High school Physics Lessons: induced current

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.

Fig.11.3

• 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:

Fig.11.4

• 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:

Fig.11.5

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

In the next section we will see the basics of an Electric generator.

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In the previous section we completed a discussion on the basics of 'effects of electric current'. In this section, we will see Electromagnetic induction.

• In a previous chapter 9, we saw that, when current flows through a conductor, a magnetic field will develop around that conductor. Naturally, a question will arise:
■ Is the reverse possible? That is., if a conductor is placed in a magnetic field, will current flow in that conductor?
To find the answer, let us do an activity. We will write it in steps:
1. Consider fig.11.1(a) below.
• A solenoid made of insulated copper wire is connected to a galvanometer.

Fig.11.1

• Galvanometer is an instrument used to detect and measure small electric currents.
(some images can be seen here)
♦ When there is no current flow, the needle is at the 'zero mark', which is at the center
♦ When there is a flow of current, the needle deflects towards the left or right of the zero mark
♦ This direction of deflection depends on the direction of flow of current
♦ If current is large, deflection will be more
♦ If current is low, deflection will be less
2. A bar magnet is kept ready at one end of the solenoid.
• We can see that, the needle of the galvanometer is at the center.
• So it is clear that, there is no current flowing in the conductor.
3. Now we can begin the trials.

Trial 1:
(i) Push the bar magnet into the solenoid. The North pole goes in. This is shown in fig.11.1(b)
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ It deflects towards the right
(ii) Now the magnet is inside the solenoid. Keep it exactly at that position with out any movement. This is shown in fig.c
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ There is no deflection. The needle is at the exact center
(iii) Pull the magnet out of the solenoid
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ It deflects towards the left
• So in a single trial (Trial 1), we did 3 steps and noted down the observation in each.
Trial 2:
• Reverse the orientation of the bar magnet. This is shown in fig.11.2(a) below:

Fig.11.2

(i) Push the bar magnet into the solenoid. Now the South pole goes in. This is shown in fig.11.2(b)
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ It deflects towards the left
(ii) Now the magnet is inside the solenoid. Keep it exactly at that position with out any movement. This is shown in fig.c
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ There is no deflection. The needle is at the exact center
(iii) Pull the magnet out of the solenoid
• Note down the 'direction of deflection' of the needle of the galvanometer
♦ It deflects towards the right
• So in a single trial (Trial 2), we did 3 steps and noted down the observation in each.

Before moving to trial 3, let us analyse the observations:
• Whenever there is a movement of the magnet, there is a deflection of the needle
That means:
■ Whenever there is a 'movement of the magnet', there is a 'flow of current' in the solenoid
• We can confirm that, current flows only when there is movement of magnet. Because, in both the trials, there is no deflection when the magnet is stationary. (Figs.11.1.c and 11.2.c)
• Another point worth noting:
■ The 'direction of deflection of needle' depends on the 'direction of motion of the magnet'
This is evident from the two trials:
• In trial 1, where the N pole goes in first:
♦ When magnet moves from left to right, needle deflects towards right
♦ When magnet moves from right to left, needle deflects towards left
• In trial 2, where the S pole goes in first:
♦ When magnet moves from left to right, needle deflects towards left
♦ When magnet moves from right to left, needle deflects towards right
■ Thus from the two trials, we get two points. We will write them as a list:

• We will have to expand the list as we do more trials.

Trial 3:
• This is an exact repetition of trial 1 (or trial 2). All the three observations have to be made.
• The only difference is that, the 'number of turns of the solenoid' is increased
What observations do we get?
• The three observations are similar to those obtained in trial 1 (or trial 2)
• The only difference is that, the needle 'deflects more'
• Greater deflection means: 'Greater current'
• So we will add point 3 to the list:

Trial 4:
• This is an exact repetition of trial 1 (or trial 2). All the three observations have to be made.
• The only difference is that, the 'number of turns of the solenoid' is decreased
What observations do we get?
• The three observations are similar to those obtained in trial 1 (or trial 2)
• The only difference is that, the needle 'deflects less'
• Lesser deflection means: 'Lesser current'
• So we will add point 4 to the list:

Trial 5:
• This is an exact repetition of trial 1 (or trial 2). All the three observations have to be made.
• The only difference is that, 'the pushing in' and 'pulling out' of the magnet is done with increased speed
What observations do we get?
• The three observations are similar to those obtained in trial 1 (or trial 2)
• The only difference is that, the needle 'deflects more'
• Greater deflection means: 'Greater current'
• So we will add point 5 to the list:

Trial 6:
• This is an exact repetition of trial 1 (or trial 2). All the three observations have to be made.
• The only difference is that, 'the pushing in' and 'pulling out' of the magnet is done with decreased speed
What observations do we get?
• The three observations are similar to those obtained in trial 1 (or trial 2)
• The only difference is that, the needle 'deflects less'
• Lesser deflection means: 'Lesser current'
• So we will add point 6 to the list:

■ In all the above trials, the solenoid was kept stationary at a fixed position. The magnet was given motion.
• We can obtain the same results if we do the reverse also. That is:
■ The magnet can be kept stationary and solenoid can be given motion.

Based on the discussions so far in this chapter, we can write:
■ Whenever there is a relative motion between magnet and solenoid, there is a flow of electricity
• Note that 'relative motion' is specified. It is important to specify those two words. Let us see the reason:
• Solenoid is kept stationary and magnet is moved → There is relative motion between solenoid and magnet → We will get a current flow
• Magnet is kept stationary and solenoid is moved → There is relative motion between solenoid and magnet → We will get a current flow
• Both solenoid and magnet are moved with same velocity → There is no relative motion between solenoid and magnet → We will not get a current flow
♦ 'Same velocity' implies that, 'both are moving with the same speed in the same direction'.
♦ This does not require a special mention because, velocity has both magnitude and direction
• Both solenoid and magnet are moved with different velocities → There is relative motion between solenoid and magnet → We will get a current flow

• Both solenoid and magnet are kept stationary → There is no relative motion between solenoid and magnet → We will not get a current flow

■ So it is clear: To get a current flow, there must be a 'relative motion'

In the next section we will see 'electromagnetic induction'.

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High school Physics Lessons

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Wednesday, May 23, 2018

Chapter 11.1 - Fleming's Right hand rule

Tuesday, May 22, 2018

Chapter 11 - Current produced in a Solenoid