagnetism with classical mechanics. However, it is compatible with special relativity, as demonstrated by Einstein's theory of relativity. According to this theory, if an observer moves alongside the charged particles, magnetic fields transform into electric fields and vice versa.
The direction of the magnetic force on a moving charge follows the Right-Hand Rule (RHR-1) and is perpendicular to the plane formed by velocity and the magnetic field. The magnitude of the force is proportional to the charge, velocity, magnetic field strength, and the sine of the angle between velocity and the magnetic field. When the velocity of a charged particle is parallel to the magnetic field, the magnetic force becomes zero, resulting in straight-line motion.
The interaction between moving charged particles and magnetic fields leads to circular or spiral motion. This phenomenon is observed in particle accelerators, where protons are kept in circular paths, and in cosmic rays, which follow spiral paths due to Earth's magnetic field. The radius of these paths provides valuable information about the mass, charge, and energy of the particle.
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Electric motors use magnetic fields to spin a shaft
The rotation of the shaft in an electric motor is achieved through the creation of a magnetic force. This force is generated by manipulating the electrical contacts on coils to reverse the direction of the current with each half turn, preventing the north and south poles from aligning. The resulting magnetomechanical attraction creates a force that drives the rotor to follow the rotating magnetic field.
Rotating magnetic fields are essential in the operation of induction machines, with the induction motor being a prime example. It consists of a stator with fixed windings and a rotor or armature with coils that are short-circuited. The arrangement of windings in the stator produces a magnetic field that rotates at a speed determined by the frequency of the alternating current. This rotating magnetic field induces a current in the rotor, causing it to rotate in a definite direction.
The concept of rotating magnetic fields was first formulated by French physicist François Arago in 1824, using a rotating copper disk and a needle, known as "Arago's rotations." Charles Babbage and John Herschel, English experimenters, built upon this by inducing rotation in Arago's copper disk with a spinning horseshoe magnet. Michael Faraday, an English scientist, later attributed this effect to electromagnetic induction.
The development of electric motors that utilize rotating magnetic fields continued with Walter Baily, an English physicist, who in 1879, replaced horseshoe magnets with electromagnets, manually turning switches on and off to demonstrate a primitive induction motor. The idea was further explored by Galileo Ferraris, an Italian physicist, and Nikola Tesla, a Serbian-American inventor, who delved into the concept of a rotating magnetic field in an AC motor.
Today, electric motors employing rotating magnetic fields are prevalent, and their ability to create a rotating field has led to the dominance of three-phase systems in the world's electric power supply.
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The Lorentz force law describes the electromagnetic force
The Lorentz force law is used in electromagnetism and is fundamental to our understanding of the interactions between electric and magnetic fields. It is described in terms of the properties of the matter involved, such as the distances between two masses or charges, rather than the electric and magnetic fields themselves. This modern concept of electric and magnetic fields first arose in the theories of Michael Faraday and was later given a full mathematical description by Lord Kelvin and James Clerk Maxwell. Maxwell's equations describe how electrically charged particles and currents give rise to electric and magnetic fields, while the Lorentz force law describes the force acting on a moving charge within these fields.
The Lorentz force law is particularly useful for understanding the behaviour of charged particles in electric and magnetic fields. For example, it explains the motion of a charged particle in a uniform magnetic field. If the velocity of the particle is perpendicular to the magnetic field, it will follow a circular trajectory. If the angle between the velocity and the magnetic field is less than 90 degrees, the particle orbit will be a helix. When the angle is zero, there is no magnetic force acting on the particle, and it will continue to move undeflected along the field lines.
The Lorentz force law also has applications in understanding the behaviour of wires carrying current in an external magnetic field. Since a current represents the movement of charges in the wire, the Lorentz force acts on the moving charges, and the magnetic forces are transferred to the wire. The force on a small length of the wire depends on the orientation of the wire with respect to the magnetic field, with the force being strongest when the current and field are perpendicular to each other.
Overall, the Lorentz force law is a fundamental concept in electromagnetism that describes the electromagnetic force acting on a moving charge in electric and magnetic fields. It has important applications in understanding the behaviour of charged particles and currents in these fields.
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Classical electromagnetism is incompatible with classical mechanics
The relationship between magnetic force and electricity has been a subject of study since ancient times. Electricity and magnetism were initially believed to be distinct forces. However, the discovery of electromagnetism revealed their interconnected nature. Electromagnetism is a fundamental force of nature, governing the interactions between charged particles. It is comprised of two intertwined phenomena: electrostatics and magnetism.
Classical electromagnetism, or classical electrodynamics, is a branch of physics that examines the interactions between electric charges and currents using an extension of the classical Newtonian model. It is a classical field theory, applicable when quantum mechanical effects are negligible. Classical electromagnetism is described by a set of equations known as Maxwell's equations, which detail the behaviour of the electromagnetic field.
However, classical electromagnetism presents challenges when reconciled with classical mechanics. One key issue arises from Maxwell's equations, which state that the speed of light in a vacuum is a universal constant dependent solely on the electrical permittivity and magnetic permeability of free space. This assertion violates Galilean invariance, a foundational principle of classical mechanics. Galilean invariance dictates that the laws of physics remain unchanged across all inertial frames of reference.
The contradiction between classical electromagnetism and classical mechanics can be contextualised through the lens of the electromagnetic theory and the principle of relativity. According to the electromagnetic theory, light consistently moves at a constant speed relative to one inertial frame. However, applying the usual rules for combining velocities would imply that light's speed is not constant relative to a moving frame, contradicting the principle of relativity, which asserts that all inertial frames are equivalent. This paradox is addressed by Einstein's theory of relativity, also known as special relativity. By embracing special relativity, classical electromagnetic theory and classical mechanics can coexist within a single self-consistent theory, albeit at the expense of abandoning the relativity principle.
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Frequently asked questions
Magnetic force is one of the four fundamental forces of nature and is responsible for the attraction or repulsion between electrically charged particles in motion.
An electric current inside a wire creates a corresponding magnetic field outside the wire. Moving electric charges create magnetic fields and can attract or repel other magnets, influencing the motion of other charged particles.
Electromagnetism is the interaction between electric and magnetic forces, combining electrostatics and magnetism. It describes how electric forces cause attraction between oppositely charged particles and repulsion between like charges, while magnetism involves interactions between charged particles in motion.
