In the previous section we we saw that there will be an interaction between a current carrying conductor and a magnet. In this section, we will discuss about it's details.
• We know that, a magnetic field will exert a force on another magnet.
• In our present case, we have a ready magnet. It is the magnetic needle.
• A new magnetic field acted upon this magnetic needle.
■ Where did this new magnetic field come from?
Ans: Whenever current passes through a conductor, a magnetic field is developed around it.
In our present case, when the switch was turned on, current passed through AB and so a magnetic field was developed around AB.
Let us learn more about this new magnetic field. We will learn it with the help of an activity:
1. Insert a conductor AB through the centre of a cardboard as shown in fig.9.3(a) below:
• The conductor should be in a vertical position.
2. Turn the switch on and allow current to pass through the conductor.
3. Using a magnetic compass or magnetic needle, draw the magnetic field lines around the conductor.
The method of drawing can be seen here.
• When we view the cardboard from the top, we will see that the magnetic lines are in the anti-clockwise direction.
• Note that, positive terminal of the battery is connected to the lower end A of the conductor. So current flowed from bottom A to top B
4. Now use another cardboard. This time, the lower end A of the conductor should be connected to the negative terminal of the battery. So the current will flow from the top B to bottom A.
5. Draw the magnetic field lines around the conductor. We will see that the lines are in the clockwise direction. This is shown in fig.9.3(b)
From the above activity the following two items are evident:
Item I: A magnetic field exists around a current carrying conductor
• Note that the cardboard can be placed at any point along the length of the conductor. We will get the same lines of force.
• That means, the same lines of force exists along the whole length of the conductor.
• So all the magnetic lines taken together will take the form of a cylinder.
This is shown in the fig.9.4 (a) below:
• If a magnet comes anywhere near that cylinder, 'an interaction' will take place.
Item II: The direction of the magnetic field depends upon the direction of the current.
• Since the direction depends on the direction of the current, we may encounter situations such as the one described below:
• A person shows us a wire AB. Then he asks:
• If current flows from A to B, what will be the direction of the magnetic field lines?
• A special rule known as Right hand thumb rule will help us to give the answer.
■ Imagine you are holding a current carrying conductor with the right hand in such a way that the thumb points in the direction of the current. The direction in which the other fingers encircle the conductor gives the direction of the magnetic field. This is the Right hand thumb rule. It is shown in fig.9.4(b) above.
• To get a better understanding of the rule, we will break it down into simpler sentences:
1. Hold the current carrying conductor in the right hand.
• Holding in the left hand will not give the required results
2. The thumb should point in the direction of the current.
3. The other four fingers should encircle the conductor.
4. Then the direction indicated by the other four fingers will give the direction of the magnetic field lines
5. Fig.9.4(c) shows the application of the rule when the current flows from top to bottom
Next we are going to see a special case. We will write it in steps.
1. Consider fig.9.5 below:
• Two current carrying conductors are kept side by side.
• The current is flowing in the same direction (bottom to top) in both of them.
• The magnetic field around each conductor is also drawn.
2. Now let us view the conductors in a direction shown by the cyan arrow.
• That is., we are standing at the tail end of the cyan arrow and looking towards the head of the arrow. 3. We can see an interesting situation:
• Look closely at the field lines between the two conductors
♦ The field lines of the left conductor are flowing away from us
♦ The field lines of the right conductor are flowing towards us
• So the field lines between the two conductors are traveling in opposite directions.
4. Is their any possibility to make them both travel in the same direction?
Let us try:
• In fig.9.6 below, the direction of current in the right side conductor is reversed.
5. Now look closely at the field lines between the two conductors
♦ The field lines of the left conductor are flowing away from us
♦ The field lines of the right conductor are also flowing away from us
• So the field lines between the two conductors are now traveling in the same direction.
6. In this situation, we get a 'special zone'
• This 'special zone' is the space between the two conductors.
♦ It is 'special' because, the magnetic field here is 'stronger'
♦ It is 'stronger' because, the fields from two conductors are in the same direction.
7. In the fig.9.6, we are having two separate conductors. That means, we have to supply current separately to them.
• Is there any possibility to make the same current I to flow through both of them? Let us try:
Consider fig.9.7 below:
• A conductor is bent into a circular shape.
• It pierces the cardboard at two points.
[The portion of the conductor below the cardboard will not be visible in the fig. So it is shown in dashed lines]
8. Connect it to a battery and turn on the switch
• At the left piercing point, the current is traveling from bottom to top
• At the right piercing point, the current is traveling from top to bottom
• Now look closely at the field lines between the two piercing points
♦ The field lines on the left side are flowing away from us
♦ The field lines on the right side are also flowing away from us
• So the field lines between the two piercing points are traveling in the same direction. We have the 'special zone'
9. Now reverse the terminal connections to the battery and turn on the switch.
• The current will flow in the opposite direction. This is shown in fig.9.7(b)
• Look closely at the field lines between the two piercing points
♦ The field lines on the left side are flowing towards us
♦ The field lines on the right side are also flowing towards us
• So the field lines between the two piercing points are traveling in the same direction. Thus in this case also, we have the 'special zone'.
• Now we can try to improve this apparatus. That is, we want to 'increase the strength of the magnetic field'.
• Of course, we can do it by increasing the intensity of the current.
• But can we do it with the same current intensity? Let us try:
1. Consider fig.9.8(a) below. One more circular shaped conductor is piercing through the cardboard.
2. A magnetic field will be produced in between the new piercing points also.
• This new magnetic field will have the same direction as the one already produced by the first circular conductor.
♦ This is because, the currents are both in the same direction.
• So the two magnetic fields will combine together. Thus we get a stronger magnetic field.
3. If we place a third circular conductor, the magnetic field will become even more stronger. This is shown in fig,9.8(b)
• We know that, a magnetic field will exert a force on another magnet.
• In our present case, we have a ready magnet. It is the magnetic needle.
• A new magnetic field acted upon this magnetic needle.
■ Where did this new magnetic field come from?
Ans: Whenever current passes through a conductor, a magnetic field is developed around it.
In our present case, when the switch was turned on, current passed through AB and so a magnetic field was developed around AB.
Let us learn more about this new magnetic field. We will learn it with the help of an activity:
1. Insert a conductor AB through the centre of a cardboard as shown in fig.9.3(a) below:
 |
| Fig.9.3 |
2. Turn the switch on and allow current to pass through the conductor.
3. Using a magnetic compass or magnetic needle, draw the magnetic field lines around the conductor.
The method of drawing can be seen here.
• When we view the cardboard from the top, we will see that the magnetic lines are in the anti-clockwise direction.
• Note that, positive terminal of the battery is connected to the lower end A of the conductor. So current flowed from bottom A to top B
4. Now use another cardboard. This time, the lower end A of the conductor should be connected to the negative terminal of the battery. So the current will flow from the top B to bottom A.
5. Draw the magnetic field lines around the conductor. We will see that the lines are in the clockwise direction. This is shown in fig.9.3(b)
From the above activity the following two items are evident:
Item I: A magnetic field exists around a current carrying conductor
• Note that the cardboard can be placed at any point along the length of the conductor. We will get the same lines of force.
• That means, the same lines of force exists along the whole length of the conductor.
• So all the magnetic lines taken together will take the form of a cylinder.
This is shown in the fig.9.4 (a) below:
 |
| Fig.9.4 |
Item II: The direction of the magnetic field depends upon the direction of the current.
• Since the direction depends on the direction of the current, we may encounter situations such as the one described below:
• A person shows us a wire AB. Then he asks:
• If current flows from A to B, what will be the direction of the magnetic field lines?
• A special rule known as Right hand thumb rule will help us to give the answer.
■ Imagine you are holding a current carrying conductor with the right hand in such a way that the thumb points in the direction of the current. The direction in which the other fingers encircle the conductor gives the direction of the magnetic field. This is the Right hand thumb rule. It is shown in fig.9.4(b) above.
• To get a better understanding of the rule, we will break it down into simpler sentences:
1. Hold the current carrying conductor in the right hand.
• Holding in the left hand will not give the required results
2. The thumb should point in the direction of the current.
3. The other four fingers should encircle the conductor.
4. Then the direction indicated by the other four fingers will give the direction of the magnetic field lines
5. Fig.9.4(c) shows the application of the rule when the current flows from top to bottom
Next we are going to see a special case. We will write it in steps.
1. Consider fig.9.5 below:
 |
| Fig.9.5 |
• The current is flowing in the same direction (bottom to top) in both of them.
• The magnetic field around each conductor is also drawn.
2. Now let us view the conductors in a direction shown by the cyan arrow.
• That is., we are standing at the tail end of the cyan arrow and looking towards the head of the arrow. 3. We can see an interesting situation:
• Look closely at the field lines between the two conductors
♦ The field lines of the left conductor are flowing away from us
♦ The field lines of the right conductor are flowing towards us
• So the field lines between the two conductors are traveling in opposite directions.
4. Is their any possibility to make them both travel in the same direction?
Let us try:
• In fig.9.6 below, the direction of current in the right side conductor is reversed.
 |
| Fig.9.6 |
♦ The field lines of the left conductor are flowing away from us
♦ The field lines of the right conductor are also flowing away from us
• So the field lines between the two conductors are now traveling in the same direction.
6. In this situation, we get a 'special zone'
• This 'special zone' is the space between the two conductors.
♦ It is 'special' because, the magnetic field here is 'stronger'
♦ It is 'stronger' because, the fields from two conductors are in the same direction.
7. In the fig.9.6, we are having two separate conductors. That means, we have to supply current separately to them.
• Is there any possibility to make the same current I to flow through both of them? Let us try:
Consider fig.9.7 below:
 |
| Fig.9.7 |
• It pierces the cardboard at two points.
[The portion of the conductor below the cardboard will not be visible in the fig. So it is shown in dashed lines]
8. Connect it to a battery and turn on the switch
• At the left piercing point, the current is traveling from bottom to top
• At the right piercing point, the current is traveling from top to bottom
• Now look closely at the field lines between the two piercing points
♦ The field lines on the left side are flowing away from us
♦ The field lines on the right side are also flowing away from us
• So the field lines between the two piercing points are traveling in the same direction. We have the 'special zone'
9. Now reverse the terminal connections to the battery and turn on the switch.
• The current will flow in the opposite direction. This is shown in fig.9.7(b)
• Look closely at the field lines between the two piercing points
♦ The field lines on the left side are flowing towards us
♦ The field lines on the right side are also flowing towards us
• So the field lines between the two piercing points are traveling in the same direction. Thus in this case also, we have the 'special zone'.
• Now we can try to improve this apparatus. That is, we want to 'increase the strength of the magnetic field'.
• Of course, we can do it by increasing the intensity of the current.
• But can we do it with the same current intensity? Let us try:
1. Consider fig.9.8(a) below. One more circular shaped conductor is piercing through the cardboard.
 |
| Fig.9.8 |
• This new magnetic field will have the same direction as the one already produced by the first circular conductor.
♦ This is because, the currents are both in the same direction.
• So the two magnetic fields will combine together. Thus we get a stronger magnetic field.
3. If we place a third circular conductor, the magnetic field will become even more stronger. This is shown in fig,9.8(b)
• So our next task is to prove this:
■ Increasing the number of circular conductors will increase the strength of the magnetic field.
• We can prove it using an activity. We will see it in the next section.
■ Increasing the number of circular conductors will increase the strength of the magnetic field.
• We can prove it using an activity. We will see it in the next section.
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