Magnetic Effects of Electric Current(part-2)

Blog number:-018
Hello everybody,
Well, I hope you all will be fine.

So, In our last session we discussed about the Introduction on  Magnetic Effects of Electric Current. In that we came to know about Electromagnet, Magnetic effect of Electric current, Magnetic field, Magnetic field due to current carrying conductor, and Right-Hand Thumb Rule.

Now, In this session we are going to discuss about "Force on a current carrying conductor in a magnetic field" and "Electromagnetic Induction".

Force on a current carrying conductor in a magnetic field

                               We have learnt that an electric current flowing through a conductor produces a magnetic field. The field so produced exerts a force on a magnet placed in the vicinity of the conductor. And according to Newton's Third Law of Motion which tells us that "For every action, there is an equal and opposite reaction". So, The magnet must also exert an equal and opposite force on the current-carrying conductor.

The force due to a magnetic field acting on a current-carrying conductor can be demonstrated through the following Experiment:-

                                   

        Take a small aluminium rod AB (of about 5 cm). Using two connecting wires suspend it horizontally from a stand, as shown in Fig. Place a strong horse-shoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. For this put the north pole of the magnet vertically below and south pole vertically above the aluminium rod. Connect the aluminium rod in series with a battery. Now pass a current through the aluminium rod from end B to end A. What do you observe? It is observed that the rod is displaced towards the left. You will notice that the rod gets displaced. Reverse the direction of current flowing through the rod and observe the direction of its displacement. It is now towards the right.Why does the rod get displaced?

        The displacement of the rod in the above Experiment suggests that a force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field. It also suggests that the direction of force is also reversed when the direction of current through the conductor is reversed. 

       Now change the direction of field to vertically downwards by interchanging the two poles of the magnet. It is once again observed that the direction of force acting on the current-carrying rod gets reversed. It shows that the direction of the force on the conductor depends upon the direction of current and the direction of the magnetic field. 

        In the Experiment, we considered the direction of the current and that of the magnetic field perpendicular to each other and found that the force is perpendicular to both of them. The three directions can be illustrated through a simple rule, called Fleming’s left-hand rule. 

Fleming’s left-hand rule

                               

According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Devices that use current-carrying conductors and magnetic fields include electric motor.

ELECTROMAGNETIC INDUCTION

          We have studied that when a current-carrying conductor is placed in a magnetic field such that the direction of current is perpendicular to the magnetic field, it experiences a force. This force causes the conductor to move. Now let us imagine a situation in which a conductor is moving inside a magnetic field or a magnetic field is changing around a fixed conductor. What will happen? To observe this effect, let us perform the following Experiment.

                                         

      Take a coil of wire AB having a large number of turns. Connect the ends of the coil to a galvanometer as shown in Fig. Take a strong bar magnet and move its north pole towards the end B of the coil. Do you find any change in the galvanometer needle? There is a momentary deflection in the needle of the galvanometer, say to the right. This indicates the presence of a current in the coil AB. The deflection becomes zero the moment the motion of the magnet stops. Now withdraw the north pole of the magnet away from the coil. Now the galvanometer is deflected toward the left, showing that the current is now set up in the direction opposite to the first. We see that the galvanometer needle deflects toward the right when the coil is moved towards the north pole of the magnet. Similarly the needle moves toward left when the coil is moved away. When the coil is kept stationary with respect to the magnet, the deflection of the galvanometer drops to zero. What do you conclude from this Experiment?

You can also check that if you had moved south pole of the magnet towards the end B of the coil, the deflections in the galvanometer would just be opposite to the previous case. When the coil and the magnet are both stationary, there is no deflection in the galvanometer. It is, thus, clear from this Experiment that motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced electric current in the circuit.

Let us now perform a variation of last Experiment in which the moving magnet is replaced by a current-carrying coil and the current in the coil can be varied.

                                      

Take two different coils of copper wire having large number of turns (say 50 and 100 turns respectively). Insert them over a non-conducting cylindrical roll, as shown in Fig. (You may use a thick paper roll for this purpose.) Connect the coil-1, having larger number of turns, in series with a battery and a plug key. Also connect the other coil-2 with a galvanometer. Plug in the key. Observe the galvanometer. Is there a deflection in its needle? You will observe that the needle of the galvanometer instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil-2

Disconnect coil-1 from the battery. You will observe that the needle momentarily moves, but to the opposite side. It means that now the current flows in the opposite direction in coil-2.

          From these observations, we conclude that a potential difference is induced in the coil-2 whenever the electric current through the coil–1 is changing (starting or stopping). Coil-1 is called the primary coil and coil-2 is called the secondary coil. As the current in the first coil changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the secondary coil also change. Hence the change in magnetic field lines associated with the secondary coil is the cause of induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction. In practice we can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it.

        we can use a simple rule to know the direction of the induced current Called Fleming’s right-hand rule. 

Fleming’s right-hand rule

                                     

Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other, as shown in Fig. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current. This simple rule is called Fleming’s right-hand rule.

Based on the phenomenon of electromagnetic induction, the devices which is designed is Electric Generator and Transformer.

So, That's all for this session. If you have any doubt related to the topic, please comment.

Thank you.

Introduction On Magnetic Effects of Electric Current

Blog number:-017
Hello everybody,
I hope you all will be fine.

         So, Uptill now, we have discussed about alot of things such as Electricity, Ohm's Law, Kirchhoff's Circuit Law, Series and Parallel Circuits. And uptill now We know that there is no relation between Electricity and Magnetic phenomenon.

Now we will see the relation between Magnetic field and Electric Current. So, before starting let's have a short discussion on Electromagnetic.

1. Magnet:-  Magnet is an object that attracts objects made of iron, cobalt and nickel. When the magnet is suspended freely,it comes to rest in North-South direction.

2. Electromagnet:-  An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. 

Electromagnet
                              

       If we take a needle wounded with copper wounded across it a number of times and a battery with a switch is connected across two end of copper wire . When the Switch of the circuit gets closed, the current flow through the circuit and a Magnetic effect is developed near the needle. And if we bring chips of iron metal. then What we observe...??

We will observe that the iron chip is being attracted by the needle. So, we can say that it behave as a magnet which is powered by electricity, called Electromagnet.

Magnetic Effects of Electric Current

                 We know that an electric current-carrying wire behaves like a magnet and we also know that When the magnet is suspended freely, it comes to rest in North-South direction and moves only in presence of any Magnet. Now combining these two points. let's see what happen. 

Compass needle is deflected on passing an electric current through a metallic conductor

                                          

            We see that the needle is deflected. What does it mean? It means that the electric current through the copper wire has produced a magnetic effect. Thus we can say that electricity and magnetism are linked to each other. Then, what about the reverse possibility of an electric effect of moving magnets? 

MAGNETIC FIELD AND FIELD LINES

            We are familiar with the fact that a compass needle gets deflected when brought near a bar magnet. A compass needle is, in fact, a small bar magnet. The ends of the compass needle point approximately towards north and south directions. The end pointing towards north is called north seeking or north pole. The other end that points towards south is called south seeking or south pole. Through various activities we have observed that like poles repel, while unlike poles of magnets attract each other.

Iron filings near the bar magnet align themselves along the field lines.

                                             

Fix a sheet of white paper on a drawing board using some adhesive material. Place a bar magnet in the centre of it. Sprinkle some iron filings uniformly around the bar magnet. A salt-sprinkler may be used for this purpose. Now tap the board gently.What do you observe?

                      The iron filings arrange themselves in a pattern. Why do the iron filings arrange in such a pattern? What does this pattern demonstrate? The magnet exerts its influence in the region surrounding it. Therefore the iron filings experience a force. The force thus exerted makes iron filings to arrange in a pattern. The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field. The lines along which the iron filings align themselves represent magnetic field lines. 

Field lines around a bar magnet

                                     

Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.

MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR

                       Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect all of them in series as shown in Fig.(a). Place the straight wire parallel to and over a compass needle. Plug the key in the circuit. Observe the direction of deflection of the north pole of the needle. If the current flows from north to south, as shown in Fig. (a), the north pole of the compass needle would move towards the east.Replace the cell connections in the circuit as shown in Fig.(b). This would result in the change of the direction of current through the copper wire, that is, from south to north.Observe the change in the direction of deflection of the needle. You will see that now the needle moves in opposite direction, that is, towards the west [Fig.(b)]. It means that the direction of magnetic field produced by the electric current is also reversed.

Magnetic field due to current through a straight conductor

                                          

              Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, and a long straight thick copper wire. Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down. Connect the copper wire vertically between the points X and Y, as shown in Fig.(a), in series with the battery, a plug and key. Sprinkle some iron filings uniformly on the cardboard. Keep the variable of the rheostat at a fixed position and note the current through the ammeter. Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight. Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire. What do these concentric circles represent ?. They represent the magnetic field lines. 

           How can the direction of the magnetic field be found?. Place a compass at a point (say P) over a circle. Observe the direction of the needle. The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire at point P. Show the direction by an arrow.

              What happens to the deflection of the compass needle placed at a given point if the current in the copper wire is changed? To see this, vary the current in the wire. We find that the deflection in the needle also changes. In fact, if the current is increased, the deflection also increases. It indicates that the magnitude of the magnetic field produced at a given point increases as the current through the wire increases.

          What happens to the deflection of the needle if the compass is moved from the copper wire but the current through the wire remains the same? To see this, now place the compass at a farther point from the conducting wire (say at point Q). What change do you observe? We see that the deflection in the needle decreases. Thus the magnetic field produced by a given current in the conductor decreases as the distance from it increases. It can be noticed that the concentric circles representing the magnetic field around a current-carrying straight wire become larger and larger as we move away from it.

Right-Hand Thumb Rule

                                                 

A convenient way of finding the direction of magnetic field associated with a current-carrying conductor is by using  Right-Hand Thumb Rule.

Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

So that's all for this session. In the next session we will discuss on Electromagnetic Induction and Force on Current Carrying Conductor.

If you have any doubt, Please comment.

Thank you.