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.
• 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:
(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
• 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 |
(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 |
• 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:
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:
• 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:
• 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:
• 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'
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