In the previous section we saw how to apply Fleming's left hand rule. In this section, we will see the working of an electric motor based on that rule.
1. Consider the arrangement in fig.9.28 below:
• We have a north pole and a south pole.
• In between the poles, we have a rectangular coil ABCD of insulated copper wire.
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 halves of a split ring.
■ Let us see the features of this split ring:
(i) A split ring is initially, an ordinary metallic ring.
(ii) A small portion from top and another small portion from bottom is removed. So it is split into two halves.
(iii) In the fig., the left half is shown in green color.
• The right half is shown in blue color.
• This difference in color is given for our learning purpose only. In reality, they need not be of different colors.
(iv) These halves are attached firmly to an axle.
• So if the ring rotates, the axle 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.
(v) The ring is not in direct contact with the axle.
• There is an insulating layer in between the ring and the axle.
• This is to prevent the flow of current from the ring to the axle.
(vi) The rings are in contact with two brushes X and Y. These brushes are stationary.
(vii) The coil ABCD is attached to the two 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 green half
• The right end of the coil (coming from 'D') is penetrating through the blue half
4. Now let us turn on the switch. This is shown in fig.9.29 below:
• The side AB is connected to the green split ring.
• This green split ring is connected to the positive terminal of the battery.
• So current flows in the direction: A → B → C → D
5. Consider the flow of current from A to B in the segment AB.
• Apply Fleming's left hand rule to AB:
♦ The forefinger of the left hand should point in the direction from N to S poles
♦ The middle finger should point in the direction A → B
♦ Then the thumb will point downwards
• Thus we can write:
■ A force will push AB down
6. Consider the flow of current from C to D in the segment CD.
• This current is in the opposite direction of the current from A to B.
• Apply Fleming's left hand rule to CD:
♦ The forefinger of the left hand should point in the direction from N to S poles
♦ The middle finger should point in the direction C → D
♦ Then the thumb will point upwards
• Thus we can write:
■ A force will push CD up
7. The 'pushing down of AB' and 'pushing up of CD' occur simultaneously
• As a result, the coil ABCD as a whole, will flip over. Then:
♦ AB will occupy the position previously occupied by CD
♦ CD will occupy the position previously occupied by AB
• This is shown in fig.9.30 below:
• The arrow at the rear end of the axle is pointing downwards. This is because, when the flipping of the coil occurs, the axle will 'turn'.
Note that, this 'turning' is only 'half of a full rotation'
8. Now we see the role of the split rings:
(i) The coil ABCD is attached to the split rings.
♦ AB is attached to the green half
♦ CD is attached to the blue half
(ii) So when ABCD flips, the two halves will also flip
♦ The green half will now occupy the position previously occupied by the blue half
♦ The blue half will now occupy the position previously occupied by the green half
(iii) Now an interesting turn of events occur:
• Blue half was previously attached to the negative terminal of the battery.
♦ But now it is attached to the positive terminal.
• So the current now flows in the direction: D → C → B → A
(iv) The initial direction of the current was: A → B → C → D
• The direction of the current now is: D → C → B → A
• So we can say: The current got reversed
■ A device that reverses the direction of current through a circuit is called a commutator
■ In an electric motor, the split ring acts as a commutator
9. Consider the flow of current from D to C in the segment CD.
♦ Applying Fleming's left hand rule, we can see that, a force will push CD down
• Consider the flow of current from B to A in the segment AB.
♦ This current is in the opposite direction of the current from D to C.
♦ Applying Fleming's left hand rule, we can see that, a force will push AB up
10. The 'pushing up of AB' and 'pushing down of CD' occurs simultaneously
• As a result, the coil ABCD as a whole, will flip over. Then
♦ AB will occupy the position previously occupied by CD
♦ CD will occupy the position previously occupied by AB
11. From the above steps we can reach the following conclusions:
• Whenever AB is on the left side, it will be pushed downwards
• Whenever AB is on the right side, it will be pushed upwards
• Whenever CD is on the left side, it will be pushed downwards
• Whenever CD is on the right side, it will be pushed upwards
■ This means that:
• Which ever segment is on the left, will be pushed downwards
• Which ever segment is on the right, will be pushed upwards
12. So there is a continuous flipping of the coil ABCD in the anti-clockwise direction
■ So we get a continuous rotation of the axle in the anti-clockwise direction
• If we attach this axle to the rotating parts of appliances like electric fans, mixers, compressors of refrigerators, MP3 players etc., those parts will also rotate.
13. The force with which this rotation of the axle takes place can be increased by the following methods:
• Increase the number of turns in the coil
• Increase the intensity of the current
• Increase the strength of the magnetic field between the N and S poles
• But these methods will increase the overall size of the motor, and it will become heavier.
14. So the rotation of the axle can be used to make various appliances 'do mechanical works' for us.
We can write:
■ An electric motor converts electrical energy to mechanical energy
1. The current enters a coil through one of the lead wires shown in the fig.9.31 below:
• This coil is called voice coil.
• It is wound on a cylinder made of materials such as cardboard. This cylinder is shown in green color.
2. The current flows through the entire spiral length of the voice coil and flows out through the other lead wire.
• Whenever such a flow of current occurs, a magnetic field will develop around the coil.
• To increase the strength of that magnetic field, a soft iron core is also placed inside the coil.
3. The voice coil is placed between the poles of permanent magnets.
• So when current flows through the coil, a force will begin to act on it.
• How much force will act?
Ans: If the speaker at the other end, speaks loudly, greater current will flow, and so the voice coil will experience greater force
• If the speaker at the other end, speaks with a low voice, only lesser current will flow, and so the voice coil will experience only a lesser force
4. Because of the varying force, the voice coil will begin to vibrate.
• This vibration will be in accordance with the nature of the speech produced by the speaker.
5. A paper cone is attached to the voice coil. So the paper cone will also begin to vibrate
• This vibration will also be in accordance with the nature of the speech produced by the speaker.
6. The vibration of the paper cone will setup similar vibrations in the surrounding air.
• Thus the speech of the speaker will be reproduced.
7. The paper cone is placed securely within a casing.
• The edge space between the paper cone and casing is covered by a materials like elastic rubber, foam etc., This is shown in magenta color in the fig. Note that, this material should be flexible. Other wise, the vibration of the paper cone will not be free.
8. The voice coil must be protected from the dust by using a dust cap.
■ At this point, some simple questions will arise in the mind of the reader:
• Which is the current that enters the voice coil?
• Where did it come from?
• How did it get the same characteristics of the speech made by the speaker at the other end?
We will see the answers to these questions in later chapters.
1. Consider the arrangement in fig.9.28 below:
 |
| Fig.9.28 |
• In between the poles, we have a rectangular coil ABCD of insulated copper wire.
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 halves of a split ring.
■ Let us see the features of this split ring:
(i) A split ring is initially, an ordinary metallic ring.
(ii) A small portion from top and another small portion from bottom is removed. So it is split into two halves.
(iii) In the fig., the left half is shown in green color.
• The right half is shown in blue color.
• This difference in color is given for our learning purpose only. In reality, they need not be of different colors.
(iv) These halves are attached firmly to an axle.
• So if the ring rotates, the axle 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.
(v) The ring is not in direct contact with the axle.
• There is an insulating layer in between the ring and the axle.
• This is to prevent the flow of current from the ring to the axle.
(vi) The rings are in contact with two brushes X and Y. These brushes are stationary.
(vii) The coil ABCD is attached to the two 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 green half
• The right end of the coil (coming from 'D') is penetrating through the blue half
4. Now let us turn on the switch. This is shown in fig.9.29 below:
 |
| Fig.9.29 |
• This green split ring is connected to the positive terminal of the battery.
• So current flows in the direction: A → B → C → D
5. Consider the flow of current from A to B in the segment AB.
• Apply Fleming's left hand rule to AB:
♦ The forefinger of the left hand should point in the direction from N to S poles
♦ The middle finger should point in the direction A → B
♦ Then the thumb will point downwards
• Thus we can write:
■ A force will push AB down
6. Consider the flow of current from C to D in the segment CD.
• This current is in the opposite direction of the current from A to B.
• Apply Fleming's left hand rule to CD:
♦ The forefinger of the left hand should point in the direction from N to S poles
♦ The middle finger should point in the direction C → D
♦ Then the thumb will point upwards
• Thus we can write:
■ A force will push CD up
7. The 'pushing down of AB' and 'pushing up of CD' occur simultaneously
• As a result, the coil ABCD as a whole, will flip over. Then:
♦ AB will occupy the position previously occupied by CD
♦ CD will occupy the position previously occupied by AB
• This is shown in fig.9.30 below:
 |
| Fig.9.30 |
Note that, this 'turning' is only 'half of a full rotation'
8. Now we see the role of the split rings:
(i) The coil ABCD is attached to the split rings.
♦ AB is attached to the green half
♦ CD is attached to the blue half
(ii) So when ABCD flips, the two halves will also flip
♦ The green half will now occupy the position previously occupied by the blue half
♦ The blue half will now occupy the position previously occupied by the green half
(iii) Now an interesting turn of events occur:
• Blue half was previously attached to the negative terminal of the battery.
♦ But now it is attached to the positive terminal.
• So the current now flows in the direction: D → C → B → A
(iv) The initial direction of the current was: A → B → C → D
• The direction of the current now is: D → C → B → A
• So we can say: The current got reversed
■ A device that reverses the direction of current through a circuit is called a commutator
■ In an electric motor, the split ring acts as a commutator
9. Consider the flow of current from D to C in the segment CD.
♦ Applying Fleming's left hand rule, we can see that, a force will push CD down
• Consider the flow of current from B to A in the segment AB.
♦ This current is in the opposite direction of the current from D to C.
♦ Applying Fleming's left hand rule, we can see that, a force will push AB up
10. The 'pushing up of AB' and 'pushing down of CD' occurs simultaneously
• As a result, the coil ABCD as a whole, will flip over. Then
♦ AB will occupy the position previously occupied by CD
♦ CD will occupy the position previously occupied by AB
11. From the above steps we can reach the following conclusions:
• Whenever AB is on the left side, it will be pushed downwards
• Whenever AB is on the right side, it will be pushed upwards
• Whenever CD is on the left side, it will be pushed downwards
• Whenever CD is on the right side, it will be pushed upwards
■ This means that:
• Which ever segment is on the left, will be pushed downwards
• Which ever segment is on the right, will be pushed upwards
12. So there is a continuous flipping of the coil ABCD in the anti-clockwise direction
■ So we get a continuous rotation of the axle in the anti-clockwise direction
• If we attach this axle to the rotating parts of appliances like electric fans, mixers, compressors of refrigerators, MP3 players etc., those parts will also rotate.
13. The force with which this rotation of the axle takes place can be increased by the following methods:
• Increase the number of turns in the coil
• Increase the intensity of the current
• Increase the strength of the magnetic field between the N and S poles
• But these methods will increase the overall size of the motor, and it will become heavier.
14. So the rotation of the axle can be used to make various appliances 'do mechanical works' for us.
We can write:
■ An electric motor converts electrical energy to mechanical energy
Moving coil loudspeaker
Now let us see the working of a Moving coil loudspeaker. We will write it in steps:1. The current enters a coil through one of the lead wires shown in the fig.9.31 below:
 |
| Fig.9.31 |
• It is wound on a cylinder made of materials such as cardboard. This cylinder is shown in green color.
2. The current flows through the entire spiral length of the voice coil and flows out through the other lead wire.
• Whenever such a flow of current occurs, a magnetic field will develop around the coil.
• To increase the strength of that magnetic field, a soft iron core is also placed inside the coil.
3. The voice coil is placed between the poles of permanent magnets.
• So when current flows through the coil, a force will begin to act on it.
• How much force will act?
Ans: If the speaker at the other end, speaks loudly, greater current will flow, and so the voice coil will experience greater force
• If the speaker at the other end, speaks with a low voice, only lesser current will flow, and so the voice coil will experience only a lesser force
4. Because of the varying force, the voice coil will begin to vibrate.
• This vibration will be in accordance with the nature of the speech produced by the speaker.
5. A paper cone is attached to the voice coil. So the paper cone will also begin to vibrate
• This vibration will also be in accordance with the nature of the speech produced by the speaker.
6. The vibration of the paper cone will setup similar vibrations in the surrounding air.
• Thus the speech of the speaker will be reproduced.
7. The paper cone is placed securely within a casing.
• The edge space between the paper cone and casing is covered by a materials like elastic rubber, foam etc., This is shown in magenta color in the fig. Note that, this material should be flexible. Other wise, the vibration of the paper cone will not be free.
8. The voice coil must be protected from the dust by using a dust cap.
■ At this point, some simple questions will arise in the mind of the reader:
• Which is the current that enters the voice coil?
• Where did it come from?
• How did it get the same characteristics of the speech made by the speaker at the other end?
We will see the answers to these questions in later chapters.
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