In the previous section we saw that, a current carrying solenoid will act as a bar magnet. We also saw the methods to increase the strength of it's magnetic field. In this section, we will see Fleming's Left hand rule.
• In the discussions so far in this chapter, we can notice the following peculiarity:
♦ The current carrying conductor was stationary. That is., it was fixed in a position
♦ The magnet was free to move
• Now we will consider the opposite situation:
♦ The magnet is stationary
♦ The current carrying conductor is free to move.
Let us do an activity. We will write it in steps:
1. In fig.9.19(a) given below, a U shaped magnet is kept stationary in a vertical position
• A straight aluminium conductor AB is suspended midway between the N and S poles of the magnet
• AB is suspended using thin wires. So it can oscillate freely
2. The thin wires are both connected to a battery through a switch
• End A is connected to the positive terminal of the battery. So current flows from A to B
3. The switch is turned on.
• The conductor is suddenly displaced to one side.
• The displacement is towards the magnet. This is shown in fig.b
■ We can say: When the magnet is stationary and conductor is movable, that conductor will move when current passes through it
■ In the previous activities we saw this: When the magnet is movable and conductor is stationary, the magnet will move when current passes through the conductor
• But what we see in fig.9.19(b) above is only one of the four possible cases. Let us see the other three:
4. Case 2: In fig.9.20 below, all the arrangements are the same as in case 1 except that, the connections to the terminals of the battery are interchanged.
• So when the switch is turned on, the current will flow from B to A in the conductor.
• We can see that, AB is now displaced away from the magnet
5. Case 3: In fig.9.21(a) below, all the arrangements are the same as in case 1 except that, the north and south poles of the magnet are interchanged. The N pole is now at top and S pole is at bottom.
• When the switch is turned on, the current will flow from A to B in the conductor.
• We can see that, AB is now displaced away from the magnet
6. Case 4: In fig.9.21(b) above, all the arrangements are the same as in case 3 except that, the connections to the terminals of the battery are interchanged.
• So when the switch is turned on, the current will flow from B to A in the conductor.
• We can see that, AB is now displaced towards the magnet
We can write a summary:
Case 1:
■ North pole above and South pole below
• Current from A to B
♦ Conductor AB moves towards the magnet
Case 2:
■ North pole above and South pole below
• Current from B to A
♦ Conductor AB moves away from the magnet
Case 3:
■ North pole below and South pole above
• Current from A to B
♦ Conductor AB moves away from the magnet
Case 4:
■ North pole below and South pole above
• Current from B to A
♦ Conductor AB moves towards the magnet
From the above four cases, we can infer the following points:
(i) A force is acting on the conductor. That is why it is being displaced
(ii) The direction of the force depends on the direction of the current.
(iii) The direction of the force depends on the direction of the magnetic field
■ So the direction of the force depends on two quantities:
• Direction of current
• Direction of magnetic field
■ Suppose a person shows us the poles of a magnet and also a conductor AB between those poles.
Then he asks us: If current flows from A to B, in which direction will the conductor move?
• To answer such questions, we can use a special rule known as Fleming's left hand rule:
Hold the forefinger, middle finger and thumb of the left hand in mutually perpendicular directions as shown in the fig.9.22 below:
IF
Forefinger indicates the direction of the magnetic field B
AND
Middle finger indicates the direction of the current I
THEN
The thumb will indicate the direction of force F
The following points should be noted while using this rule:
• Only left hand should be used. If we use the right hand, required results will not be obtained
• The forefinger, middle finger and thumb should be kept perpendicular to each other
• A 3D model of the fingers is shown in the fig.9.23 below:
• The advantage of making such a model is that, it can be aligned to any required direction that we want
Let us now apply the model to the four cases that we saw above.
Case 1:
1. Consider fig.9.24 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing upwards
2. Current is flowing from A to B. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 2:
1. Consider fig.9.25 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger in the model is pointing upwards
2. Current is flowing from B to A. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 3:
1. Consider fig.9.26 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger in the model is pointing downwards
2. Current is flowing from A to B. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 4:
1. Consider fig.9.27 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger in the model is pointing downwards
2. Current is flowing from B to A. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
• In the discussions so far in this chapter, we can notice the following peculiarity:
♦ The current carrying conductor was stationary. That is., it was fixed in a position
♦ The magnet was free to move
• Now we will consider the opposite situation:
♦ The magnet is stationary
♦ The current carrying conductor is free to move.
Let us do an activity. We will write it in steps:
1. In fig.9.19(a) given below, a U shaped magnet is kept stationary in a vertical position
 |
| Fig.9.19 |
• AB is suspended using thin wires. So it can oscillate freely
2. The thin wires are both connected to a battery through a switch
• End A is connected to the positive terminal of the battery. So current flows from A to B
3. The switch is turned on.
• The conductor is suddenly displaced to one side.
• The displacement is towards the magnet. This is shown in fig.b
■ We can say: When the magnet is stationary and conductor is movable, that conductor will move when current passes through it
■ In the previous activities we saw this: When the magnet is movable and conductor is stationary, the magnet will move when current passes through the conductor
• But what we see in fig.9.19(b) above is only one of the four possible cases. Let us see the other three:
4. Case 2: In fig.9.20 below, all the arrangements are the same as in case 1 except that, the connections to the terminals of the battery are interchanged.
 |
| Fig.9.20 |
• We can see that, AB is now displaced away from the magnet
5. Case 3: In fig.9.21(a) below, all the arrangements are the same as in case 1 except that, the north and south poles of the magnet are interchanged. The N pole is now at top and S pole is at bottom.
 |
| Fig.9.21 |
• We can see that, AB is now displaced away from the magnet
6. Case 4: In fig.9.21(b) above, all the arrangements are the same as in case 3 except that, the connections to the terminals of the battery are interchanged.
• So when the switch is turned on, the current will flow from B to A in the conductor.
• We can see that, AB is now displaced towards the magnet
We can write a summary:
Case 1:
■ North pole above and South pole below
• Current from A to B
♦ Conductor AB moves towards the magnet
Case 2:
■ North pole above and South pole below
• Current from B to A
♦ Conductor AB moves away from the magnet
Case 3:
■ North pole below and South pole above
• Current from A to B
♦ Conductor AB moves away from the magnet
Case 4:
■ North pole below and South pole above
• Current from B to A
♦ Conductor AB moves towards the magnet
From the above four cases, we can infer the following points:
(i) A force is acting on the conductor. That is why it is being displaced
(ii) The direction of the force depends on the direction of the current.
(iii) The direction of the force depends on the direction of the magnetic field
■ So the direction of the force depends on two quantities:
• Direction of current
• Direction of magnetic field
■ Suppose a person shows us the poles of a magnet and also a conductor AB between those poles.
Then he asks us: If current flows from A to B, in which direction will the conductor move?
• To answer such questions, we can use a special rule known as Fleming's left hand rule:
Hold the forefinger, middle finger and thumb of the left hand in mutually perpendicular directions as shown in the fig.9.22 below:
 |
| Fig.9.22 Source: Wikimedia commons |
Forefinger indicates the direction of the magnetic field B
AND
Middle finger indicates the direction of the current I
THEN
The thumb will indicate the direction of force F
The following points should be noted while using this rule:
• Only left hand should be used. If we use the right hand, required results will not be obtained
• The forefinger, middle finger and thumb should be kept perpendicular to each other
• A 3D model of the fingers is shown in the fig.9.23 below:
 |
| Fig.9.23 |
Let us now apply the model to the four cases that we saw above.
Case 1:
1. Consider fig.9.24 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing upwards
2. Current is flowing from A to B. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 2:
1. Consider fig.9.25 below:
 |
| Fig.9.25 |
2. Current is flowing from B to A. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 3:
1. Consider fig.9.26 below:
 |
| Fig.9.26 |
2. Current is flowing from A to B. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
Case 4:
1. Consider fig.9.27 below:
 |
| Fig.9.27 |
2. Current is flowing from B to A. The middle finger in the model is pointing in this direction
3. When the above two directions are fixed, there is only one possible direction in which the thumb can point
• The conductor AB is indeed moving in that direction of the thumb
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