So far, we discussed two passive properties of electronic components – resistance and capacitance and have reviewed the associated passive electronic components resistors and capacitors. We learned how these components affect current and voltage in a circuit, their electrical properties, and their various commercial types.
A third and final passive property of electronic components that remains to discuss is inductance. Resistance, capacitance, and inductance are called passive properties as due to the effect of these electrical properties, current or voltage cannot be generated or amplified. Inductance appears in an electronic component due to the magnetic effects of the electric current. Inductors are electronic devices that are specially designed to offer inductance in a circuit. Electricity and magnetism are two aspects of the same energy — electromagnetic energy. To understand inductance, it is important first to understand magnetism and electromagnetism.
Naturally occurring magnets like loadstones have fascinated humans since ancient times. At some point, it was discovered that magnets have two poles. The north-seeking pole of a magnet is called its North Pole, and the south-seeking pole is called South Pole. That is why naturally occurring magnets have been used to make compasses. It was also discovered that like poles of a magnet repel each other and unlike poles attract each other.
The magnets are also known to attract iron pieces and some other metals. The space around a magnet within which a magnet can attract or repel other magnets, or attract iron and some other metals is termed as a magnetic field. The scientists have modeled a magnetic field to consist of magnetic field lines or magnetic flux lines. The flux lines are imaginary lines of a magnetic field along which iron filling or tiny pieces of iron get aligned around a magnet. The magnetic force is applied along the flux lines, and the strength of the magnetic field is determined by the concentration of flux lines in a magnetic field. The magnetic field in the form of flux lines around a typical bar magnet is shown in the figure below.
As the science and technology advanced, electricity and magnetism found to be connected. When current flows through a straight wire passing through a perpendicular plane and iron fillings are placed on the plane, the iron filings align themselves in concentric circles around the wire. This shows that the current flowing through a straight wire have circular magnetic field lines around it. If the wire is wrapped in a loop and current flows through it, magnetic field lines similar to a bar magnet are observed. When the number of loops of the wire is increased, the magnetic field (force) is strengthened by more flux lines concentrated in the magnetic field. Like magnetic force is strongest near poles in a magnet where magnetic flux lines are converged (come closer) at the poles, the magnetic field around a current carrying wire is strongest near the wire. The flux lines around a current carrying straight wire are shown in the following figure:
The flux lines through a loop of wire are shown in the following figure:
Actually, magnetism is just another aspect of electricity. When electric charges are stationary, they exert an attractive electrostatic force on opposite charges and repulsive electrostatic force on the same charges. When electric charges are in motion, they exert a magnetic force on other charges. So, the magnetic force is just a manifestation of the electrostatic force. In other words, when charges are in motion, their electrostatic force is converted into magnetic force. As charges exert an electrostatic or magnetic force on each other all the time, it can be said that electrical energy, magnetism, and mechanical energy are naturally interconvertible.
Dipoles and Monopoles
Any magnet has two opposite poles. The pair of opposite magnetic poles is called dipole. A lone pole is called monopole. Monopoles do not exist in nature as magnetic flux lines always connect two opposite poles in a closed loop (like in a bar magnet). However, moving electric charges exerting magnetic force can be assumed to be monopoles where flux lines spread around the charge in concentric circles.
When a charge is stationary, it exerts an electrostatic force which is radial and equally distributed in all directions. This can be seen as equally distributed radial electric field lines either pointing inwards for negative charge or pointing outwards for positive charge. The stationary charges have no magnetic field or magnetic flux lines around them.
When a charge moves with a constant velocity, the electric field lines are still radial and straight pointing inwards or outwards, but are not uniformly distributed. Due to motion of charge, some of the electrostatic energy is converted to magnetic energy and concentric circular magnetic flux lines appear along a plane perpendicular to the direction of the motion of charge.
When a charge accelerates, the electric field lines are still radial, but are converged and concentrated near the charge. Due to acceleration of the charge, the magnetic field around the charge gets distorted and it emits electromagnetic energy in the form of electromagnetic radiations. So, on acceleration of a charge, some of its energy is lost in the form of electromagnetic waves.
Strength of magnetic field
The magnitude of a magnetic field is expressed in a unit – ‘Weber’. It is abbreviated as ‘Wb’. The magnitude of weak magnetic fields is expressed in a unit – ‘Maxwell’. One Weber is equivalent to 108 Maxwell. The intensity of the magnetic force in a magnetic field is determined by the flux density or intensity of the magnetic field at that point. The flux density at a point is the number of magnetic lines per square meter or per square centimeter at that point in the magnetic field. The flux density or magnetic field intensity is measured in units – Tesla and Gauss. One Tesla of magnetic field intensity is defined as one Weber of magnetic field per square meter. One Gauss of magnetic field intensity is defined as one Maxwell of magnetic field per square centimeter. One Tesla is equivalent to 104 Gauss.
Permeability, paramagnetism, ferromagnetism and diamagnetism
The magnetic flux lines can pass through all materials. In fact, earth itself is a giant magnet. The bar magnets aligning along the north and south poles of the earth is the same as the iron fillings aligning along the north and south poles of a bar magnet. However, different materials respond differently to a magnetic field.
Some materials get magnetized in a magnetic field and get demagnetized as the magnetic field is removed. Such materials are called paramagnetic materials. Some paramagnetic materials are magnetized such that they remain magnetized even after removing the magnetic field. Such materials are called ferromagnetic materials. Some materials oppose magnetic field and do not get magnetized by a magnetic field. Such materials are called diamagnetic materials.
When paramagnetic or ferromagnetic materials are held free in a magnetic field, they align themselves along the magnetic flux lines. Like iron filings align along the magnetic flux lines of a bar magnet and bar magnets align along the magnetic flux lines of the earth. When diamagnetic materials are held free in a magnetic field, they align perpendicular to the magnetic flux lines opposing any magnetization.
When magnetic flux lines pass through paramagnetic and ferromagnetic materials, they come closer within the material and their intensity is increased. While when magnetic flux lines pass through diamagnetic materials, they get farther within the material and their intensity is decreased.
Most of the metals are paramagnetic and most of the non-metals are diamagnetic. Iron, Nickel, Cobalt and other rare earth metals are ferromagnetic. A number of alloys of iron, nickel, other rare earth metals with other elements (permalloys) and ferrites (compounds made of iron, oxygen and other elements) are also great ferromagnetic materials. Lodestone, which is a naturally occurring magnet, is a ferrite compound of iron and oxygen.
The ability of a material to support magnetic flux inside it is called its permeability. Permeability is an indication of the degree of magnetization that a material can obtain in response to a magnetic field. The permeability of vacuum and air is considered 1. The permeability of an iron core can be 60 to 8000 depending upon its purity. So, when magnetic field lines passed through an iron core, they can strengthen by 60 to 8000 times. The permalloys can have permeability up to 1,000,000.
Cause of magnetism
What causes magnetism? Why some materials get magnetized by a magnetic field while some do not?
All materials are made up of atoms. All atoms have a nucleus which is positively charged due to protons and have negatively charged electrons revolving around the nucleus. The electrons spin around their own axis at the same time. Most of the electrons in atoms exist in pairs where electrons in each pair spin in opposite directions canceling their magnetic effect. When there are unpaired electrons in an atom or molecule, it has a net magnetic field making the atom/molecule a microscopic magnet. Paramagnetic and ferromagnetic materials are made up of atoms or molecules which have unpaired electrons.
Any charge carrier when moves along or parallel to magnetic flux lines, it experiences no force while when moving in a direction not parallel to magnetic flux lines, experience a force perpendicular to both magnetic field and its velocity vector. The unpaired electrons when exposed to a magnetic field experience a force perpendicular to the direction of their velocity and direction of the magnetic field. This causes these unpaired electrons to move such that they produce a magnetic field along the applied magnetic flux lines. So, paramagnetic and ferromagnetic materials get magnetized in a magnetic field due to unpaired electrons creating a net magnetic field inside the material.
In diamagnetic materials, there are no unpaired electrons. The electrons revolving in their atoms do experience a force due to applied magnetic field, due to which, they produce a weak magnetic field opposing the applied magnetic field. That is why, diamagnetic materials when held free in a magnetic field, align themselves perpendicular to the direction of applied magnetic flux lines.
So, it should be noted that magnetism in permanent magnets also happens only due to the motion of unpaired electrons.
Retentivity or Remanence is the measure of ability of a material to remain magnetized after removing an external magnetic field. Paramagnetic materials immediately get demagnetized as the applied magnetic field is removed while ferromagnetic materials retain magnetism even after removing the external magnetic field. Retentivity is expressed as percentage of flux density retained by the material after removing the external magnetic field. Suppose, if on applying an external magnetic field, a material has a flux density of y Tesla and on removing the external magnetic field, it retains a flux density of x Tesla, then the Retentivity will be 100 * x/y.
We have already seen that when a current flows through a loop of wire, a magnetic field is induced in it. Whenever current varies or changes direction, this magnetic field opposes the flow of current by inducing a voltage in reverse polarity. In other words, we can say that the energy that was stored by the coil in the form of a magnetic field is converted back to electrical field opposing the change in magnitude or direction of the current. The property of a material, by virtue of which, it opposes any change in magnitude or direction of current through it, is called Inductance.
The unit of Inductance is Henry. Inductance is said to be one Henry when the current through a coil is changing at a rate of 1 ampere per second inducing a voltage of 1 volt across it. Henry is a very large unit. Practically, inductance is usually expressed in Milli-Henry or Micro-Henry.
The inductance can be a useful phenomenon in electronic circuits. The electronic devices designed to offer inductance in a circuit are called inductors.
We will discuss more inductors and their signal behavior in the next article.
|Basic Electronics 17 – Magnetism, Electromagnetism and Inductance|
|Basic Electronics 17 – Magnetism, Electromagnetism and Inductance|