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Physics ISU - Electromagnetism
Transcript of Physics ISU - Electromagnetism
WATCHING! Electromagnetic waves are produced whenever electric charges are
accelerated. The accelerated charge loses energy that is carried away in the electromagnetic wave.
If the electric charge is accelerated in periodic motion, the frequency of the
electromagnetic waves produced is exactly equal to the frequency of oscillation of the charge.
All electromagnetic waves travel through a vacuum at a common speed c,
calculated as 3.0x10 m/s, and obey the wave equation c = f
Electromagnetic waves consist of oscillating electric and magnetic fields in a constant phase relation, perpendicular to each other, and both at 90 degrees to the direction of propagation of the wave, which is shown below. A French physicist and mathematician from the 17 and 1800's by the name of André-Marie Ampère, who is regarded as the founder of electrodynamics, or whats now known as electromagnetism.
The magnetic field in space around an electric current is
proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source.
The relationship between current flowing through any
conductor and the strength of the magnetic field it produces is
given by, the equation, Ampere's Law, Also, if l is the distance traveled through the magnetic field by each of the charges in the time , l = v . The force that a conductor experiences in a magnetic field is due to the motion of charged particles moving through it. If the charged particles within the conductor were stationary, it would carry no current and experience no force.
Consider a particle of charge q, moving with velocity , at an angle to a magnetic field . If n particles pass a given point in a time, they create an electric current, which is given by the expression, , the strength of the magnetic field.
, the length of the conductor in a magnetic field.
, the current flowing through the conductor. Also, if the conductor cuts across the magnetic field lines at an angle of theta, for a straight conductor and horseshoe magnet, =90
, the angle between and
Combining all these factors have an effect on the magnitude of , we can write the expression, Conductor in a Magnetic Field
On of the simplest examples of a conductor carrying current in a magnetic field occurs when a straight wire carries current, through a magnetic field, between poles of a horseshoe magnet. MAGNETIC FIELDS
Electrodynamics is the branch of mechanic concerned with the interaction of electric currents with magnetic fields or with other electric currents
When electricity passed through a wire, a magnetic field is created around the wire, if the wire is looped the electric field is increased
If a conductor is wounded into a coil with many loops, it is called a solenoid, the magnetic field of a solenoid is the sum of the magnetic fields of all the loops. Electromagnetism is the interaction between electric currents and magnetic fields.
Hans Christian Oersted (1777-1851) has been credited with finding the basic relationship between electricity and magnetism.
Hans formulated the basic principles of electromagnetism: Moving electric charges produce electric fields
Electromagnetism is related to the electromagnetic force that causes the attraction and repulsion of electrically charged particles.
When electrically charged particles, such as electrons, are put into motion, they create a magnetic field. When these particles are made to oscillate, they create electromagnetic radiation. This can include radio waves, visible light, or x-rays, depending on the frequency of the oscillation. Electromagnetism When a piece of unmagnetised material touches or is brought near to the pole of a permanent magnet, it becomes a magnet itself. The magnetism is induced.
Ferromagnetism is a physical phenomenon exhibited by materials such as iron, nickel and cobalt that become magnetized in a magnetic field and retain their magnetism.
Possible only when atoms are arranged in a lattice
Ferromagnetic materials obtain their strong magnetic properties due to magnetic domains.
A magnetic domain is region in which the magnetic fields of atoms are grouped together and aligned.
When a ferromagnet is raised to its curie point it loses its ability to magnetize however when cooled they regain their magnetic properties. Induced Magnetism The Earths magnetic field is produced by electrical currents circulating Earths metallic core.
Electric currents flow in a circular loop creating a magnetic field that is stronger in the centre.
A physicist in the 1600's, Sir William Gilbert, had discovered that the magnetic fields around the Earth are similar to that of a bar magnet.
The Dynamo Effect
A geophysical theory that reveals the source of the Earths main magnetic field
As a result the electric current produces a magnetic field which
interacts with fluid motion to create a secondary magnetic field stronger than the original Earth's Magnetic Fields Ferromagnetic substances are composed of a large number of tiny regions called magnetic domains.
Each domain behaves like a tiny bar magnet
When the material is in an unmagnetized state millions of domains are oriented at random (right)
If the material is placed in strong enough magnetic field some domains slightly align with the external field.(left)
As a result there is a preferred orientation of the domains causing the material to behave like a magnet. Domain Theory of Magnetism Magnetic Forces on Conductors
and Moving Charges BY:
MOHAMEDULLAH PAYENDA What are Magnetic fields ?
Regions of influence exerted by a magnetic force, normally focused along two poles.
How are Magnetic Fields created ?
A magnetic field is produced when charges from one material attract charges that are emitted from another material, this sets electrical charges in motion and a magnetic field is formed as a result of this activity.
Where are Magnetic Fields found ?
Magnetic fields exist everywhere around us, created by televisions, radio, microwave ovens and used in applications such as radar and security systems Magnetic Fields? Electrodynamics Magnetic Field's
of Conductors Magnetic Field of a Straight Conductor
When an electric current flows through a long, straight wire,
the magnetic field created creates a circular path around the centre of the wire.
Magnetic Field of a Loop
When a wire is bent to form a U shape, the magnetic fields are
concentrated on the two ends, which indicates a stronger magnetic field in the inside. Solenoids The magnetic field produced by a current has three distinct characteristics. The field can be turned on or off, have its direction reversed, or have its strength changed. Unlike Earth's magnetic field, you can turn a magnetic field produced by a current on or off.
In addition you can change the direction of the current of the magnetic field by reversing the direction of the current.
You can also change the strength of a magnetic field produced by a current, by the way the wire is looped and configured into a solenoid. The basic law of magnetism states that two magnetic fields will interact to produce forces of attraction, with opposite poles, and repulsion, with similar poles. Obtaining expression for
magnetic force acting
on a conductor o Sample Question A straight conductor carries a current of 30
A through a magnetic field, which has a length of 20 cm, when the magnetic field intensity is 1.50 T. Calculate the magnitude of the force on the conductor, when the angle between it and the magnetic field is 45 degrees. The magnetic filed is measured in tesla (T), 1 T is the magnetic field strength when a conductor with a current of 1 A, and a length of 1 m at an angle of 90 to the magnetic field, experiences a force of 1 N, as a result, the value of k will
always be 1, when the appropriate units are used. With that, the final equation becomes, o = (1.50 T)(30 A)(0.20 m)(sin 45)
= 3.18 N Force on a Moving Charge Using the previous equation, , we can derive a new equation for the force of moving charge. Sample Question In an electric conductor, where magnetic forces
are present, determine the magnitude and
direction of the magnetic force on a proton, with mass 1.6x10 C moving horizontally to the north at 8.6x10 m/s, as it enters a magnetic field of 1.2 T, pointing vertically upward. 4 -19 = (1.2 T)(1.6x10 C)(8.6x10 m/s) sin 90
= 1.7 x 10 N [EAST] o 4 -19 -14 Ampere's Law Key Notes Lorentz force is the force experienced by a
point charge moving along a wire that is in a magnetic field; the force is at right angles to both the current and the magnetic field.
To find the direction that the magnetic field is going the “right
hand rule” can be used. If a right hand is taken and wrapped around a wire with the thumb pointing in the direction of the electrical current then the fingers will be pointed in the direction of the magnetic field around the wire. Faraday's Law Faraday's Law was implemented by a man named Michael Faraday, who has made major contributions and discoveries in the areas of electricity and electromagnetism. Through many experiments, Faraday developed, what's now known
as Faraday's Law of Induction, which states that, the induced electromotive force in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
Any change in the magnetic environment of a coil of wire will cause a
voltage, or electromotive force (emf) to be induced in the coil. No matter how the change is produced, the voltage will be generated. Lenz’s states when an emf is generated by a change in magnetic flux according to Faraday's Law Electromagnetic
Waves One of the great scientific achievements of the 19th century was the discovery that waves of electromagnetic energy could travel through space. In 1864, a Scottish physicist and mathematician, James Clerk Maxwell summarized his theories about electromagnetic fields as four basic relationships, which are now known as Maxwell's Equations of Electromagnetism. Maxwell Laws The first law states that the distribution of an electric charge, in space, is related to the electric field it produces.
The second law states that magnetic field lines are continuous, and do not have a beginning or an end, whereas electric field lines begin and end on electric charges.
The third law, which was a derivative of Faraday's Law, states that an electric field can produce a magnetic field, with moving electric charges, so a changing electric field should produce a changing magnetic field.
The fourth law, which was a correction to Ampere's Law, states that a changing magnetic field can produce a changing electric field, and
hence an induced current and potential difference in a conductor
in the changing field. Electromagnetic
Waves Waves and Heinrich Hertz Electromagnetic waves are formed when an electric field interchange with a magnetic field. The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.
Heinrich Hertz, a German physicist, applied Maxwell's theories to the
production and reception of radio waves, where Hertz is used as the unit of frequency for radio waves.
Electromagnetic radiation includes radio waves, microwaves, infrared light,
visible light, ultraviolet light, x-rays, and gamma rays. All of these types of radiation can be thought of as waves, with a few important properties like speed, frequency, and wavelength. Electromagnetic
Waves Maxwell had also states some characteristics
about electromagnetic waves. Electromagnetic waves exhibit the properties of interference, diffraction, polarization, and refraction, and can carry linear and angular momentum. Their intensity is proportional to the square of the magnitude of the electric or magnetic field amplitude, and to the square of the frequency. 8 The Curie Point French chemist Pierre Curie (1859-1906) discovered a principle of magnetism called the Curie Point.
Pierre Curie discovered that ferromagnetic materials have a critical temperature at which the material loses its ferromagnetic behavior.
Ferromagnetism covers the area of normal magnetism that people usually relate to magnetism
All materials and magnets that are attracted to magnets are ferromagnetic materials.
British scientist J.J Thompson (1856-1940) studied cathode rays in depth, focusing on their deflection by electric and magnetic fields.
Thompson drilled a hole in the anode to allow a fine beam of cathode rays to pass through, when a magnetic field was applied it caused the beam to be deflected.
The direction of deflection was the result of a negatively charged particle moving from cathode to anode thus Thompson concluded that cathode rays consisted of negatively charged particles moving at high speeds, he called the particles electrons. The Oscilloscope is a device commonly used in the laboratory to analyse and measure electrical signals, a major component of the Oscilloscope is the cathode ray tube.
A major components in a cathode ray tube consists of an electron “gun”, horizontal deflection plates (or coils) and a fluorescent screen.
The cathode of the electron is heated by the nearby filament in a process called thermionic emission.
Thermionic emission is the emission of electrons from a heated source. Oscilloscope Work leading to the discovery of the electron began with the study of the discharge of electricity between two electrodes in a nearly evacuated glass tube.
It was found that when a large potential difference is applied to the electrodes, a bright glowing area is observed near the anode, together with a much darker area adjacent to the cathode.
When a metal target was placed in front of a cathode a sharp shadow was cast, it seemed like something was emitted from the cathode ,travelling in straight lines this became known as cathode rays. Cathode Rays Electromagnetic Wave Radiations