From chapter electricity we know a little bit about electric field E⃗ and how it is produced at all points around it.
Likewise magnets produces magnetic field at all points round it.
We already learned about heating effects of electric current in previous chapter about electricity. Now in this chapter we shall study about another phenomenon related to current that is magnetic effects of current.
An electric current carrying wire behaves like a magnet:- So, we already know that an electric current-carrying wire behaves like a magnet. Let us now describe an experiment that shows the presence of magnetic field near a current carrying wire. ACTIVITY showing presence of magnetic field near current carrying wire:-
This experiment is also called Oersted Experiment.
First take a straight thick copper wire and place it between the points X and Y in an electric circuit, as shown in Fig. 1. Figure 1: compass needle is deflected when it is placed near a current carrying wire
Now we place a small compass near to this copper wire.
After placing the needle note the position of its needle.
Now insert the key into the plug to close the circuit and pass the current through the circuit.
Since the current is flowing in the circuit now observe the change in the position of the compass needle.
In the above activity we observed that the needle of the compass gets deflected when it is placed near the current carrying conductor. The result of this activity implies that current flowing through copper wire is producing a magnetic effect. Thus we can say that electricity and magnetism are linked to each other.
Who was Hans Christian Oersted? Hans Christian Oersted (1777-1851) through his experiments showed that electricity and magnetism are related to each other. His research later used in radio, television etc. The unit of magnetic field strength is name Oersted in his honour.
Magnets and magnetism
Magnets have been source of curiosity for ages. You can find magnets in places like laboratories, in toys, magnetic stickers that stick in refrigerator etc.
So, what is a magnet? The substances which have the property of attracting small pieces of iron, nickel and cobalt etc. are called magnets and this property of attraction is called magnetism.
Most of the metals like copper, gold, silver aluminium etc. are not attracted to magnets.
Magnets are found naturally in certain natural rocks and can also be made artificially by certain methods.
Artificially made permanent magnets are made in various shapes like bar, rod, disk, ring etc.
In a bar, rod and horse shoe magnets north and south poles are dictated by letters N and S or North Pole is indicated by a dot.
In disk and ring magnets one face is North Pole and another face is South Pole.
Permanent magnets are commonly used as a direction finding compass.
What are magnetic poles? Magnetic poles refer to the two areas of a magnet where the magnetic effects are the strongest. The poles are generally termed as the north and south poles.
Properties of magnet:
Attracts objects of iron, cobalt and nickel.
Force of attraction of a magnet is greater at its poles then in the middle.
Like poles of magnets repel each other while unlike poles of magnets attract each other.
A free suspended magnet always point towards north and south direction.
The pole of a magnet which points toward north direction is called North Pole or north seeking.
The pole of a magnet which points toward south direction is called South Pole or south seeking.
Magnetic Field
When we bring two magnets near each either they attract each other or they repel each other.
We can explain this force of attraction and repulsion between two magnets using the concept of Magnetic Field.
Magnets produces magnetic field in the space around it, which exerts force on any other magnet placed in this region. So it is the region around magnet within which its influence can be experienced.
What is magnetic Field? The space around a magnet in which the force of attraction and repulsion due to the magnet can be detected is called the magnetic field.
IMPORTANT NOTE:- Each point in the field of any magnet has a particular strength and magnetic field at each point has definite direction. How do you find the direction of the magnetic field due to magnet at a point near it? The direction of the magnetic field due to a magnet at a point near it can be found by placing a magnetic compass at that point. The compass needle gets deflected when it is placed near the magnet.
More about compass
The simplest compass is a magnetized metal needle mounted in such a way that it can spin freely.
Needle of a compass is a small bar magnet. This is the reason it gets deflected when we place it in the field of other magnet.
Compass needles are lightweight because earth has very week magnetic field. To show up the effect of that tiny magnetic field it should have even less effect due to gravity (note:- that gravitational force is much stronger then the force produced by earth’s magnetic field).
Compass needles are mounted on frictionless bearings because in this case there would be less frictional resistance for the magnetic force to overcome.
The ends of a compass needle points approximately in North and south directions.
About earth’s magnetism
The earth has a magnetic field which we call as the earth’s magnetic field.
The magnetic field is tilted slightly from the Earth’s axis.
The core of earth is filled with molten iron (Fe) which give Earth its very own magnetic field.
This large magnetic field protects the Earth from space radiation and particles such as the solar wind.
The region surrounding Earth where its magnetic field is located is termed as the Magnetosphere.
Earth has a magnetic field that has a shape similar to that of a large bar magnet.
To the north is the magnetic north pole, which is really the south pole of Earth’s bar magnet. (It is because this pole attracts the north pole of the compass magnet)
Magnetic Field Lines
Magnetic field surrounding the magnet and the force it exerts are depicted using imaginary curved lines with arrow called magnetic field lines.
Ways of obtaining magnetic field lines around a bar magnet
(A) Iron filings demonstration
Procedure:-
Fix a sheet of white paper on a drawing board using some adhesive material.
Place a bar magnet in the centre of it.
Sprinkle some iron filings uniformly around the bar magnet (Fig. 2). A salt-sprinkler may be used for this purpose.
Now tap the board gently.
Iron filings near the bar magnet align themselves along the field lines.
Fig. 2 Iron filings near the bar magnet align themselves along the field lines.This happens because the magnet exerts its influence in the region surrounding it. Therefore the iron filings experience a force. The force thus exerted makes iron filings to arrange in a pattern. The lines along which the iron filings align themselves represent magnetic field lines. (B) Demonstration using magnetic compass With this demonstration you can draw the field lines of a bar magnet yourself
Procedure of this activity:-
Take a small compass and a bar magnet.
Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.
Mark the boundary of the magnet.
Place the compass near the north pole of the magnet.
Here you will notice that the south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet.
Mark the position of two ends of the needle.
Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole.
In this way, proceed step by step till you reach the south pole of the magnet as shown in Fig. 3. Fig. 3 Drawing a magnetic field line with the help of a compass needle.
Join the points marked on the paper by a smooth curve. This curve represents a field line.
Repeat the above procedure and draw as many lines as you can. You will get a pattern shown in Fig. 4. These lines represent the magnetic field around the magnet. These are known as magnetic field lines. Fig. 4 Field lines around a bar magnet
Observe the deflection in the compass needle as you move it along a field line. The deflection increases as the needle is moved towards the poles.
Observations
Direction in which compass needle points is the direction of the magnetic field.
The strength of the magnetic field is inversely proportional to the distance between the field lines.
Magnetic field lines never cross each other. It is unique at every point in space.
Magnetic field lines begin at the north pole of a magnet and terminate on the South Pole.
Properties of magnetic field lines
All field lines are closed curves.
Outside the magnet field lines emerge from North Pole and merge at South Pole.
Inside a magnet, the direction of field lines is from South Pole to its north pole.
Field lines never intersect each other.
Field lines are closed together near the poles and spread out away from them.
The field is stronger where the field lines are more closely spaced. So, the field is stronger near the poles then at other points.
Why do field lines never intersect each other? If two lines were to intersect each other, then a compass needle placed at the point of interaction would point in two different directions which is not possible.
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