Electromagnetism

Electromagnetism Plasma lamp The central sphere is an electrode and the glass sphere is filled with gases and driven by an alternating current.

Contents • Electricity By Sala Luca • Magnets and the magnetic force By Meroni Davide • Electromagnetism By Casati Denis

Electricity Electricity is the science associated with the presence and flow of electric charges. Electricity is due to several types of physics: • Electric Charge • Electric Current • Electric Field • Electric Potential

Electric Charge Electricity is the flow of electric charges. The basic units of charge are the proton and electron: the proton charge is positive while the electron charge is negative. Two particles which have the same charges, positive or negative, repel each other, while two particles which have different charges attract each other according to Coulomb’s law: the charges on electrons and protons, which are equal and opposite, are defined as: e=1,6*10^-19 Coulombs

Charles-Augustin de Coulomb The presence of charges gives rise to the electromagnetic force. These phenomena were investigated by Charles-Augustin de Coulomb, who deduced that a charge manifests itself in two opposing forms: like-charged objects repel and opposite-charged objects attract.

where r is the separation distance and ke is a proportionality constant. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb's law: The scalar form of Coulomb's law is an expression for the magnitude and sign of the electrostatic force between two idealized point charges, small in size compared to their separation. This force (F) acting simultaneously on point charges (q1) and (q2), is given by:

A positive force implies it is repulsive, while a negative force implies it is attractive. The coulomb (symbol: C) is the SI derived unit of electric charge.

Electric Current The movement of electric charge is known as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

Hans Christian Ørsted One of the most important discoveries related to current was made accidentally by Hans Christian Ørsted in 1820. During a lecture, Ørsted noticed a compass needle deflected from magnetic north when an electric current from a battery was switched on and off, confirming a direct relationship between electricity and magnetism.He said that an electric current produces a circular magnetic field as it flows through a wire

André-Marie Ampère André Marie Amperé in France discovered that the fundamental nature of magnetism was not associated with magnetic poles or iron magnets, but with electric currents.

Electric field Michael Faraday The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The field may be visualised by a set of imaginary lines of force whose direction at any point is the same as that of the field. The field lines are the paths that a point positive charge would seek to make as it was forced to move within the field; they are however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines. Field lines emanating from stationary charges have several key properties: firstly they originate at positive charges and terminate at negative charges; secondly they must enter any good conductor at right angles and finally they may never cross nor close in on themselves.

Electric Potential An electric field has the special property that it is conservative, which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated. The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.

Magnetism - History of magnetism; • Compass; • Magnets; • Magnetic force; • Magnetic field lines; • Earth’s magnetic field is fading; • Magnetic shield; • Magnetic fields on the Sun; - Right-hand rule;

History Of Magnetism In ancient China, the earliest literary reference to magnetism lies in a 4th century BC book called Book of the Devil Valley Master. The ancient Chinese scientist Shen Kuo (1031–1095) was the first person to write of the magnetic needle compass. Alexander Neckham, by 1187, was the first in Europe to describe the compass and its use for navigation. In 1269, Peter Peregrinus de Maricourt wrote the Epistola de magnete, the first extant treatise describing the properties of magnets. In 1282, the properties of magnets and the dry compass were discussed by Al-Ashraf, a physicist, astronomer, and geographer. In 1600, William Gilbert published his De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth). In this work he describes many of his experiments and he concluded that the Earth was itself magnetic and that this was the reason compasses pointed north. An understanding of the relationship between electricity and magnetism began in 1819 with work by Hans Christian Oersted.

Compass A compass is a navigational instrument that measures directions in a frame of reference that is stationary relative to the surface of the Earth. The frame of reference defines the four cardinal directions (or points) – north, south, east, and west. Usually, a diagram called a compass rose, which shows the directions (with their names usually abbreviated to initials), is marked on the compass. There are different types of compass: the magnetic compass contains a magnet that interacts with the earth's magnetic field and aligns itself to point to the magnetic poles; the gyro compass (sometimes spelled with a hyphen, or as one word) contains a rapidly spinning wheel whose rotation interacts dynamically with the rotation of the earth.

Magnets A magnetized bar has its power concentrated at two ends, its poles known as north(N) and south(S). The N end will repel the N end of another magnet, S will repel S, but N and S attract each other. The region where this is observed is colled magnetic field. Either pole can also attract iron objects such as pins and paper clips. That is because under the influence of a magnet, each pin or paper clip becomes itself a temporary magnet.

Magnetic Force In 1821 Hans Christian Oersted (1777-1851) in Denmark found that an electric current produced a magnetic force. Andrè-Marie Ampère (1775-1836) in France discovered that the fundamental nature of magnetism was associated with electric currents. The magnetic force was a force between electric currents.

Magnetic Field Lines Michael Faraday proposed a method for visualizing magnetic fields. Field lines of a bar magnet are commonly illustrated by iron filings sprinkled on a sheet of paper held over a magnet. The filings line up in the space of the field.

Earth’s Magnetic Field Is Fading Earth’s magnetic field is fading. Today is about 10 percent weaker than it was when German mathematician Gauss started keeping tabs on it in 1845, scientists say. If the trend continues, the field may collapse altogether and then reverse. Compasses would point south instead of north.

Magnetic Shield The geo-dynamo is the mechanism that creates our planet’s magnetic field, maintains it, and causes it to reverse. Earth’s geo-dynamo creates a magnetic field that shields most of the habited parts of our planet from charged particles that come mostly from the sun. The field deflects the speeding particles toward Earth’s Poles. Without our planet’s magnetic field, Earth would be subjected to more cosmic radiation. The increase could knock out power grids, scramble the communications systems on spacecraft.

Magnetic Fields On The Sun Many of the interesting features observed on the Sun by Yohkoh are magnetic. Indeed, much of the structure of the Sun's corona is shaped by the magnetic field, just like the pattern of the iron filings. Although it varies over time and from place to place on the Sun, the Sun's magnetic field can be very strong. Inside sunspots, the magnetic field can be several thousand times the strength of the Earth's magnetic field.

Right-hand rule

Right-hand rule In mathematics and physics, the right-hand rule is a common mnemonic device in order to understand conventions for vectors in 3 dimensions. It was invented, for use in electromagnetism, by British physicist John Ambrose Fleming in the late 19th century.

A different form of the right-hand rule is used in situations where a vector must be assigned to the rotation of a body, a magnetic field or a fluid. When a rotation is specified by a vector, and it is necessary to understand the way in which the rotation occurs, the right-hand grip rule is applicable. This version of the rule is used in two applications of Ampère's circuital law: 1. an electric current passes through a solenoid (a coil forming the shape of a straight tube, a helix, is called a solenoid), creating a magnetic field. When you wrap your right hand around the solenoid with your fingers in the direction of the current, your thumb points in the direction of the magnetic north pole; 2. an electric current passes through a straight wire. Here, the thumb points in the direction of the current (from positive to negative), and the fingers point to the direction of the magnetic lines of flux.

Electromagnetism • Faraday’s law • Electromagnet • Pick up • AC/DC Generator • Electric Circuits • Maxwell

Electromagnetism Electromagnetism is the branch of science concerned with the forces that occur between electrically charged particles. In electromagnetic theory these forces are explained using electromagnetic fields. Electric fields are the cause of several common phenomena, such as electric potential (such as the voltage of a battery) and electric current (such as the flow of electricity through a flashlight). Electromagnetism manifests as both electric fields and magnetic fields. Both fields are different aspects of electromagnetism. In fact, if a change occurs in an electric field, a magnetic field is generated, and viceversa if a change occurs in an magnetic field, a electric field is generated. This effect is called electromagnetic induction, and is the basis of operation for electrical generators, induction motors, and transformers.

Faraday's Law We suppose that we have a spire; if we take a magnet and we move it in and out,a current is created in the spire.This current is called the induced current.This current is generated even if we keep holding the magnet and move the spire.If this current in the coil is generated, we say that there is an electromotive force.

Electromagnet An electromagnet is a type of magnet where the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off. Electromagnets are widely used as components of other electrical devices, such as Pick up in an electric guitar. The magnetic field of all the turns of wire passes through the center of the coil, creating a strong magnetic field there. A coil forming the shape of a straight tube (a helix) is called a solenoid. Much stronger magnetic fields can be produced if a "core" of ferromagnetic material, such as soft iron, is placed inside the coil. The ferromagnetic core increases the magnetic field up to thousands of times the strength of the field of the coil, due to the high magnetic permeability (μ0) of the ferromagnetic material. This is called a ferromagnetic-core or iron-core electromagnet. The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current. However, a continuous supply of electrical energy is required to maintain the field.

The strenght of the magnetic field around the coil can be increased by: Using a soft iron core Using more turns of wire on the coil Using a bigger current If we reverse the direction of the current, we reverse the magnetic field direction.

Electromagnets in the Pick-up The most elementary form of a pick-up is made by a fixed bar magnet and a very small copper wire (the size of a hair) that is wrapped around.

The wire turns around the bar several times (thousands), thus creating an electric coil. The magnet coil generates a magnetic field around itself. As the pickup is placed just below, the strings of the guitar interact with it. When the string is still, the magnetic field is inert. As soon as we touch the string, the shape of the field is altered. The alteration of the lines of force in the magnetic field causes a small pulse of electrical energy, which then reach the amplifier in the form of alternating current. The movement of the vibrating string is always different: it depends on the note that is being played.

Back in Black

AC Generator If we turn a coil in a magnetic field, we produce motional emfs (electromagnetic forces) in both sides of the coil. The component of the velocity, perpendicular to the magnetic field, changes sinusoidally with the rotation: the voltage, which has been generated, is sinusoidal or AC. This process can be described in terms of Faraday's law:we see that the rotation of the coil continually changes the magnetic flux through the coil and therefore it generates a voltage.

DC Generator The essential difference between an AC and a DC generator is the nature of the connection between the rotor coils and the external circuit. In a DC generator, the brushes run on a split-ring commutator which reverse the connection between the coil and the external circuit for every half-turn of the coil.The voltage in the external circuit fluctuates between zero and a maximum, while the current flows in a constant direction.

Electric circuits An electric circuit is an interconnection of electric components. The components in an electric circuit can take many forms, which can include elements such as resistors, capacitors, switches, transformers and electronics. The resistor is the simplest element of passive circuit : it resists the current that flows through it, dissipating its energy as heat. The resistance is a consequence of the motion of charge through a conductor. Ohm's law is a basic law of circuit theory: the current through a conductor between two points is directly proportional to the potential difference across the two points.

The capacitor consists of two conducting plates separated by a thin insulating layer; If the charges on the plates are +q and −q, and V gives the voltage between the plates, then the capacitance is given by: I is the current through the conductor in units of amperes. V is the potential difference measured across the conductor in units of volts. R is the resistance of the conductor in units of ohms, symbolised by the Greek letter Ω. The unit of capacitance is the farad F

where v denotes the voltage in volts and i the current in amperes. The voltage across an inductor is equal to the product of its inductance and the time rate of change of the current through it. The inductor is a conductor, usually a coil of wire, that stores energy in a magnetic field in response to the current through it. When the current changes, the magnetic field does too, inducing a voltage between the ends of the conductor. The relationship between the self inductance L of an electrical circuit in henries, voltage and current is:

Maxwell

James Clerk Maxwell Electromagnetic waves were analysed theoretically by James Clerk Maxwell in 1864. Maxwell developed a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. Maxwell's equations describe how electric charges and electric currents act as sources for the electric and magnetic fields: of the four equations, two of them, Gauss’ law and Gauss' law for magnetism, describe how the fields are emanated from charges, while the Ampère's law (with Maxwell's correction) and Faraday's law describe how the fields 'circulate' around their respective sources.

Paradox of Ampere theory On the left: CB = J0 I In the centre: CB = 0 On the rigth: CB = J0 I When we use the word centre, we have to consider that we are inside the space of the capacitor. How does the current cross the void without a wire? Between the two plates, there is only an electric field. Maxwell, using the Gauss’ law, was able to correct the Ampere’s equation and combine every fundamental equations of electricity and magnetism. Furthermore Maxwell noted also the trend of CB e CE : these move in a sinusoidal way along planes that are perpendicular to each other.

Maxwell equations Gauss's law Gauss's law for magnetism Maxwell–Faraday equation(Faraday's law of induction) Ampère's circuital law(with Maxwell's correction)

Maxwell also discovered the speed of these waves with the relation: If we are in the void, this speed becomes: C=299 792,458 km/s The speed of light!!!

If we change the frequency, we have different types of waves:

T h e d e n

Electromagnetism