In the previous section we saw that current will flow in a solenoid when there is a relative motion between that solenoid and a bar magnet. In this section, we will see the 'reason for the formation of that current'. We will also see some applications of such a current.
1. When we push the magnet into the solenoid, the 'magnetic flux in contact with the solenoid' increases.
2. But we cannot continue the push indefinitely. At some point, we have to stop the push and get ready to pull.
3. Consider the instant at which the push is stopped. At that instant, we saw that, there is no current flow
4. While we pull out, the 'magnetic flux in contact with the solenoid' decreases. Then also the current begins to flow (in the reverse direction).
5. So we can say:
• The current begins to flow in the solenoid, whenever the flux increases
• The current begins to flow in the solenoid, whenever the flux decreases
6. 'Increase' and 'decrease' indicate something which is 'not constant'. It is a 'change'.
So we can say:
• The current begins to flow in the solenoid, whenever the flux changes
■ Whenever there is a 'change in the magnetic flux' linked with a coil, a voltage is induced in that coil. This phenomenon is called electromagnetic induction.
• 'A voltage is induced' means that, there is a potential difference between the two ends of the coil.
• As a result of this potential difference, a current begins to flow in the coil.
• The voltage thus induced is called induced emf.
• The current which begins to flow is called induced current.
■It was the great scientist Michael Faraday who discovered the relation between electricity and magnetism. He is regarded as the Father of electricity.
Let us do an activity to help us learn more about this 'induced current'.
1. In fig.11.3 below, a conductor AB is placed inside a magnetic field.
• It is kept perpendicular to the direction of the field.
2. AB is connected to a galvanometer
• The galvanometer helps to detect any flow of current in the circuit
3. The red wires indicate that the conductor has 'flexibility of movement'.
• That is., we can move the conductor in any direction we want
4. Initially, the galvanometer shows zero reading.
• That means, initially no current is flowing through the conductor
5. Now we can begin the trials:
Trial 1: Move the conductor vertically upwards. This is shown in fig.11.4(a) below:
• Observation: The needle of the galvanometer deflects to the right
• From this observation, we can write:
The direction of the current is from B to A
6. Trial 2: Move the conductor vertically downwards. This is shown in fig.11.4(b)
• Observation: The needle of the galvanometer deflects to the left
• From this observation, we can write:
The direction of the current now is from A to B
■ Suppose a person shows us the poles of a magnet and also a conductor AB between those poles.
Then he asks us: If we move AB upwards, in which direction will the current flow?
• To answer such questions, we can use a special rule discovered by British physicist John Ambrose Fleming.
• It is known as Fleming's right hand rule:
Hold the forefinger, middle finger and thumb of the right hand in mutually perpendicular directions as shown in the fig.11.5 below:
IF
Forefinger indicates the direction of the magnetic field
AND
Thumb indicates the direction of motion of the conductor
THEN
The middle finger will indicate the direction of current
The following points should be noted while using this rule:
• Only right hand should be used. If we use the left 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.11.6 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 two cases that we saw above.
Case 1:
1. Consider fig.11.7 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing from N to S pole
2. Motion of the conductor is upwards. So the thumb is pointing in the upward direction
3. When the above two directions are fixed, there is only one possible direction in which the middle finger can point
• The current in the conductor AB is indeed flowing in that direction of the middle finger
Case 2:
1. Consider fig.11.8 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing from N to S pole
2. Motion of the conductor is downwards. So the thumb is pointing in the downward direction
3. When the above two directions are fixed, there is only one possible direction in which the middle finger can point
• The current in the conductor AB is indeed flowing in that direction of the middle finger
1. When we push the magnet into the solenoid, the 'magnetic flux in contact with the solenoid' increases.
2. But we cannot continue the push indefinitely. At some point, we have to stop the push and get ready to pull.
3. Consider the instant at which the push is stopped. At that instant, we saw that, there is no current flow
4. While we pull out, the 'magnetic flux in contact with the solenoid' decreases. Then also the current begins to flow (in the reverse direction).
5. So we can say:
• The current begins to flow in the solenoid, whenever the flux increases
• The current begins to flow in the solenoid, whenever the flux decreases
6. 'Increase' and 'decrease' indicate something which is 'not constant'. It is a 'change'.
So we can say:
• The current begins to flow in the solenoid, whenever the flux changes
■ Whenever there is a 'change in the magnetic flux' linked with a coil, a voltage is induced in that coil. This phenomenon is called electromagnetic induction.
• 'A voltage is induced' means that, there is a potential difference between the two ends of the coil.
• As a result of this potential difference, a current begins to flow in the coil.
• The voltage thus induced is called induced emf.
• The current which begins to flow is called induced current.
■It was the great scientist Michael Faraday who discovered the relation between electricity and magnetism. He is regarded as the Father of electricity.
Let us do an activity to help us learn more about this 'induced current'.
1. In fig.11.3 below, a conductor AB is placed inside a magnetic field.
 |
| Fig.11.3 |
2. AB is connected to a galvanometer
• The galvanometer helps to detect any flow of current in the circuit
3. The red wires indicate that the conductor has 'flexibility of movement'.
• That is., we can move the conductor in any direction we want
4. Initially, the galvanometer shows zero reading.
• That means, initially no current is flowing through the conductor
5. Now we can begin the trials:
Trial 1: Move the conductor vertically upwards. This is shown in fig.11.4(a) below:
 |
| Fig.11.4 |
• From this observation, we can write:
The direction of the current is from B to A
6. Trial 2: Move the conductor vertically downwards. This is shown in fig.11.4(b)
• Observation: The needle of the galvanometer deflects to the left
• From this observation, we can write:
The direction of the current now is from A to B
■ Suppose a person shows us the poles of a magnet and also a conductor AB between those poles.
Then he asks us: If we move AB upwards, in which direction will the current flow?
• To answer such questions, we can use a special rule discovered by British physicist John Ambrose Fleming.
• It is known as Fleming's right hand rule:
Hold the forefinger, middle finger and thumb of the right hand in mutually perpendicular directions as shown in the fig.11.5 below:
 |
| Fig.11.5 |
Forefinger indicates the direction of the magnetic field
AND
Thumb indicates the direction of motion of the conductor
THEN
The middle finger will indicate the direction of current
The following points should be noted while using this rule:
• Only right hand should be used. If we use the left 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.11.6 below:
Let us now apply the model to the two cases that we saw above.
Case 1:
1. Consider fig.11.7 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing from N to S pole
2. Motion of the conductor is upwards. So the thumb is pointing in the upward direction
3. When the above two directions are fixed, there is only one possible direction in which the middle finger can point
• The current in the conductor AB is indeed flowing in that direction of the middle finger
1. Consider fig.11.8 below:
• Direction of the magnetic field is always from the north pole to south pole. So the forefinger is pointing from N to S pole
2. Motion of the conductor is downwards. So the thumb is pointing in the downward direction
3. When the above two directions are fixed, there is only one possible direction in which the middle finger can point
• The current in the conductor AB is indeed flowing in that direction of the middle finger
Did you know that in a magnetic field, a current-carrying conductor would meet force? It is known as electromagnetic force, or emf. This trait is important to all modern electric motors and generators. So we know there's a force operating on the current-carrying conductor, but how do we know which direction it's pulling? If only a simple process could be applied in almost every situation. Fleming left hand rule will suffice.
ReplyDeleteSimilarly, when a conductor is activated in a magnetic field, an induced current is present. Power direction may be easily found by applying Fleming Right hand rule.