MAGNETIC EFFECT OF CURRENT

 

Chapter 13 :  MAGNETIC EFFECT OF CURRENT

Introduction

In the earlier lesson you have learnt that, electricity is an important part in today’s world of industrialization. Our life is incomplete without it. Whether we work in an office or at home, every thing depends upon the availability of electricity. Appliances like the electric bulb, fan, television, refrigerator, washing machine, motor, radio, everything works due to electricity.

When electric current passes through current carrying conductor or coil then a magnetic field is produced around it. The working of appliances like electric bell is based on this principle. As opposite to this if a continuous change in magnetic field is produced then electric current can be produced. This is how electricity and magnetism have become synonymous today. Transmission of electric current takes place from the distant electricity generation stations through high tension wires, through transformers to homes. This chapter deals with the meaning of safe use of electricity. Along with this the same concepts related to magnetism are explained through simple activities that one can perform on their own.

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?
In the year 1820 a scientist from Denmark named H.C. Oersted observed this effect for the first time. 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.

 MAGNET AND ITS PROPERTIES

Magnet has always been a thing of awe use and attraction for humans. According to history, the use of magnets were discovered by the ancient Greeks during the period of Greek Civilization. 

Fig 2. Natural magnet 

They found stones which were able to attract iron and nickel like other substances. This naturally occurring stones (see Fig. 2) which was discovered then is called as ‘lodestone’. This is an oxide of iron (Fe3O4). The property of attraction of small particles of iron towards lodestone is called as ‘magnetism’. It has been often seen that the magnetic force of attraction of these natural magnets is much less and thus, these magnets cannot be use for practical purposes. Strong magnets made of iron, nickel and lead are made artificially and used for practical purposes. Those magnets are also called as permannent magnet. So, a magnet is a material or object that produces a magnetic field which is responsible for a force that pulls or attacts on other materials. These strong magnets can be made in various shapes and creats its own persistent magnetic field. The magnets that are commonly available in different shapes are: 

(a) Bar magnet 

(b) Horseshoe magnet 

(c) Cylindrical magnet 

(d) Circular magnet

 (e) Rectangular magnet

Properties of Magnets 

 we can list the properties of magnetic as follows:

 1. Attracts iron towards itself. 

2. Freely suspended magnet always rests at the north-south direction. 3. Like poles repel while unlike poles attract.

4. If iron pieces are brought near a strong magnet they also start behaving as magnets.

5. The poles of a magnet cannot be separated.

ACTIVITY

Take one magnetic needle, two bar magnets, some iron filling, an alpin and do the experimental study of properties of magnet.

 Following step may followed: 

1. Tie a string at the middle of a bar magnet and hang it with the help of a hook. This bar always rests at the same direction. With the help of the magnetic needle, Circular Magnet Cylindrical Magnet Rectangular Magnet Horse Shoe Magnet Bar Magnet   find out the direction. By this you will be able to prove that a bar magnet always rests in the north-south direction.

Fig.  (i) 

2. Take iron fillings near the bar magnet. They stick to the magnet. Thus, magnet attracts iron. You would observe that the amount of iron fillings near the poles is maximum while at the middle is negligible.

Fig.  (ii) 

3. If you bring any pole of a bar magnet near the pole of a freely suspended bar magnet, then either it will attract or repel the same. Opposite poles of a magnet attract each other while like poles (north-north or south-south) repel each other.

Fig.  (iii) 

 4. Take an iron alpin near a bar magnet leave it there for sometime. You will find that the alpin has acquired magnetic properties and iron fillings start sticking to the ends of the alpin as well.

Fig. (iv)

 5. Break the bar magnet into smaller pieces. Now observe that magnetic properties are retained in the pieces as well. Thus, the two poles of a magnet cannot be separated.

Fig. (v)

MAGNETIC FIELD

 Keep a small magnetic needle near a bar magnet. The magnetic needle rotates and stops in a particular direction only. This shows that a force acts on the magnetic needle that makes it rotate and rest in a particular direction only. This force is called torque. The region around the magnet where the force on the magnetic needle occurs and the needle stops at a specific direction, is called a magnetic field. The direction of magnetic field is represented by magnetic line of forces. 

the direction of magnetic needle changes continuously and it takes the curved path while moving from north to south. This curved path is known as magnetic line of forces. Tangent line draw at any point on magnetic line of force, represent the direction of magnetic field at that point. These magnetic line of forces have following properties: 

1. Magnetic line of forces always start from north pole and end at south pole of the magnet.

2. These line of forces never intersect each other.

3. Near the poles magnetic lines are very near to each other which shows that magnetic field at the poles is stronger as compare to other parts. 

Earth as a Magnet and its Magnetic field 

Our Earth itslef acts as a giant magnet with south magnetic pole somewhere in the Arctic and north magnetic pole in Antaractic. The Earth also behaves like a bar magnet. It’s hot liquid centre core contains iron and as it moves, it creates an electric current that cause a magnetic field around the Earth. The Earth has a north and south magnetic pole. These poles are not same with the geographic north and south poles on a map and tilted at an angle of 11.3 degree with repect to it. Due to this, if a magnetic needle is suspended freely, it rests in the north-south direction and is useful for navigation.

Magnetic field Due to a Current Carrying Conductor:

Magnetic field due to current through a straight conductor:

A current carrying straight conductor has magnetic field in the form of concentric circles; around it. Magnetic field of current carrying straight conductor can be shown by magnetic field lines.

The direction of magnetic field through a current carrying conductor depends upon the direction of flow of electric current. The direction of magnetic field gets reversed in case of a change in the direction of electric current.

Let a current carrying conductor be suspended vertically and the electric current is flowing from south to north. In this case, the direction of magnetic field will be anticlockwise. If the current is flowing from north to south, the direction of magnetic field will be clockwise.

Right Hand Thumb Rule:

The direction of magnetic field; in relation to direction of electric current through a straight conductor can be depicted by using the Right Hand Thumb Rule. It is also known as Maxwell’s Corkscrew Rule.

If a current carrying conductor is held by right hand; keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.

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As per Maxwell’s corkscrew rule, if the direction of forward movement of screw shows the direction of current, then the direction of rotation of screw shows the direction of magnetic field.

Properties of Magnetic Field:

• The magnitude; of magnetic field increases with increase in electric current and decreases with decrease in electric current.
• The magnitude of magnetic field; produced by electric current; decreases with increase in distance and vice-versa. The size of concentric circles of magnetic field lines increases with distance from the conductor, which shows that magnetic field decreases with distance.
• Magnetic field lines are always parallel to each other.
• No two field lines cross each other.

Electromagnetism

Magnetic field due to current through a circular loop:

In case of a circular current carrying conductor, the magnetic field is produced in the same manner as it is in case of a straight current carrying conductor.

In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally; at the centre, the arcs of big circles would appear as a straight lines.

The direction of magnetic field can be identified using Right Hand Thumb’s Rule. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction; at the top of the loop. Moreover, it would be in anticlockwise direction at the bottom of the loop.

Clock Face Rule:

 A current carrying loop works like a disc magnet. The polarity of this magnet can be easily understood with the help of clock face rule. If the current is flowing in anti-clockwise direction, then the face of the loop shows north pole. On the other hand, if the current is flowing in clockwise direction, then the face of the loop shows south pole.

Magnetic field and number of turns of coil: 

Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.

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Magnetic Field due to a current in a Solenoid:

Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of cylinder.

A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the north pole and another end behaves as the south pole. Magnetic field lines are parallel inside the solenoid; similar to a bar magnet; which shows that magnetic field is same at all points inside the solenoid.

By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.