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What is Electromagnetic Induction and Faraday’s Law
We have already studied that an electric current can produce magnetism. The reverse of this is also true. That is, magnetism (or magnets) can produce electric current. The production of electricity from magnetism is called electromagnetic induction.
For example, when a straight wire is moved up and down rapidly between the two poles of a horseshoe magnet, then an electric current is produced in the wire. This is an example of electromagnetic induction. Again, if a bar magnet is moved in and out of a coil of wire, even then an electric current is produced in the coil. This is also an example of electromagnetic induction.
The current produced by moving a straight wire in a magnetic field (or by moving a magnet in a coil) is called induced current. The phenomenon of electromagnetic induction was discovered by a British scientist Michael Faraday and an American scientist Joseph Henry independently in 1831.
The process of electromagnetic induction has led to the construction of generators for producing electricity at power stations. Before we describe experiments to demonstrate the phenomenon of electromagnetic induction, we should know something about the galvanometer which we will be using now.
A galvanometer is an instrument which can detect the presence of electric current in a circuit. It is connected in series with the circuit. When no current is flowing through a galvanometer, its pointer is at the zero mark (in the centre of semi¬circular scale). When an electric current passes through the galvanometer, then its pointer deflects (or moves) either to the left side of zero mark or to the right side of the zero mark, depending on the direction of current. We will describe the experiments now.
1. To Demonstrate Electromagnetic Induction by Using a Straight Wire and a Horseshoe-Type Magnet
In Figure (a), we have a straight wire AB held between the poles N and S of a horseshoe magnet (which isVU-shaped magnet). The two ends of wire are connected to a current-detecting instrument called galvanometer. When the wire AB is held standstill between the poles of the horseshoe magnet, then there is no deflection in the galvanometer pointer. This shows that no current is produced in the wire when it is held stationary in the magnetic field.
1. Let us move the wire AB upwards rapidly between the poles of the horseshoe magnet [see Figure (a)]. When the wire is moved up, there is a deflection in the galvanometer pointer showing that a current is produced in the wire AB momentarily which causes the deflection in galvanometer [see Figure(a)]. The deflection lasts only while the wire is in motion. Thus, as the ivire is moved up through the magnetic field, an electric current is produced in it.
2. We now move the wire AB downwards rapidly between the poles of the horseshoe magnet [see Figure(b)].When the wire is moved down, the galvanometer pointer again shows a deflection, but in the opposite direction (to the left side) [see Figure(b)]. This means that when the wire is moved down in the magnetic field, even then an electric current is produced in it. But when the direction of movement of wire is reversed (from Up to down), then the direction of current produced in the wire is also reversed.
If we move the wire AB up and down continuously between the poles of the horse-shoe magnet, then a continuous electric current will be produced in the wire. But the direction of this electric current will change rapidly as the direction of movement of the wire changes. This is because when the wire moves up, then the current in it will flow in one direction but when the wire moves down, then the current in it will flow in opposite direction. We will see the pointer of galvanometer move to and fro rapidly as the current in the wire changes direction of flow continuously. The electric current produced in the wire (which changes direction continuously) is called alternating current or a.c.
The above experiment shows that when the direction of motion of wire in the magnetic field is reversed, then the direction of induced current is also reversed. Please note that the direction of induced current in the wire can also be reversed by reversing the positions of the poles of the magnet which means that the direction of induced current can also be reversed by reversing the direction of magnetic field. We will now discuss why the movement of a wire in the magnetic field produces an electric current in the wire.
When a wire is moved in a magnetic field between the poles of a magnet, then the free electrons present in the wire experience a force. This force makes the electrons move along the wire. And the movement of these electrons produces current in the wire. We are spending energy (from our body) in moving the wire up and down in the magnetic field. So, it is the energy spent by us in moving the wire in the magnetic field which is getting converted into electrical energy in the wire and producing an electric current in the-wire.
Thus, the movement of a wire in a magnetic field can produce electric current. So, we can generate electricity by moving a wire continuously in the magnetic field between the poles of a magnet. This principle is used in producing electricity through generators. A generator uses the movement (or rotation) of a rectangular coil of wire between the poles of a horseshoe magnet to produce an electric current (or electricity). Thus, the phenomenon of electromagnetic induction is used in the production of electricity by a generator.
In the above experiment we have seen that when a wire is moved between the poles of a fixed magnet, then an electric current is produced in the wire. The reverse of this is also true. That is, if a wire (in the form of a coil) is kept fixed but a magnet is moved inside it, even then a current is produced in the coil of wire. This point will become more clear from the following experiment.
2. To Demonstrate Electromagnetic Induction by Using a Coil and a Bar Magnet
In Figure (a), we have a fixed coil of wire AB. The two ends of the coil are connected to a current-detecting instrument called galvanometer. Now, when a bar magnet is held standstill inside the hollow coil of wire, then there is no deflection in the galvanometer pointer showing that no electric current is produced in the coil of wire when the magnet is held stationary in it.
When a bar magnet is moved quickly into a fixed coil of wire AB, then a current is produced in the coil. This current causes a deflection in the galvanometer pointer [see Figure (a)], Similarly, when the magnet is moved out quickly from inside the coil, even then a current is produced in the coil [see Figure (b)]. This current also causes a deflection in the galvanometer pointer but in the opposite direction (showing that when the direction of movement of magnet changes, then the direction of current produced in the coil also changes). So, the current produced in this case is also alternating current or a.c.
The production of electric current by moving a magnet inside a fixed coil of wire is also a case of electromagnetic induction. The concept of a fixed coil and a rotating magnet is used to produce electricity on large scale in big generators of power houses. Please note that the condition necessary for the production of electric current by electromagnetic induction is that there must be a relative motion between the coil of wire and a magnet. Out of the coil of wire and a magnet, one can remain fixed but the other has to rotate (or move).
After performing a large number of experiments, Faraday and Henry made the following observations about electromagnetic induction :
- A current is induced in a coil when it is moved (or rotated) relative to a fixed magnet.
- A current is also induced in a fixed coil when a magnet is moved (or rotated) relative to the fixed coil.
- No current is induced in a coil when the coil and magnet both are stationary relative to one another.
- When the direction of motion of coil (or magnet) is reversed, the direction of current induced in the coil also gets reversed.
- The magnitude of current induced in the coil can be increased : studygear
(a) by winding the coil on a soft iron core,
(b) by increasing the number of turns in the coil,
(c) by increasing the strength of magnet, and
(d) by increasing the speed of rotation of coil (or magnet).
Fleming’s Right-Hand Rule for the Direction of Induced Current
The direction of induced current produced in a straight conductor (or wire) moving in a magnetic field is given by Fleming’s right-hand rule. According to Fleming’s right-hand rule : Hold the thumb, the forefinger and the centre finger of your right-hand at right angles to one another [as shown in Figure (b)]. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.
Suppose the direction of magnetic field is directed from east to west as shown by arrow AB in Figure (a), and the direction of motion of conductor is vertically downwards, as shown by the arrow AC in Figure (a). Then to find out the direction of induced current in the conductor, we hold the thumb, the forefinger and centre finger of our right-hand mutually at right angles to one another.
We adjust the right hand in such a way that the forefinger points from east to west (to represent the magnetic field), and the thumb points vertically downwards (to represent the direction of motion), then we will find that our centre finger points towards north [Figure (b)] and this gives the direction of induced current. Thus, the induced current in this case will be towards north as represented by arrow AD in Figure (a). Please note that Fleming’s right-hand rule is also called dynamo rule.
Direct Current and Alternating Current
Before we discuss the construction and working of an electric generator, it is necessary to know the meaning of direct current and alternating current. This is discussed below. If the current flows in one direction only, it is called a direct current. Direct current is written in short form as D.C. (or d.c.) The current which we get from a cell or a battery is direct current because it always flows in the same direction.
The positive (+) and negative (-) polarity of a direct current is fixed. Some of the sources of direct current (or d.c.) are dry cell, dry cell battery, car battery and d.c. generator. If the current reverses direction after equal intervals of time, it is called alternating current. Alternating current is written in short form as A.C. (or a.c.). Most of the power stations in India generate alternating current. The alternating current produced in India reverses its direction every \(\frac{1}{100}\) second.
Thus, the positive (+) and negative (-) polarity of an alternating current is not fixed. Some of the sources which produce alternating current (or a.c.) are power house generators, car alternators and bicycle dynamos. An important advantage of alternating current (over direct current) is that alternating current can be transmitted over long distances without much loss of electrical energy. Both a.c. and d.c. can be used for lighting and heating purposes. But radios and televisions, etc., need a d.c. supply. The radios and televisions have a special device inside them which changes the a.c. supplied to them into d.c.