Chapter 9.1 - Magnetic field around Current carrying Conductor

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.

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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

• 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) 

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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

• 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.

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• 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

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Next we are going to see a special case. We will write it in steps.
1. Consider fig.9.5 below:

Fig.9.5

• 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. 

Fig.9.6

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:

Fig.9.7

• 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'.

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• 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

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)

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• 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.

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Chapter 9 - Magnetic effect of Electric current

In the previous section we completed a discussion on electric circuits. In this section, we will discuss about the relation between electricity and magnetism.

Let us do an activity:
1. Place a pivoted magnetic needle on the table. 
• It will come to rest in the North-south direction. This is shown in fig.9.1(a) below:

Fig.9.1

 2. Arrange a conductor AB above the magnetic needle
• The conductor must be above the magnetic needle  
• The conductor must be parallel to the magnetic needle
    ♦ That is., the conductor must be aligned in the North-south direction
• The conductor must be close to the magnetic needle
3. Connect the conductor to a battery through a switch
• End A should be connected to the positive terminal of the battery
4. Now we can begin the trials
Trial 1:
(i) Turn on the switch
(ii) Note down the observation:
• The north pole of the needle deflects towards the west. This is shown in fig (b)  
(iii) Turn off the switch
(iv) Note down the observation:
• The needle returns to the north-south orientation
■ We can write the conclusion:
• In this trial end A, is connected to the positive terminal. So current flows from A to B
• Direction of motion of the north pole (N) of the magnetic needle is anti-clockwise.
• When there is no current in AB, the needle does not deflect
Trial 2:
(i) Connect end B to the positive terminal of the battery
(ii) Turn on the switch
(iii) Note down the observation:
• The north pole of the needle deflects towards the east. This is shown in fig (c) 
(iv) Turn off the switch
(v) Note down the observation:
• The needle returns to the north-south orientation
■ We can write the conclusion:
• In this trial, end B is connected to the positive terminal. So current flows from B to A
• Direction of motion of the north pole (N) of the magnetic needle is clockwise.
• When there is no current in AB, the needle does not deflect
5. Before doing the next trial, we have to make a small change in the arrangements:
• Bring the conductor below the needle. This is shown in fig (d)
• All other arrangements are the same:
    ♦ The conductor must be parallel to the magnetic needle
    ♦ The conductor must be close to the magnetic needle
• End A should be connected to the positive terminal of the battery
6. Now we can begin the trials
Trial 3:
(i) Turn on the switch
(ii) Note down the observation:
• The north pole of the needle deflects towards the east. This is shown in fig (e)  
(iii) Turn off the switch
(iv) Note down the observation:
• The needle returns to the north-south orientation
■ We can write the conclusion:
• In this trial, end A is connected to the positive terminal. So current flows from A to B
• Direction of motion of the north pole (N) of the magnetic needle is clockwise.
• When there is no current in AB, the needle does not deflect
Trial 4:
(i) Connect end B to the positive terminal of the battery
(ii) Turn on the switch
(iii) Note down the observation:
• The north pole of the needle deflects towards the west. This is shown in fig (f) 
(iv) Turn off the switch
(v) Note down the observation:
• The needle returns to the north-south orientation
■ We can write the conclusion:
• In this trial, end B is connected to the positive terminal. So current flows from B to A
• Direction of motion of the north pole (N) of the magnetic needle is anti-clockwise.
• When there is no current in AB, the needle does not deflect

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The trials are complete. Let us write a summary:
■ Conductor above the needle
• Current from A to B
    ♦ Magnetic needle deflects in anti-clockwise direction
• Current from B to A
    ♦ Magnetic needle deflects in clockwise direction
■ Conductor below the needle
• Current from A to B
    ♦ Magnetic needle deflects in clockwise direction
• Current from B to A
    ♦ Magnetic needle deflects in anti-clockwise direction
■ When there is no current in AB, the needle does not deflect

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So it is clear:
• There is some sort of interaction between current and magnet
• Also this interaction depends on the direction of the current
• Our next aim is to study this interaction in detail. Then we will be able to put it to practical use.
■ Before we go into those details we need to be able to answer an interesting question:
• A person shows us a wire AB and a magnetic needle. Then he asks:
• If current flows from A to B, in which direction will the needle deflect?
• A special rule known as Ampere's swimming rule will help us to give the answer
■ Ampere's swimming rule states that, if a man swims along the wire carrying current such that his face is always towards the magnetic needle with current entering his feet and leaving his head then the North Pole of the magnetic needle is always deflected towards his left hand.
• To get a better understanding of the rule, we will break it down into simpler sentences:
1. A man should swim along the given wire. That is:
• The wire should not be above the swimmer
• The wire should not be below the swimmer
• The wire should pass through the swimmer. We can write:
    ♦ The wire should enter the swimmer's body through the feet
    ♦ The wire should leave the swimmer's body through the head
2. The swimmer must always be able to see the needle.
• That is., the needle should not be placed behind the swimmer
3. The direction of the current is important:
• The current should enter the swimmer's body through his feet
• The current should leave the swimmer's body through his head
4. The wire should be placed in the North-South direction
• So the current can flow in either one of the two directions below:
    ♦ From north to south
    ♦ From south to north
• Then, when the switch is off, the needle will be at rest in the North-South direction 
■ If the above four conditions are satisfied, when the switch is turned on, the needle will deflect towards the left hand of the swimmer 
Let us see the four possible cases:
Case 1:
■ Conductor above the needle
• Current from south to north. This is shown in fig.9.2(a) below:

Fig.9.2

• It is easy to satisfy all the four conditions. 
• However, condition 2 needs special attention
    ♦ The needle should be placed below the surface of the water. 
    ♦ Then only the swimmer will see the needle. 
    ♦ Because the swimming is done on the surface of water, conductor passes through the swimmer and needle is below the conductor
Case 2:
■ Conductor above the needle
• Current from north to south. This is shown in fig.9.2(b)
• It is easy to satisfy all the four conditions. 
• However, condition 2 needs special attention
    ♦ Just as in case 1, the needle should be placed below the surface of the water. 
    ♦ Then only the swimmer will see the needle. 
    ♦ Because the swimming is done on the surface of water, conductor passes through the swimmer and needle is below the conductor
Case 3:
■ Conductor below the needle
• Current from south to north. This is shown in fig.9.2(c)
• It is easy to satisfy all the four conditions. 
• However, condition 2 needs special attention
    ♦ The needle should be placed above the surface of the water. 
    ♦ Because the swimming is done on the surface of water, conductor passes through the swimmer and needle is above the conductor
    ♦ Swimming should be done in 'backstroke style'. Then only the swimmer will see the needle. 
Case 4:
■ Conductor below the needle
• Current from north to south. This is shown in fig.9.2(d)
• It is easy to satisfy all the four conditions. 
• However, condition 2 needs special attention
    ♦ Just as in case 3, the needle should be placed above the surface of the water. 
    ♦ Because the swimming is done on the surface of water, conductor passes through the swimmer and needle is above the conductor
    ♦ Swimming should be done in 'backstroke style'. Then only the swimmer will see the needle. 
■ In all four cases, when the switch is turned on, the needle will deflect towards the left hand of the swimmer

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Now we can get back to the original discussion. That is., we can discuss the details about the interaction between current flow and magnets. We will see it in the next section.

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