Physics SS 3 Week 7

Topic: MAGNETIC FIELD

INTRODUCTION

Magnetic field has a region around a magnet in which the influence of the magnet can be felt or detected.
The area around a magnet in which it can attract or repel objects or in which a magnetic force can be detected is called the magnetic field of the magnet.

Patterns of Magnetic Field

Magnetic field patterns can conveniently be observed using iron fillings. The magnet is placed on paper and the iron fillings are sprinkled lightly on the paper around the magnet. The paper is now tapped gently and the iron fillings will be seen to turn and settle in definite directions. The pattern of the magnetic field and for other magnetic arrangements are as depicted below Magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H. B refers to magnetic flux density, and H to magnetic field strength. Magnetic flux density is most commonly defined in terms of the Lorentz force it exerts on moving electric charges
Alternative names for B
Magnetic flux density, Magnetic induction, Magnetic field
Alternative names for H
Magnetic field intensity, Magnetic field strength, Magnetic field, Magnetizing field
The magnetic field can be defined in several equivalent ways based on the effects it has on its environment.
Often the magnetic field is defined by the force it exerts on a moving charged particle. It is known from experiments in electrostatics that a particle of charge q in an electric field E experiences a force F = qE. However, in other situations, such as when a charged particle moves in the vicinity of a current-carrying wire, the force also depends on the velocity of that particle. Fortunately, the velocity dependent portion can be separated out such that the force on the particle satisfies the Lorentz force law;
F = q(E + v x B).
Here v is the particle’s velocity and × denotes the cross product.
The vector B is termed the magnetic field, and it is defined as the vector field necessary to make the Lorentz force law correctly describe the motion of a charged particle. This definition allows the determination of B in the following way.
The command, “Measure the direction and magnitude of the vector B at such and such a place,” calls for the following operations: Take a particle of known charge q. Measure the force on q at rest, to determine E. Then measure the force on the particle when its velocity is v; repeat with v in some other direction. Now find a B that makes [the Lorentz force law] fit all these results—that is the magnetic field at the place in question.
SI units, B is measured in teslas (symbol: T) and correspondingly ΦB (magnetic flux) is measured in webers (symbol: Wb) so that a flux density of 1 Wb/m2 is 1 tesla. The SI unit of tesla is equivalent to (newton•second)/(coulomb•metre). In Gaussian-cgs units, B is measured in gauss (symbol: G). (The conversion is 1 T = 10,000 G.) The H-field is measured in amperes per metre (A/m) in SI units.

Temporary and Permanent Magnets

Series of experiments carried out have shown that iron, when brought close to a magnetic material, the magnetic property is lost easily.
When iron nails are clung to a magnet, they form a single file in their arrangement, but when the magnet bearing heavy iron nail to form an arrangement is removed, every other nail looses their property, thereby falling off. The conclusion is that iron easily magnetizes and also demagnetizes, where strong magnetism is required for a short time. Examples of temporary magnet are soft iron and electromagnet.

Temporary magnets are employed in the following devices: electric bells, induction coil, telephone ear-piece, magnetic relay, etc.
Steel is not easily magnetized to a magnetic material, because it takes time for the magnetic molecules in steel to be arranged. Steel is not easily demagnetized because it retains its magnetic properties even after the removal of magnets. Steel keeps its magnetism much longer than iron because of these differences in their magnetic properties. Therefore steel is used and most preferred for making permanent magnets. Examples of permanent magnets are steel alloy, etc.
Permanent magnets are employed in the following devices: electric motors, D.C Dynamo, radio loud speaker, aerials of transistor, etc.

Differences between electromagnet and ordinary magnet

Electromagnets                                                                              Ordinary Magnets
2 They are made from cast iron.                                              They are made from steel.
3 They are temporary magnets.                                               They are permanent magnets.

Magnetization

Magnetization is the process by which magnetic material is attracted to a single magnet. An example is an experiment using a bar of horse-shoe magnet and many nails. The first nail clings to the magnet followed by second, third, fourth, etc until the magnet force could no longer retain.

Methods of making magnets

1. Single touch method:
This is done by continuous stroking of a steel bar by a permanent magnet. The magnet is raised each time it reaches the end of the steel bar. A stage is reached when the last touch of the stroking process produces or results in a pole opposite.
2. Method of divided touch (Double stroke):
Using the divided method, each half of the steel is stroked continuously in opposite directions by the opposite poles N ad S of the two bar magnets. Divided touch stroking starts in the middle. The same principle is followed in single touch method.
3. Hammering in the earth’s field:
A magnet is made through the influence of the earth’s field (magnetic). The bar is first placed in a north- south direction and inclined at angle 700, to the horizontal axis. The upper part of the magnet is hammered repeatedly. It is found that the lower part has a weak North pole.
4. Electrical method:
This involves using electrical method which is the best way of making magnet. A solenoid of a long coil of insulated copper wire is connected to a battery at both terminals or end points. A steel bar is placed inside the solenoid, current is switched on for few seconds and switched off. A test after the experiment will show that the steel bar is found to be a magnet with North and South poles.
NB: This position of the poles depends on the direction of the current. When the current is clockwise in the coil, the bar has a South Pole at its end and vice-versa.

Demagnetization

This simply means the removal or loss of magnetism from a magnetic material i.e. destroying magnetism. Demagnetism, on the other hand is a process by which the property or substance is removed, causing a breakdown in the magnetic circuit. This can be done through the following methods:
1. Heating method: When a magnet is heated until it is red hot and placed in .a East-West direction, the magnetic property is lost and would no more behave like a magnet again.
2. Hammering method: The magnet is repeatedly hammered while pointing in an E-W direction, that is, about 900 to the earth’s magnetic field of direction. Hammering randomly disorganizes the arrangement of its magnetic property.
3. Electric method: The best way to demagnetize a magnet is by electrical method. A magnet is connected to an AC source and current through a steel bar placed inside a solenoid coil pointing in the direction of E-W, after some seconds, the magnets are taken away from the solenoid and are placed a distance away from the solenoid.

Earth’s Magnetic Field

The earth magnetism: The reason for the earth’s magnetism is not understood, though it is generally agreed that the earth contains the electrical charges. Field also shows the same similarity to the production of North and South poles by a magnet inside the earth, slightly inclined to the geographical axis. The behaviour is like the south-seeking pole pointing towards the North pole.
If a magnet placed in a uniform sphere of non-magnetic material is mounted on an axis joining the point on the N and S poles of the earth, the line of force will run as follows. Below are very key information as regards the magnetic field of the earth.
As per the most established theory it is due to the rotation of the earth where by the various charged ions present in the molten state in the core of the earth rotate and constitute a current.
(1) The magnetic field of earth is similar to one which would be obtained if a huge magnet is assumed to be buried deep inside the earth at its centre.
(2) The axis of rotation of earth is called geographic axis and the points where it cuts the surface of earth are called geographical poles (Ng, Sg). The circle on the earth’s surface perpendicular to the geographic axis is called equator.
(3) A vertical plane passing through the geographical axis is called geographical meridian.
(4) The axis of the huge magnet assumed to be lying inside the earth is called magnetic axis of the earth. The points where the magnetic axis cuts the surface of earth are called magnetic poles. The circle on the earth’s surface perpendicular to the magnetic axis is called magnetic equator.
(5) Magnetic axis and Geographic axis don’t coincide but they make an angle of 17.5° with each other.
(6) Magnetic equator divides the earth into two hemispheres. The hemisphere containing south polarity of earth’s magnetism is called northern hemisphere while the other, the southern hemisphere.
(7) The magnetic field of earth is not constant and changes irregularly from place to place on the surface of the earth and even at a given place in varies with time too.
(8) Direction of earth’s magnetic field is from S (geographical south) to N (Geographical north). Elements of Earth’s Magnetic Field

The magnitude and direction of the magnetic field of the earth at a place are completely given by certain quantities known as magnetic elements.
(1) Magnetic Declination: It is the angle between geographic and the magnetic meridian planes.

Declination at a place is expressed at or depending upon whether the north pole of the compass needle lies to the east or to the west of the geographical axis.
(2) Angle of inclination or Dip (ø): It is the angle between the direction of intensity of total magnetic field of earth and a horizontal line in the magnetic meridian.
(3) Horizontal component of earth’s magnetic field (BH): Earth’s magnetic field is horizontal only at the magnetic equator. At any other place, the total intensity can be resolved into horizontal component (BH) and vertical component (BV).

Magnet

What we typically refer to as a magnet, ie. a material that spontaneously produces a magnetic field, is in fact a ferromagnet. The name comes from the region where ferromagnetic stones were found in ancient Greek times, but magnets were also known in the same time period in India and China.
A compass is a freely suspended ferromagnet that can be used for navigation, or, as we will use it in this lecture, to determine the direction of a magnetic field.

Poles

We are familiar with the idea that magnet has poles, and that like poles repel and unlike poles attract. What is a Pole?

Poles always come in pairs, magnetic monopoles would be highly theoretically interesting, but have not been observed in experiment. A magnetic monopole would be the magnetic equivalent of charge and would act as a source or sink of magnetic field.
We can think of a magnet as having a particular magnetization direction, and we can then understand why if we break a magnet we end up with the creation of another pair of poles. In the context of the bar magnet we can consider the poles to describe the ends of a magnetic material.

As with electric fields it can be useful to draw lines which reflect the magnetic field at a point. Field lines point from North to South.
The lack of magnetic monopoles means that magnetic field lines do not begin or end anywhere. So in the case of a bar magnet we can see that the field lines that we can measure outside the magnet continue within it to close the loop. Earth’s magnetic field

The fact that a compass works demonstrates that the Earth has a magnetic field. The magnetic field of the earth is also important in shielding the earth from cosmic radiation. We should note that the magnetic North Pole is actually a magnetic south pole, and the magnetic South Pole is actually a magnetic north pole.
The Earth’s magnetic poles move around, and in fact can flip their direction completely. This might suggest to us that the magnetic field, unlike in our bar magnet, is due to some kind of dynamic process. In fact the Earth’s magnetic field is due to electric currents in the outer liquid core. So we should discuss how an electric current gives rise to a magnetic field.

Magnetic field due to a current carrying wire

If we put current through a wire we can measure that it produces a magnetic field. The direction of the field can be determined by a right hand rule. The experiment we will perform today was first performed by Hans Christian Oersted who first noticed the effect on a compass due to a current during a lecture in 1820.

Magnetic field due to a current carrying loop

If we make the wire in to a loop we can again apply the right hand rule to determine the direction of the magnetic field. We can compare the field distribution to the one we measured for the bar magnet.

Force on a current carrying wire

Having seen that a current carrying wire produces a magnetic field, we can now see whether a magnetic field exerts a force on a current carrying wire. In doing so we will be able to produce a definition of the magnetic field.
The direction of the force can be determined in a Jumping wire experiment. Further experiment would reveal that the force on a wire is always perpendicular to both the current and the field, so we can see that we can use a right hand rule to determine the direction of the force.
I make my parallel fingers the field lines, my thumb the current and the force is the direction of the palm of my hand (the direction I would push).

Magnitude of magnetic field

If we consider wire of which length lies within a magnetic field we find that the force depends on as well as the current. We can write an equation that contains this information as well as the right hand rule for the direction that we identified earlier. We can give the length of the wire a direction and make it a vector. The current is then defined to be positive when it flows in the direction of the length vector. The force is then or in the diagram below

We can also chop the length up in to infinitesimal pieces which produce infinitesimal forces to accommodate a wire that changes its direction with respect to a magnetic field, or a non-uniform magnetic field.
Questions
1. Which is not true about electromagnets?