ELECTROMAGNETISM 2

There are still so many concepts and applications of electromagnetism to explore… is how the last blog ended, thus am here with another electromagnetism blog! Let’s dive right into a really cool concept - electromagnetic induction. 

In the previous blog, we discussed the magnetic effect of electric current - current (moving charge) produces a magnetic field around it. In this blog we’re going to discuss the opposite - producing electric current with a magnetic field. Michael Faraday thought of this concept, performed many experiments, and observed that when there is a change in number of magnetic field lines linked with a conductor, (also called magnetic flux) an electromotive force or emf (potential difference) is developed (which causes current to flow) between the ends of the conductor which lasts as long as there is a change in number of magnetic field lines. This phenomenon is called electromagnetic induction. 

An experiment to demonstrate and understand this phenomenon can be performed using a solenoid (formed by winding a copper wire in a spiral on a paper cylinder), a centre zero galvanometer (instrument for measuring electrical current in which the needle will point to the center of the scale when no current is flowing), and a magnet. 

After connecting the two ends of the solenoid to the galvanometer and placing the magnet nearby along the axis of the solenoid, the experiment is set up. At this point, the pointer of the galvanometer reads 0 that means no current flows in the wire.

When the magnet with its north pole facing the solenoid is moved towards it, the galvanometer deflects towards the right indicating a flow of current in that direction. 

When the motion of the magnet is stopped, the pointer of the galvanometer reads 0. 

When the magnet is moved away from the solenoid, the galvanometer deflects towards the left indicating a flow of current in the opposite direction. 

When the magnet with its south pole facing the solenoid is moved towards it, the galvanometer deflects towards the left indicating a flow of current in the opposite direction. 

When the motion is performed by the solenoid instead of the magnet, exactly similar observations are obtained. 

When the magnet is moved back and forth rapidly, its strength is increased, the area of cross section of the coil or the number of turns in the coil is increased, the deflection in the galvanometer is more in the particular direction. From this we can conclude that these factors affect the magnitude of current flowing in the coil. 

The observation that only on moving the magnet or solenoid, the galvanometer shows deflection, and current flows leads to the conclusion that a current flows in the coil only when there is a relative motion between the coil and magnet (either moves). Another conclusion from observations of different motions of the magnet is that the direction of current is affected by the direction of motion of the magnet or solenoid, and the polarity of the magnet.  

Michael Faraday explained the reason for these observations with the concept of electromagnetic induction. What is it that is changing with the motion of the magnet and causing an emf to be induced? The magnetic flux linked with (or the number of magnetic field lines passing through) the coil!

If the north pole of the magnet is moved towards the coil, the magnetic flux linked with the coil increases which causes an emf to be induced. The increase in magnetic flux is because the direction of magnetic field lines is from north pole to south pole. It is as if magnetic field lines are coming out from the magnet through the north pole and on moving it towards the solenoid more magnetic field lines are able to pass through. 

Similarly if the north pole of the magnet is moved away from the coil, the magnetic flux linked with the solenoid decreases. 

ELectromagnetic induction does not violate the law of conservation of energy as electrical energy is not being created. The mechanical energy spent to move the magnet and thus change the magnetic flux is converted to electrical energy in the form of current. 

These observations and explanation were summarised by Faraday in the two laws of electromagnetic induction:

1. Whenever there is a change in magnetic flux linked with a coil, an e.m.f. is induced. The e.m.f. induced lasts so long there is a change in magnetic flux linked with the coil.

2. The magnitude of e.m.f. induced is directly proportional to the rate of change of

magnetic flux linked with the coil (change in magnetic flux with time). If magnetic flux changes at a constant rate, a steady e.m.f. is produced.

Direction of induced emf

This section talks about two rules that answer multiple Why questions regarding the experiment - Why did the galvanometer deflect towards the right and not left when the north pole of the magnet was moved towards the solenoid ? Why did it deflect in the opposite direction (left) when the magnet was moved away (direction of motion changed)? When the south pole (polarity changed) of the magnet was moved towards the solenoid, why did the galvanometer deflect towards the left? The direction of induced emf (or induced current) can be determined with Fleming’s Right Hand Rule and Lenz’s Law. 

Fleming’s Right Hand Rule requires us to stretch the thumb, central finger and forefinger of our right hand mutually perpendicular to each other. If the forefinger indicates the direction of magnetic field and the thumb indicates the direction of motion of the conductor; then the central finger will indicate the direction of induced current.

According to Lenz's law, the direction of induced e.m.f. (or induced current) is such that it opposes the cause which produces it. The cause of emf is the motion of the magnet. If the magnet is moving towards the solenoid, the solenoid will try to repel the magnet. For this the end of the solenoid facing the magnet must be of the same polarity (like poles repel). 

For example, in the second observation of the experiment the north pole of the magnet was moved towards the solenoid. The solenoid tried to repel the magnet by making the end facing the magnet of north polarity. Thus, induced current at this end flows in an anticlockwise direction (clock rule). The current at the other end flows in a clockwise direction as it acts as the south pole. For these, the current must be flowing in the solenoid from right to left and in the wire from left to right, causing the galvanometer to deflect towards the right. We have answered the first why question!

The second why question can be answered in a similar way! In the third observation the north pole of the magnet was moved away from the solenoid. The solenoid tried to attract the magnet by making the end facing the magnet of south polarity. Thus, induced current at this end flows in a clockwise direction. The current at the other end ,acting as the north pole, flows in an anticlockwise direction. For these, the current must be flowing in the solenoid from left to right and in the wire from right to left, causing the galvanometer to deflect towards the left. 

The third why question is now a piece of cake! In the fourth observation the south pole of the magnet was moved towards the solenoid. The solenoid tried to repel the magnet by making the end facing the magnet of south polarity. Thus, induced current at this end flows in a clockwise direction. The current at the other end ,acting as the north pole, flows in an anticlockwise direction. For these, the current must be flowing in the solenoid from left to right and in the wire from right to left, causing the galvanometer to deflect towards the left. 

Lenz’s law, just like electromagnetic induction, is based on the law of conservation of energy. The mechanical energy spent to move the magnet against the opposing force is converted to electrical energy in the form of current. 

Transformers 

These devices are commonly used to change the voltage (220 V of mains) to provide suitable voltage to different appliances. They increase or decrease the magnitude of alternating emf. Why alternating emf? As its magnitude can change. Voltage/emf in case of direct current is of constant magnitude. 

Transformers work on the principle of electromagnetic induction, and the magnetic effect of electric current can also be seen. Transformers make use of two coils - primary and secondary coils - having different numbers of turns. The alternating e.m.f., of which magnitude is to be changed, is applied across the primary coil and the appliance in which output is to be obtained is connected across the secondary coil. When there is a change in the magnetic field lines due to varying current (alternating current) in the primary coil, the number of magnetic field lines linked with the secondary coil changes and so an induced varying current of different magnitude flows in the secondary coil (as number of turns are different - a factor which changes magnitude of current). As magnitude of current varies, emf also varies. 

The primary and secondary coils of the transformer are winded on the two arms of a simple rectangular core, enabling the change in magnetic field lines of one to affect the number linked with the other. 

The ratio of the number of turns in the secondary coil (Nₛ) to the number of turns in the primary coil (Nₚ) is called the turns ratio (Nₛ/Nₚ). It is equal to the ratio of emf across the secondary coil to the emf across the primary coil (Nₛ/Nₚ = Eₛ/Eₚ). This can be understood as follows - Emf across the secondary coil varies as the emf across the primary coil varies (as magnitude of emf of input varies, the magnitude of emf of output varies similarly) so their ratio will be constant. The value of turns ratio and emf across the primary coil determine the emf in secondary coil- (Nₛ/Nₚ) x Eₚ = Eₛ. The emf across the primary coil determines the initial value and the turns ratio determines the increase or decrease. For an ideal transformer, the energy loss is zero, so output power is equal to input power - EₛIₛ = EₚIₚ → Iₚ/Iₛ = Eₛ/Eₚ = Nₛ/Nₚ. 

There are two types of transformers: 

Step up transformer: The transformer used to change a low alternating voltage to a high alternating voltage (Eₛ>Eₚ). Following the fact that Eₛ/Eₚ = Nₛ/Nₚ , the number of turns in the secondary coil is more than that in the primary coil (Nₛ>Nₚ). The turns ratio (Nₛ/Nₚ) > 1. Following the fact that Iₚ/Iₛ = Eₛ/Eₚ , the current in the primary coil is more than that in the secondary coil (Iₚ > Iₛ).  Thus, to reduce resistance and loss of energy as heat, the wire in the primary coil is made thicker (increasing area of cross section).

Step down transformer: The transformer used to change a high alternating voltage to a low alternating voltage (Eₚ>Eₛ). The number of turns in the primary coil is more than that in the secondary coil (Nₚ>Nₛ). The turns ratio < 1. The current in the secondary coil is more than that in the primary coil (Iₛ > Iₚ).  Thus, to reduce resistance, the wire in the secondary coil is made thicker.  

Chargers for laptops and mobile phones use step down transformers. Taking an example of running a 30 W, 20V (Eₛ) charger with mains of 240V (Eₚ). 

Eₛ/Eₚ = Nₛ/Nₚ → 20/240 = Nₛ/Nₚ → 1/12 = Nₛ/Nₚ → (1/12) x Nₚ = Nₛ - number of turns in secondary coil is one-twelfth of number of turns in primary coil. Assuming this to be an ideal transformer, input power = output power = 30W. 

Iₚ = Pₚ/Eₚ → 30/240 = 0.125 A. Iₛ = Pₛ/Eₛ → 30/20 = 1.5 A. Since Iₛ>Iₚ, wire of secondary coil is made thicker than that of primary coil. 

To conclude, we explored the amazing concept of electromagnetic induction, and understood it with an experiment and Faraday's laws. Not to forget Fleming’s rule and Lenz’s law that taught us about the direction of induced current. Finally, we learnt about devices that apply the principle of electromagnetic induction -  transformers. That’s it for this blog!