Материал: Андрианова English Thesaurus of Student-Physicist 2013

106

A general procedure for deriving equations to calculate the magnetic field due to a long, straight wire carrying a current was proposed by the French scientist Andre-Marie Ampere (1775-1836); it provides a relation between the current in an arbitrarily shaped wire and the magnetic field produced by the wire.

Magnetic force between two parallel conductors

A magnetic force acts on a current-carrying conductor when the conductor is placed in an external magnetic field. Because a conductor carrying a current creates a magnetic field around itself, it is easy to understand that two current-carrying wires placed close together exert magnetic forces on each other. Parallel conductors carrying currents in the same direction attract each other parallel conductors carrying currents in opposite directions repel each other.

Magnetic fields of current loops and solenoids

The strength of the magnetic field set up by a piece of wire carrying a current can be enhanced at a specific location if the wire is formed into a loop. If a long, straight wire is bent into a coil of several closely spaced loops, the resulting device is a solenoid, often called an electromagnet. This device is important in many applications because it acts as a magnet only when it carries a current.

Magnetic domains

The magnetic field produced by a current in a coil of wire gives us a hint as to what might cause certain materials to exhibit strong magnetic properties. The magnetic properties of many materials can be explained by the fact that an electron not only circles in an orbit, but also spins on its axis like a top, with spin magnetic moment. In atoms containing many electrons, the electrons usually pair up with their spins opposite each other, so that their fields cancel each other. That is why most substances are not magnets. However, in certain strongly magnetic materials, such as iron, cobalt, and nickel, the magnetic fields produced by the electron spins don't cancel completely. Such materials are said to be ferromagnetic. In ferromagnetic materials, strong coupling occurs between neighboring atoms, forming large groups of atoms with spins that are aligned. Called domains, the sizes of these groups typically range from about 10~4 cm to 0.1 cm.

1.Where do we use magnets?

2.How can you describe magnets?

107

3.What do we mean by saying a "north-seeking" pole and a "southseeking" pole?

4.What do we need to determine the direction of the magnetic

force?

5.What magnetic field affects charged particle?

6.What do you know about the experiment carried out by Oersted in 1820?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3.Fill in the gaps with the missing words from the list: magnetized magnetic material, magnetic field line, magnetic force,

magnetic south pole, right-hand rule

1.A magnetic field also surrounds a properly _____.

2.The magnetic field of a bar magnet can be traced with the aid of a compass, defining a _____.

3.The north pole of a magnet in a compass points north because it's attracted to the Earth's _____ - located near the Earth's geographic north pole.

108

4.The total _____ on the wire is the sum of all the magnetic forces on the individual charges producing the current.

5.Application of the _____ at any point shows that the magnetic force is always directed toward the center of the circular path.

UNIT 20

INDUCED VOLTAGES AND INDUCTANCE

Read the following texts. Study the thesaurus on fig. 20, answer the questions after the texts

In 1819, Hans Christian Oersted discovered that an electric current exerted a force on a magnetic compass. Although there had long been speculation that such a relationship existed, Oersted's finding was the first evidence of a link between electricity and magnetism. Because nature is often symmetric, the discovery that electric currents produce magnetic fields led scientists to suspect that magnetic fields could produce electric currents. Indeed, experiments conducted by Michael Faraday in England and independently by Joseph Henry in the United States in 1831 showed that a changing magnetic field could induce an electric current in a circuit. The results of these experiments led to a basic and important law known as Faraday's law.

Induced emf and magnetic flux

An experiment first conducted by Faraday demonstrated that a current can be produced by a changing magnetic field. The apparatus consists of a coil connected to a switch and a battery. We will refer to this coil as the primary coil and to the corresponding circuit as the primary circuit. The coil is wrapped around an iron ring to intensify the magnetic field produced by the current in the coil. A second coil, at the right, is wrapped around the iron ring and is connected to an ammeter. This is called the secondary coil, and the corresponding circuit is called the secondary circuit. It's important to notice that there is no battery in the secondary circuit.

At first glance, you might guess that no current would ever be detected in the secondary circuit. However, when the switch in the primary circuit is suddenly closed, something amazing happens: the ammeter measures a current in the secondary circuit and then returns to zero! When the switch

109

is opened again, the ammeter reads a current in the opposite direction an again returns to zero. Finally, whenever there is a steady current in the primal circuit, the ammeter reads zero. From observations such as these, Faraday concluded that an electric current could be produced by a changing magnetic field.

In order to evaluate induced emfs quantitatively, we need to understand what factors affect the phenomenon. While changing magnetic fields always induce electric fields, there are also situations in which the magnetic field remains constant, yet an induced electric field is still produced. The best example of this is an electric generator: A loop of conductor rotating in a constant magnetic field creates an electric current.

The physical quantity associated with magnetism that creates an electric field is a changing magnetic flux. SI unit: weber (Wb).

Faraday's law of induction

The usefulness of the concept of magnetic flux can be made obvious by another simple experiment that demonstrates the basic idea of electromagnetic induction. If a magnet is moved toward the loop, the ammeter reads a current in one direction. If the magnet is moved away from the loop, the ammeter reads a current in the opposite direction. If the magnet is held stationary and the loop is moved either toward or away from the magnet, the ammeter also reads a current. From these observations, it can be concluded that a current is set up in the circuit as long as there is relative motion between the magnet and the loop. The same experimental results are found whether the loop moves or the magnet moves. We call such a current an induced current, because it is produced by an induced emf.

In each case, an emf is induced in a circuit when the magnetic flux through the circuit changes with time. It turns out that the instantaneous emf induced in a circuit equals the negative of the rate of change of magnetic flux with respect to time through the circuit. This is Faraday's law of magnetic induction.

The polarity determines which of two different directions current will flow in a loop, a direction given by Lenz's law: The current caused by the induced emf travels in the direction that creates a magnetic field with flux opposing the change in the original flux through the circuit.

110