In the previous section we saw the current through a circular conductor. We also saw that, increasing the number of circular conductors will increase the strength of the magnetic field. In this section, we will see an activity to prove this.
1. Consider the circular conductor shown in fig.9.9(a) below.
• The two points at which it pierces through the cardboard are named as A and B.
• The conductor must be 'straight up'. That is., it must be perfectly vertical.
• Also the cardboard must be perfectly horizontal.
2. The line through A and B should be aligned exactly in the North-South direction
3. The exact midpoint between A and B is named as C
• Through C, a perpendicular line XY is drawn
4. Place a magnetic needle exactly centred at C. This is shown in fig.b
[Note that, the magnetic needle should be mounted on a pivot. Then only it will rotate freely. In the fig. however, the pivot is not shown. Also the connections of the conductor to the battery is not shown in the fig. These are to avoid clutter and thus increase clarity]
5. Now we can turn on the switch. The current flows from A to B. This is shown in fig.9.10(a) below:
• The needle deflects towards the west.
■ This is due to the force exerted by the circular conductor
6. Now gradually move the needle towards X.
• The movement should be along the line XY
• We can see that the deflection gradually decreases.
♦ The needle is moving back towards the normal North-South orientation.
• This is because, as the distance from the conductor increases, the force exerted by it decreases.
7. Keep moving the needle.
• We will reach a particular point at which the needle is back in the exact North-South orientation.
8. Mark this point as O1. This is shown in fig.9.11(a) below:
• Measure the distance CO1 and note it down.
■ We can write: Beyond the point O1, the conductor has no influence on the needle
9. Turn off the switch. Add a second circular conductor parallel to AB. This is shown in fig.b. Repeat the experiment.
• Ensure that the current I is same as that in the previous trial. This can be done using a rheostat.
[It is important to ensure that the currents are the same. Because, we want to prove that, the magnetic field becomes stronger because of an additional conductor. It must not be due to the increase in current]
• Find the point O2 and mark it.
• O2 is the point beyond which, the 'combined action the two circular conductors' have no influence on the needle.
10. Measure the length of CO2 and note it down
• We will see that, CO2 is greater than CO1
■ This is because, the combined action produced a stronger magnetic field. This stronger magnetic field can exert a force to a greater distance.
• If we add a third circular conductor, even if the current is the same, we will find that CO3 is greater than CO2
So we achieved our objective. We proved that:
■ Increasing the number of circular conductors will increase the strength of the magnetic field.
• But now a problem arises:
We need to figure out a method to connect those circular conductors together. Then only the 'same current from a single battery source' will flow through them.
• Consider the conductor in fig.9.12(a) below:
• The current flowing upwards through end X will come downwards through the end Y.
♦ This is possible because the conductor is wound in the form of a spring coil.
• This coil can be made to pierce through a cardboard. It is shown in fig (b).
• When we look from above the cardboard, it will appear as two separate circular conductors.
♦ But there is interconnection below the card board.
• If we want three circular conductors, one more turn can be added to the coil. This is shown in fig.9.13 below:
• In fact we can add a large number of turns. This is shown in fig.9.14 below:
■ When the 'number of turns' of the coil is increased, the strength of the magnetic field is increased.
• Such a coil is indeed used in many electrical devices. It is called a solenoid.
Some images can be seen here.
• It is used in situations where a strong magnetic field is required.
■ The official definition is:
Solenoid is a conducting coil wound in the shape of a spring.
• We know that each individual turn of the solenoid will have it's own magnetic field.
• So the fields of adjacent turns will combine together to give a strong magnetic field
• The fig.9.15 below shows a comparison between the following two items:
(a) The magnetic field lines around a solenoid
(b) The magnetic field lines around a bar magnet
• The details of the solenoid in the above fig. is obtained from wikimedia commons here
• The details of the bar magnet in the above fig. is obtained from wikimedia commons here
■ Comparing the two, we can see that, they are identical.
That is.,
• The magnetic field lines around a solenoid
Is identical to:
• The magnetic field lines around a bar magnet
So a current carrying solenoid will act as a bar magnet. Our next task is to find it's poles
There are 3 methods to find the poles. They are described below:
Method 1:
• Bring a magnetic needle near one end of the current carrying solenoid
♦ If the north pole of the needle gets attracted, then that end of the solenoid is it's south pole
♦ If the south pole of the needle gets attracted, then that end of the solenoid is it's north pole
Method 2:
We will write this method in steps:
1. Consider the solenoid in fig.9.16 (a) below:
• The current goes up through X and comes down through Y
2. Imagine we are standing at the tail end of the cyan arrow. And we are looking towards the head end of the cyan arrow
• We will see that, the current I is flowing in the clockwise wise direction
3. Imagine we are standing at the tail end of the red arrow. And we are looking towards the head end of the red arrow
• We will see that, the current I is flowing in the anti-clockwise wise direction
4. The opposite of the above two will happen when the current is reversed. This is shown in fig.b
• At end X, the the current is flowing in the anti-clockwise direction.
• And at end Y, the current is flowing in the clockwise direction.
5. So we find an important property:
■ Even if current is flowing normally from one end of the solenoid to the other end, when looked from either ends, one is clockwise and the other is anti-clockwise.
• This property can be related to the polarity. The rule is:
(i) Hold a solenoid against your face.
(ii) Note the direction of current at the end nearest to your face.
• If the direction is clockwise, then that end is the south pole.
• If the direction is anti-clockwise, then that end is the north pole.
Method 3:
1. Consider the solenoid in the above fig.9.16(a).
• Imagine that the following two conditions are satisfied:
(i) You are holding it in the right hand.
(ii) The thump is pointing towards the end X.
2. If the above two conditions are satisfied, the other fingers will be pointing in a 'direction opposite to the direction of current'.
• Such an 'opposite pointing' is not allowable for this method to work
• We want 'pointing in the same direction'. So what do we do?
3. Note that, only right hand should be used. Let the thumb point towards Y. We will write the steps again:
• Imagine that the following two conditions are satisfied:
(i) You are holding it in the right hand.
(ii) The thump is pointing towards the end Y.
4. If the above two conditions are satisfied, the other fingers will be pointing 'in the same direction of current'.
• So this is acceptable.
• In this situation, the end towards which the thumb points is the north pole.
So we can write a summary of this third method:
■ Imagine the solenoid being held by the right hand. If the four fingers encircling the solenoid shows the direction of the current, the thumb indicates the north pole.
So we have seen the basic properties of a solenoid. Now we will see methods to increase the magnetic strength of a solenoid.
Method 1:
• We know that the solenoid has a large number of turns.
• We have also seen that, when current is turned on, 'a combination of magnetic fields' of the individual turns takes place.
• So if the 'number of turns' is increased, the strength of the magnetic field will increase.
Method 2:
• Use a soft iron core. Let us see how it is done:
• We know that the solenoid has a large number of turns.
• If these turns are made around a soft iron core, then the strength of the magnetic field will increase. This is shown in the fig.9.17 below:
■ The following properties of soft iron makes it suitable for the core:
• It has greater magnetic susceptibility
♦ That is., soft iron is 'attracted more' into a magnetic field than other metals
• It has greater magnetic permeability
♦ That is., soft iron has a greater 'tendency to support the formation of magnetic fields within it's body' than other metals
1. In the fig.9.18(a) below, for the bulb to glow, current must flow from A to B.
2. But between A and B, a MCB is placed.
• The cyan rectangle shows the body of the MCB.
• The circuit will be complete only if the current successfully come out of the MCB.
3. The current entering the MCB is lead to the coil shown in red colour.
• So the current flows through all the turns of the coil.
• A soft iron core is placed inside the coil.
• The current emerging from the coil passes through the conductor shown in green colour.
• After passing through the green conductor, the current emerges from the MCB and flows into the bulb. Thus the circuit is completed.
4. But when there is a fault in the circuit as shown in fig.b, or when there is an over load, excess current flows through the circuit.
• Then a strong magnetic field is developed in the coil.
• The soft iron core will get magnetized. That is., the soft iron core will become a temporary magnet.
• So it will be attracted towards the green conductor. Because of the forward motion of the soft iron core, the green conductor will be displaced from it's original position.
• Thus the circuit will break. Current will no longer flow through the circuit.
1. Consider the circular conductor shown in fig.9.9(a) below.
 |
| Fig.9.9 |
• The conductor must be 'straight up'. That is., it must be perfectly vertical.
• Also the cardboard must be perfectly horizontal.
2. The line through A and B should be aligned exactly in the North-South direction
3. The exact midpoint between A and B is named as C
• Through C, a perpendicular line XY is drawn
4. Place a magnetic needle exactly centred at C. This is shown in fig.b
[Note that, the magnetic needle should be mounted on a pivot. Then only it will rotate freely. In the fig. however, the pivot is not shown. Also the connections of the conductor to the battery is not shown in the fig. These are to avoid clutter and thus increase clarity]
5. Now we can turn on the switch. The current flows from A to B. This is shown in fig.9.10(a) below:
 |
| Fig.9.10 |
■ This is due to the force exerted by the circular conductor
6. Now gradually move the needle towards X.
• The movement should be along the line XY
• We can see that the deflection gradually decreases.
♦ The needle is moving back towards the normal North-South orientation.
• This is because, as the distance from the conductor increases, the force exerted by it decreases.
7. Keep moving the needle.
• We will reach a particular point at which the needle is back in the exact North-South orientation.
8. Mark this point as O1. This is shown in fig.9.11(a) below:
 |
| Fig.9.11 |
■ We can write: Beyond the point O1, the conductor has no influence on the needle
9. Turn off the switch. Add a second circular conductor parallel to AB. This is shown in fig.b. Repeat the experiment.
• Ensure that the current I is same as that in the previous trial. This can be done using a rheostat.
[It is important to ensure that the currents are the same. Because, we want to prove that, the magnetic field becomes stronger because of an additional conductor. It must not be due to the increase in current]
• Find the point O2 and mark it.
• O2 is the point beyond which, the 'combined action the two circular conductors' have no influence on the needle.
10. Measure the length of CO2 and note it down
• We will see that, CO2 is greater than CO1
■ This is because, the combined action produced a stronger magnetic field. This stronger magnetic field can exert a force to a greater distance.
• If we add a third circular conductor, even if the current is the same, we will find that CO3 is greater than CO2
So we achieved our objective. We proved that:
■ Increasing the number of circular conductors will increase the strength of the magnetic field.
• But now a problem arises:
We need to figure out a method to connect those circular conductors together. Then only the 'same current from a single battery source' will flow through them.
• Consider the conductor in fig.9.12(a) below:
 |
| Fig.9.12 |
♦ This is possible because the conductor is wound in the form of a spring coil.
• This coil can be made to pierce through a cardboard. It is shown in fig (b).
• When we look from above the cardboard, it will appear as two separate circular conductors.
♦ But there is interconnection below the card board.
• If we want three circular conductors, one more turn can be added to the coil. This is shown in fig.9.13 below:
 |
| Fig.9.13 |
 |
| Fig.9.14 |
• Such a coil is indeed used in many electrical devices. It is called a solenoid.
Some images can be seen here.
• It is used in situations where a strong magnetic field is required.
■ The official definition is:
Solenoid is a conducting coil wound in the shape of a spring.
• We know that each individual turn of the solenoid will have it's own magnetic field.
• So the fields of adjacent turns will combine together to give a strong magnetic field
• The fig.9.15 below shows a comparison between the following two items:
(a) The magnetic field lines around a solenoid
(b) The magnetic field lines around a bar magnet
 |
| Fig.9.15 |
• The details of the bar magnet in the above fig. is obtained from wikimedia commons here
■ Comparing the two, we can see that, they are identical.
That is.,
• The magnetic field lines around a solenoid
Is identical to:
• The magnetic field lines around a bar magnet
So a current carrying solenoid will act as a bar magnet. Our next task is to find it's poles
There are 3 methods to find the poles. They are described below:
Method 1:
• Bring a magnetic needle near one end of the current carrying solenoid
♦ If the north pole of the needle gets attracted, then that end of the solenoid is it's south pole
♦ If the south pole of the needle gets attracted, then that end of the solenoid is it's north pole
Method 2:
We will write this method in steps:
1. Consider the solenoid in fig.9.16 (a) below:
 |
| Fig.9.16 |
2. Imagine we are standing at the tail end of the cyan arrow. And we are looking towards the head end of the cyan arrow
• We will see that, the current I is flowing in the clockwise wise direction
3. Imagine we are standing at the tail end of the red arrow. And we are looking towards the head end of the red arrow
• We will see that, the current I is flowing in the anti-clockwise wise direction
4. The opposite of the above two will happen when the current is reversed. This is shown in fig.b
• At end X, the the current is flowing in the anti-clockwise direction.
• And at end Y, the current is flowing in the clockwise direction.
5. So we find an important property:
■ Even if current is flowing normally from one end of the solenoid to the other end, when looked from either ends, one is clockwise and the other is anti-clockwise.
• This property can be related to the polarity. The rule is:
(i) Hold a solenoid against your face.
(ii) Note the direction of current at the end nearest to your face.
• If the direction is clockwise, then that end is the south pole.
• If the direction is anti-clockwise, then that end is the north pole.
Method 3:
1. Consider the solenoid in the above fig.9.16(a).
• Imagine that the following two conditions are satisfied:
(i) You are holding it in the right hand.
(ii) The thump is pointing towards the end X.
2. If the above two conditions are satisfied, the other fingers will be pointing in a 'direction opposite to the direction of current'.
• Such an 'opposite pointing' is not allowable for this method to work
• We want 'pointing in the same direction'. So what do we do?
3. Note that, only right hand should be used. Let the thumb point towards Y. We will write the steps again:
• Imagine that the following two conditions are satisfied:
(i) You are holding it in the right hand.
(ii) The thump is pointing towards the end Y.
4. If the above two conditions are satisfied, the other fingers will be pointing 'in the same direction of current'.
• So this is acceptable.
• In this situation, the end towards which the thumb points is the north pole.
So we can write a summary of this third method:
■ Imagine the solenoid being held by the right hand. If the four fingers encircling the solenoid shows the direction of the current, the thumb indicates the north pole.
So we have seen the basic properties of a solenoid. Now we will see methods to increase the magnetic strength of a solenoid.
Method 1:
• We know that the solenoid has a large number of turns.
• We have also seen that, when current is turned on, 'a combination of magnetic fields' of the individual turns takes place.
• So if the 'number of turns' is increased, the strength of the magnetic field will increase.
Method 2:
• Use a soft iron core. Let us see how it is done:
• We know that the solenoid has a large number of turns.
• If these turns are made around a soft iron core, then the strength of the magnetic field will increase. This is shown in the fig.9.17 below:
 |
| Fig.9.17 Source: Wikimedia commons |
• It has greater magnetic susceptibility
♦ That is., soft iron is 'attracted more' into a magnetic field than other metals
• It has greater magnetic permeability
♦ That is., soft iron has a greater 'tendency to support the formation of magnetic fields within it's body' than other metals
Now we will see the working of a Miniature Circuit Breaker, which is commonly known as MCB.
We will write it in steps:1. In the fig.9.18(a) below, for the bulb to glow, current must flow from A to B.
 |
| Fig.9.18 |
• The cyan rectangle shows the body of the MCB.
• The circuit will be complete only if the current successfully come out of the MCB.
3. The current entering the MCB is lead to the coil shown in red colour.
• So the current flows through all the turns of the coil.
• A soft iron core is placed inside the coil.
• The current emerging from the coil passes through the conductor shown in green colour.
• After passing through the green conductor, the current emerges from the MCB and flows into the bulb. Thus the circuit is completed.
4. But when there is a fault in the circuit as shown in fig.b, or when there is an over load, excess current flows through the circuit.
• Then a strong magnetic field is developed in the coil.
• The soft iron core will get magnetized. That is., the soft iron core will become a temporary magnet.
• So it will be attracted towards the green conductor. Because of the forward motion of the soft iron core, the green conductor will be displaced from it's original position.
• Thus the circuit will break. Current will no longer flow through the circuit.
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