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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