violencia filioparental
5. Violencia a los mayores que residen en el hogar: maltrato a los mayores
The molecular theory of magnetism was given by Weber and modified later by Ewing.
According to this theory:
1. Every molecule of a magnetic substance (whether magnetized or not) is a complete in itself, having a north pole and a south pole of equal strength.
2. In an unmagnetized substance, the molecular magnets are randomly oriented such that they form closed chains (Fig. 1.42). The North-pole of one molecular magnet cancels the effect of South-pole of the other so that the resultant magnetism of the unmagnetized specimen is zero.
3. On magnetizing the substance, the molecular magnets are realigned so that North-poles of all molecular magnets point in one direction and South-North-poles of all molecular magnets point in opposite direction (Fig. 1.43).
Fig. 1.41: Repulsion and attraction by magnets
The extent of magnetization of the specimen is the extent of realignment of the molecular magnets.
4. When all the molecular magnets are fully aligned, the substance is said to be saturated with magnetism.
5. At all stages, the strengths of the two poles developed will always be equal.
6. On heating the magnetized specimen, molecular magnets acquire some kinetic energy.
Some of the molecules may get back to the closed chain arrangement. That is why magnetism of the specimen would reduce on heating.
Magnetic lines of force: The concept of magnetic lines of force or simply the field lines was developed to visualize the effect of the magnetic field. The magnetic field lines represent the magnetic field in the same way as the electric field lines represent an electric field.
The magnetic lines of force do not exist in reality. They are only hypothetical lines, which enable us to understand certain phenomena in magnetism. To draw these lines, we have to take a test object which is a magnetic dipole such as a small compass needle.
If we imagine a number of small compass needles around a magnet, each compass needle experiences a torque due to the field of the magnet. The torque acting on a compass needle aligns it in the direction of the magnetic field. The path along which the compass needles are aligned is known as magnetic lines of force. It should be clearly understood that tangent to a field line at any point P gives the direction of magnetic field B at that point (Fig. 1.44).
Properties of magnetic lines: Following are some of the important properties of the magnetic lines of force:
1. Magnetic lines of force are closed continuous curves; we may imagine them to be extending through the body of the magnet.
2. Outside the body of the magnet, the direction of magnetic lines of force, is from North-pole to South-North-pole (Fig. 1.45).
3. The tangent to magnetic lines of force at any point gives the direction of magnetic field at that point.
4. No two magnetic lines of force can intersect each other (Fig. 1.46).
5. Magnetic lines of force contract longitudinally and they dilate laterally.
Fig. 1.42: Unmagnetized magnet Fig. 1.43: Magnetized magnet
Fig. 1.44: Tangent to a magnetic line of
force Fig. 1.45: Magnetic line of force
Fig. 1.46: Direction of magnetic lines of force
6. Crowding of magnetic lines of force represents stronger magnetic field and vice-versa (Fig. 1.47).
It should be clearly understood that there is one funda mental difference between electricity and magnetism. Where as in electricity, an isolated charge can exist, in magnetism, an isolated pole does not exist. The simplest magnetic struc ture that can exist is only a magnetic dipole, charac terized by magnetic dipole moment M→. Thus for mapping magnetic field, the simplest test object is a dipole. That is why in the definition of B→ above, we have used the word ‘hypothetical’ isolated north pole. However, this definition of B→ (corres ponding to definition of E→ ) enables us to simplify some calculations.
Thus, magnetic dipole is characterized by a vector M→ in place of a scalar charge q in electricity. We shall show that in an external magnetic field, the dipole experiences a torque (unlike the force experienced by charge q in electric field). The effect of torque is to align the dipole along the external magnetic field. The directive property of a magnet is attri-buted to the torque acting on the magnetic dipole due to earth’s magnetic field.
Each electric line of force starts from a positive charge and ends at a negative charge.
It should be clearly understood that the electric lines are discontinuous only in the sense that no such lines exist inside a charged body. However, from a positively charged body to a negatively charged body, there is no discontinuity in the electric lines of force. In magnetism, as there are no monopoles, therefore, the magnetic field lines will be along closed loops with no starting or ending. The magnetic lines of force would pass through body of the magnet. At very far off points, the field lines due to an electric dipole and a magnetic dipole will appear identical.
Remember that electric lines of force are discontinuous, whereas magnetic lines of force are closed continuous curves.
Magnetic dipole: A magnetic dipole consists of two unlike poles of equal strength and separated by a small distance. For example, a bar magnet, a compass needle, etc. are magnetic dipoles. An atom of a magnetic material behaves as a dipole due to electrons revolving around the nucleus. Magnetic dipole moment is defined as the product of pole strength and the distance between the two poles. This distance between the poles is called magnetic length and is represented by 2l. If m is the strength of each pole, then magnetic dipole moment (M) is
M = m (2l)
Magnetic dipole moment is a vector quantity directed from South-to-North-pole. The SI units of M are joule/tesla or ampere-metre2 (Fig. 1.48).
The direction of magnetic moment (M) is from south to north. This corresponds to the electric dipole moment (p) of an electric dipole from negative charge to positive charge.
Gauss’s theorem (or Gauss’s law) in magnetism: According to Gauss’s theorem, the surface integral of electrostatic field E over a closed surface S is equal to 1/εo times the total charge q inside the surface, where εo is absolute electrical permitti vity of free space, i.e.
§ E→. ds→ = q/ε°
Fig. 1.47: Crowding of magnetic lines of force
Fig. 1.48: Magnetic dipole
Fig. 1.49: Magnetic field lines
If an electric dipole was enclosed by the surface, equal and opposite charges in the dipole add up to zero. Therefore, surface integral of electric field of a dipole over a closed surface enclosing an electric dipole is zero, i.e.
§ E→. ds→ = 0
Whereas, electric field can be produced by isolated charge, the magnetic field is produced only by a magnetic dipole. This is because isolated magnetic poles do not exist.
Hence magnetic analogue equation is as follows:
§ B→. ds→ = 0
That is surface integral of magnetic field over a surface (closed or open) is always zero, i.e. the net magnetic flux ψB through any surface S is always zero. This is called Gauss’s law in magnetism. In terms of magnetic field lines, the law means that there are as many lines entering S, as are leaving it (Fig. 1.49).
Magnetic field of earth: Sir William Gilbert was the first to suggest in the year 1600, that earth itself is a huge magnet. His statement was based on the following evidence:
1. A magnet suspended from a thread and free to rotate in a horizontal plane comes to rest along the north-south direction. On disturbing, the magnet returns quickly to its north-south direction again this is as if huge bar magnet lies along the diameter of the earth. The North pole of this fictitious magnet must be toward geographic south so as to attract South pole of the suspended magnet and vice-versa.
2. When a soft iron piece is buried under the surface of earth in the north-south direction, it is found to acquire the properties of a magnet after sometime.
3. When we draw field lines of a magnet, we come across neutral points. At these points, magnetic field due to the magnet is neutralized or cancelled exactly by the magnetic field of earth. If earth had no magnetism of its own, we would never observe neutral points.
The branch of physics which deals with the study of mag netism of earth is called terrestrial magnetism or geomagne tism.
It has been established that earth’s magnetic field is fairly uniform. The strength of this field is approximately 10-4 tesla or 1 gauss. The field is not confined only to earth’s surface.
It extends upto a height nearly 5 times the radius of the earth.
Cause of earth’s magnetism: The exact cause of earth’s magnetism is not yet known.
However, some important postulates in this respect are as follows:
1. The earth’s magnetism may be due to molten charged metallic fluid in the core of earth.
The radius of this core is about 3500 km with the rotation of earth, the fluid also rotates resulting in the development of currents in the core of earth. These currents magnetize the earth.
2. According to Prof Brackett, earth’s magnetism may be due to rotation of earth about its axis. This is because every substance is made of charged particles (protons and electrons). Therefore, a substance rotating about an axis is equivalent to circulating currents, which are responsible for its magnetization.
3. In the outer layers of earth’s atmosphere, gases are in the ionised state, primarily on account of cosmic rays. As earth rotates, strong electric currents are set up due to move-ment of (charged) ions. These currents might be magnetizing the earth.
electromaGnetIc InductIon
Michael Faraday in UK and Joseph Henry in USA observed that an emf is produced across the ends of a conductor when the number of magnetic lines of force associated with the conductor changes. The emf lasts so long as this change continues. This phenomenon of generating an emf by changing the number of magnetic lines of force associated with the conductor is called electromagnetic induction (EMI). The emf so developed is called induced emf. If the conductor is in the form of a closed circuit, a current flows in the circuit.
This is called induced current.
The phenomenon of EMI is the basis of power generators, dynamos, transformers, etc.
and hence it is important.
Magnetic flux: The magnetic flux Φ through any surface held in a magnetic field is measured by the total number of magnetic lines of force crossing the surface. The unit of magnetic flux is weber (Wb). One weber is the amount of magnetic flux over an area of 1 m2 held uniform to a uniform magnetic field of one tesla. Also, magnetic flux is a scalar quantity.