In the previous section we saw Fleming's right hand rule and it's application. In this section, we will see the Electric generator. Before that, we will see some basics about Alternating current.
Let us do two separate experiments:
Experiment 1:
1. Consider the circuit in fig.11.9(a) below:
• This type of circuit is already familiar to us.
• A cell which can be used in a torch light or clock is connected in series with a galvanometer and a rheostat
• The function of the rheostat is to provide a suitable resistance so that, excessive current will not flow through the galvanometer.
2. This experiment has only one trial.
Trial 1: Turn on the switch
3. Note down the observation:
• The needle of the galvanometer deflects to one side.
♦ The 'side to which deflection occurs' is not important for our present experiment
• What is important is this: Once the deflection occurs, the needle stays in that deflected position.
• As long as the switch is kept in 'on' position, the needle stays in that deflected position
4. So we can write two points:
(i) Since there is deflection, it can be confirmed that, current is flowing in the circuit
(ii) Since the needle stays in the deflected position, it can be confirmed that, the current flows in one direction only
Experiment 2:
1. Consider the arrangement in fig.11.9(b). A solenoid is connected to a galvanometer
2. In this experiment also, there is only one trial
Trial 1: The bar magnet is moved in and out continuously in the solenoid
3. Note down the observation:
• The needle continuously deflects to both sides alternately.
• As long as there is to and fro motion of the magnet, the needle also continues it's 'to and fro' deflection
4. So we can write two points:
(i) Since there is deflection, it can be confirmed that, current is flowing in the solenoid
(ii) Since there is continuous 'to and fro' deflection, it can be confirmed that, the current changes the direction continuously
■ A current that flows only in one direction continuously is called a direct current (DC)
■ A current that changes it's direction at regular intervals of time is called an alternating current (AC)
• It may be noted that, AC is produced due to some of our 'limitations':
• If we can move the bar magnet continuously in one direction, we will get DC
• But for such a movement, the length of the solenoid will have to be very large.
• Such a long solenoid cannot be made.
• So only possibility is 'to and fro' motion
1. Consider the arrangement in fig.11.10 below:
• We have a north pole and a south pole.
• In between the poles, we have a rectangular coil ABCD of insulated copper wire.
■ This coil is called armature
2. We know that the direction of the magnetic field will be from the N pole to the S pole.
• So the segment AB of the coil is perpendicular to the magnetic field
• The segment CD is also perpendicular to the magnetic field
3. The ends of the coil are connected to the two rings R1 and R2
■ Let us see the features of these rings:
(i) R1 is shown in blue color
• R2 is shown in green color.
• This difference in color is given for our learning purpose only. In reality, they need not be of different colors.
(ii) Both the rings are attached firmly to an axle.
• So if the axle rotates, the rings will also rotate.
• An arrow is shown at the rear end of the axle.
♦ This is for our learning purpose only. It shows the rotation of the axle.
(iii) The rings are not in direct contact with the axle.
• There is an insulating layer in between each ring and the axle.
• This is to prevent the flow of current from the ring to the axle.
(iv) The rings are in contact with two brushes B1 and B2. These brushes are stationary.
(v) The coil ABCD is attached to the rings. This attachment is clear from the following two points seen in the fig.:
• The left end of the coil (coming from 'A') is penetrating through the blue ring R1
• The right end of the coil (coming from 'D') is penetrating through the green ring R2
♦ Though this end is penetrating through R1 also, the area of contact is insulated
реж We can see that, at R1, the coil is passing through a PVC pipe
♦ Because of this insulation, the following two currents will not enter the ring R1:
(i) Current flowing out of the coil through 'D'
(ii) Current flowing into the coil through 'D'
■ The two rings are called slip rings
■ Recall that, the rings used in an electric motor are split rings. See fig.9.28.
• The difference between the two is:
♦ Split rings are used where we want a change in the direction of current
♦ Slip rings do not cause any change in current
4. A galvanometer is connected in the circuit to detect the flow of any current and also to detect the direction of the flow
5. A handle is attached so that, the axle can be turned easily
6. Now let us turn the handle. We will turn it in the clock-wise direction. This is shown in fig.11.11 below:
• The handle is turned
As a result:
• The axle, which is attached to the handle, will also turn in the clockwise direction
As a result:
• The blue and green rings, which are attached to the axle, will turn in the clock wise direction
As a result:
• The coil ABCD, which is attached to the rings will turn in the clockwise direction
7. When the coil turn, two things happen simultaneously:
(i) Segment AB goes up
(ii) Segment CD goes down
8. Now, since AB is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from A to B
• In a similar way, since CD is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from C to D
■ So the two currents will add up. We get a current flow in the direction: A → B → C → D
9. After reaching 'D', the current flows out of the armature and into the rings.
• But it does not branch at the blue ring. Because there is PVC insulation
• So all the current flows into the green ring.
• From the green ring, the current flows into the brush B2
• From brush B2, it flows into the galvanometer. So the needle deflects to the left
• After coming out of the galvanometer, the current flows into the brush B1
• From B1, it enters the blue ring
• From the blue ring, it flows back into the armature through the point 'A'
Thus the circuit is complete.
10. The current that we saw just now was formed due to the upward motion of the segment AB
(and also due to the downward motion of the segment CD)
• But AB cannot continue to move upwards indefinitely
• Neither can CD continue to move downwards indefinitely
11. So what happens to them?
• After 'half a rotation',
♦ AB occupies the position previously occupied by CD
♦ CD occupies the position previously occupied by AB
This is shown in fig.11.12 below:
■ So one 'half rotation' is over.
• During that half rotation, we obtained current in the direction A → B → C → D
• We reached the stage shown in fig.11.12
• Let us see what happens next:
12. Due to the continued rotation of the handle,
(i) Segment CD goes up
(ii) Segment AB goes down
13. Now, since CD is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from D to C
• In a similar way, since AB is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from B to A
■ So the two currents will add up. We get a current flow in the direction: D → C → B → A
14. After reaching 'A', it flows out of the armature and into the blue ring
• The current entering the blue ring has only one option: Flow into the brush B1
♦ It cannot flow into the armature because of the PVC insulation
• Current flowing out of B1 will flow into the galvanometer. So the needle deflects to the right
• Then it enter brush B2
• From brush B2, it enter the green ring
• The current in green ring has only one option: Flow into the armature
• This current which enters the armature from the green ring is flowing back into the armature.
♦ It can not branch into B1 or the blue ring because of the PVC insulation
• So all the current flows back into the armature through D
Thus the circuit is complete
15. We obtained this current D → C → B → A due to the downward motion of the segment AB
(and also due to the upward motion of the segment CD)
• But AB cannot continue to move downwards indefinitely
• Neither can CD continue to move upwards indefinitely
• Thus we reach back to the stage which we saw previously in fig.11.11. This completes the second 'half rotation'
16. The first 'half rotation' and the second 'half rotation' together forms one 'full rotation'
• Thus segment AB reaches back to where it initially was
16. As we again continue to turn the handle, we get a series of 'full rotations' of the armature
• Each full rotation consists of two 'half rotations'
♦ In the first 'half rotation', we get current in the direction A → B → C → D
♦ In the second 'half rotation', we get current in the direction D → C → B → A
17. Thus we get a continuous supply of current
• A current which changes it's direction at equal intervals of time
■ It is an alternating current (AC)
■ If we use split rings (as we saw in the case of electric motor), we can obtain 'direct current'. The details can be seen here.
• In the above figs., a handle was used for rotating the armature. We can use a turbine instead. This is shown in fig.11.13 below:
■ If water is allowed to fall on the blades of the turbine, it will rotate
• Since the turbine is attached to the axle of the generator, it will also rotate and we will get electricity.
• The water should be allowed to fall from a height. In 'Hydroelectric power stations', the required water is stored in huge dams
■ Instead of water, 'steam under high pressure' can be used to fall on the blades.
• For that, the blades will have to be of special shapes
• The required steam is produced by heating water using coal as fuel.
• Electricity is produced in 'Thermal power plants' using this method
• But burning coal can cause environmental pollution
Let us do two separate experiments:
Experiment 1:
1. Consider the circuit in fig.11.9(a) below:
 |
| Fig.11.9 |
• A cell which can be used in a torch light or clock is connected in series with a galvanometer and a rheostat
• The function of the rheostat is to provide a suitable resistance so that, excessive current will not flow through the galvanometer.
2. This experiment has only one trial.
Trial 1: Turn on the switch
3. Note down the observation:
• The needle of the galvanometer deflects to one side.
♦ The 'side to which deflection occurs' is not important for our present experiment
• What is important is this: Once the deflection occurs, the needle stays in that deflected position.
• As long as the switch is kept in 'on' position, the needle stays in that deflected position
4. So we can write two points:
(i) Since there is deflection, it can be confirmed that, current is flowing in the circuit
(ii) Since the needle stays in the deflected position, it can be confirmed that, the current flows in one direction only
Experiment 2:
1. Consider the arrangement in fig.11.9(b). A solenoid is connected to a galvanometer
2. In this experiment also, there is only one trial
Trial 1: The bar magnet is moved in and out continuously in the solenoid
3. Note down the observation:
• The needle continuously deflects to both sides alternately.
• As long as there is to and fro motion of the magnet, the needle also continues it's 'to and fro' deflection
4. So we can write two points:
(i) Since there is deflection, it can be confirmed that, current is flowing in the solenoid
(ii) Since there is continuous 'to and fro' deflection, it can be confirmed that, the current changes the direction continuously
■ A current that flows only in one direction continuously is called a direct current (DC)
■ A current that changes it's direction at regular intervals of time is called an alternating current (AC)
• It may be noted that, AC is produced due to some of our 'limitations':
• If we can move the bar magnet continuously in one direction, we will get DC
• But for such a movement, the length of the solenoid will have to be very large.
• Such a long solenoid cannot be made.
• So only possibility is 'to and fro' motion
AC Generator
AC generator is a device used for the continuous production of alternating current. It's working can be explained using Fleming's right hand rule. Let us see the details:1. Consider the arrangement in fig.11.10 below:
 |
| Fig.11.10 |
• In between the poles, we have a rectangular coil ABCD of insulated copper wire.
■ This coil is called armature
2. We know that the direction of the magnetic field will be from the N pole to the S pole.
• So the segment AB of the coil is perpendicular to the magnetic field
• The segment CD is also perpendicular to the magnetic field
3. The ends of the coil are connected to the two rings R1 and R2
■ Let us see the features of these rings:
(i) R1 is shown in blue color
• R2 is shown in green color.
• This difference in color is given for our learning purpose only. In reality, they need not be of different colors.
(ii) Both the rings are attached firmly to an axle.
• So if the axle rotates, the rings will also rotate.
• An arrow is shown at the rear end of the axle.
♦ This is for our learning purpose only. It shows the rotation of the axle.
(iii) The rings are not in direct contact with the axle.
• There is an insulating layer in between each ring and the axle.
• This is to prevent the flow of current from the ring to the axle.
(iv) The rings are in contact with two brushes B1 and B2. These brushes are stationary.
(v) The coil ABCD is attached to the rings. This attachment is clear from the following two points seen in the fig.:
• The left end of the coil (coming from 'A') is penetrating through the blue ring R1
• The right end of the coil (coming from 'D') is penetrating through the green ring R2
♦ Though this end is penetrating through R1 also, the area of contact is insulated
реж We can see that, at R1, the coil is passing through a PVC pipe
♦ Because of this insulation, the following two currents will not enter the ring R1:
(i) Current flowing out of the coil through 'D'
(ii) Current flowing into the coil through 'D'
■ The two rings are called slip rings
■ Recall that, the rings used in an electric motor are split rings. See fig.9.28.
• The difference between the two is:
♦ Split rings are used where we want a change in the direction of current
♦ Slip rings do not cause any change in current
4. A galvanometer is connected in the circuit to detect the flow of any current and also to detect the direction of the flow
5. A handle is attached so that, the axle can be turned easily
6. Now let us turn the handle. We will turn it in the clock-wise direction. This is shown in fig.11.11 below:
 |
| Fig.11.11 |
As a result:
• The axle, which is attached to the handle, will also turn in the clockwise direction
As a result:
• The blue and green rings, which are attached to the axle, will turn in the clock wise direction
As a result:
• The coil ABCD, which is attached to the rings will turn in the clockwise direction
7. When the coil turn, two things happen simultaneously:
(i) Segment AB goes up
(ii) Segment CD goes down
8. Now, since AB is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from A to B
• In a similar way, since CD is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from C to D
■ So the two currents will add up. We get a current flow in the direction: A → B → C → D
9. After reaching 'D', the current flows out of the armature and into the rings.
• But it does not branch at the blue ring. Because there is PVC insulation
• So all the current flows into the green ring.
• From the green ring, the current flows into the brush B2
• From brush B2, it flows into the galvanometer. So the needle deflects to the left
• After coming out of the galvanometer, the current flows into the brush B1
• From B1, it enters the blue ring
• From the blue ring, it flows back into the armature through the point 'A'
Thus the circuit is complete.
10. The current that we saw just now was formed due to the upward motion of the segment AB
(and also due to the downward motion of the segment CD)
• But AB cannot continue to move upwards indefinitely
• Neither can CD continue to move downwards indefinitely
11. So what happens to them?
• After 'half a rotation',
♦ AB occupies the position previously occupied by CD
♦ CD occupies the position previously occupied by AB
This is shown in fig.11.12 below:
 |
| Fig.11.12 |
• During that half rotation, we obtained current in the direction A → B → C → D
• We reached the stage shown in fig.11.12
• Let us see what happens next:
12. Due to the continued rotation of the handle,
(i) Segment CD goes up
(ii) Segment AB goes down
13. Now, since CD is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from D to C
• In a similar way, since AB is given a motion, a current will begin to flow in it
♦ Applying Fleming's right hand rule, the direction of this current will be from B to A
■ So the two currents will add up. We get a current flow in the direction: D → C → B → A
14. After reaching 'A', it flows out of the armature and into the blue ring
• The current entering the blue ring has only one option: Flow into the brush B1
♦ It cannot flow into the armature because of the PVC insulation
• Current flowing out of B1 will flow into the galvanometer. So the needle deflects to the right
• Then it enter brush B2
• From brush B2, it enter the green ring
• The current in green ring has only one option: Flow into the armature
• This current which enters the armature from the green ring is flowing back into the armature.
♦ It can not branch into B1 or the blue ring because of the PVC insulation
• So all the current flows back into the armature through D
Thus the circuit is complete
15. We obtained this current D → C → B → A due to the downward motion of the segment AB
(and also due to the upward motion of the segment CD)
• But AB cannot continue to move downwards indefinitely
• Neither can CD continue to move upwards indefinitely
• Thus we reach back to the stage which we saw previously in fig.11.11. This completes the second 'half rotation'
16. The first 'half rotation' and the second 'half rotation' together forms one 'full rotation'
• Thus segment AB reaches back to where it initially was
16. As we again continue to turn the handle, we get a series of 'full rotations' of the armature
• Each full rotation consists of two 'half rotations'
♦ In the first 'half rotation', we get current in the direction A → B → C → D
♦ In the second 'half rotation', we get current in the direction D → C → B → A
17. Thus we get a continuous supply of current
• A current which changes it's direction at equal intervals of time
■ It is an alternating current (AC)
■ If we use split rings (as we saw in the case of electric motor), we can obtain 'direct current'. The details can be seen here.
• In the above figs., a handle was used for rotating the armature. We can use a turbine instead. This is shown in fig.11.13 below:
 |
| Fig.11.13 |
• Since the turbine is attached to the axle of the generator, it will also rotate and we will get electricity.
• The water should be allowed to fall from a height. In 'Hydroelectric power stations', the required water is stored in huge dams
■ Instead of water, 'steam under high pressure' can be used to fall on the blades.
• For that, the blades will have to be of special shapes
• The required steam is produced by heating water using coal as fuel.
• Electricity is produced in 'Thermal power plants' using this method
• But burning coal can cause environmental pollution
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