Introduction
Category : 10th Class
Introduction
There are various effects of electric current. One of such effect we have studiedin the previous chapter of electricity. One of the other effect of electric currentis the magnetic effect of electric current. We will study about the magneticeffect produced by the electric current flowing through the wire. Let us take astraight copper wire and place it between the point A and B in an electriccircuit, as shown in the figure given below.
The wire AB is placed perpendicular to the plane of the paper. Place a magnetic compass near the wire and see the position of the needles. Pass the current through the circuit and observe the change in position of the needle. This change in position of the needle shows that, as the current passes through the wire magnetic effect is produced. This magnetic effect produced by electric current is called electromagnetism. The concept of electromagnetism was introduced by Hans Christian Oersted in 1820.
A magnet is an object which attracts pieces of iron like material towards itself. It is found in various shapes and size. The most commonly used magnetic is the bar magnetic and horseshoe magnetic.
A bar magnetic is a rectangular bar of uniform cross section which attracts piece of iron, steel, nickel and cobalt etc. It has two pole near its end, north pole and south pole. If we suspend a bar magnetic freely with the help of a thread it always comes to rest in geographical north- south direction. The other properties of bar magnet is that like pole repel each other and unlike pole attract each other. It is used in many electronic devices like radio, TV, MRI, etc.
Magnetic Field
The magnet create a field around itself where magnetic effect can be felt. It is known as magnetic field. We can also define it as the space around the magnet where magnetic effect can be felt. The magnetic field has both magnitude and direction. The direction of magnetic field at a point is the direction of the resultant force acting on a hypothetical north pole, placed at that point. The north end of the needle of a compass indicates the direction of magnetic field at a point where it is placed.
Magnetic Field Lines
Magnetic field lines are the lines which is drawn around the magnetic fieldstarting from North Pole and ending at South Pole. It also gives the direction ofthe magnetic field. Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is one in which a north pole of the compass needle moves inside. Therefore, it is taken by convention that the field lines emerges from North Pole and merge at the South Pole. The direction of the field lines is from its south pole to its north pole. The magnetic field lines are closed curves. The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger where the field lines are closer and weakest where the field lines are spacers.
The magnetic line of forces has the following properties:
The earth itself behave like a huge bar magnetic. The south pole of the earth's magnet is in geographical north because it attracts North Pole of the suspended magnet and North Pole of the earth’s magnet is in geographical south as it attracts South Pole of the magnet. Thus magnetic South Pole is in geographical North Pole and magnetic north pole is in geographical South Pole of the earth. The axis of earth's magnetic field is inclined at an angle of about 15° with the geographical axis. The earth's magnetic field is due to the magnetic effect of current, produced at the core of the earth, due to high temperature, pressure and the magnetic material present in the core of the earth.
The two ends of the magnet are called:
(a) Poles
(b) Axis
(c) Field Lines
(d) Field
(e) None of these
Answer: (a)
The device shown in the figure is called:
(a) Poles
(b) Axis
(c) Field Lines
(d) Field
(e) None of these
Answer: (a)
The angle between the axis of earths magnetic field and the geographical axis is:
(a) 30°
(b) 15°
(c) 45°
(d) 60°
(e) None of these
Answer: (b)
Which one of the following is the not the properties of permanent magnet?
(a) Like pole repels each other
(b) Unlike pole attract each other
(c) Magnetic line of force do not intersect each other
(d) Magnetic poles can be separated
(e) none of these
Answer: (d)
Which one of the following is not a magnetic material?
(a) Nickel
(b) Cobalta
(c) Uranium
(d) Iron
(e) None of these
Answer: (c)
Magnetic Field Due to a Current Carrying Conductors
We known that the current produces the magnetic effects and this can be detected with the help of a magnetic compass. Any current carrying conductor produces magnetic effects around it. The importance of the magnetic effect of the current lies in the fact, that, it gives rise to the mechanical forces. For example, the working of the electric motors, generators, telephone, radio, washing machine, etc are the application of mechanical force of the current. This magnetic effect of the current is called the electromagnetism. Take a straight copper wire and connect it in series with a cell of 1.5 volt. Place this wire parallel to and over the magnetic compass needle and switch on the circuit. You will observe the deflection in the compass needle. If the current flows from north to south, then the north pole of the needle would move towards the east. Now if we reverse the direction of current in the wire i.e. from south to north, then again we observe the change in the direction of the deflection of the needle. This time it will deflect towards the west. It means that if we change the direction of the current, the direction of the magnetic field also changes.
Magnetic Field Due to a Straight Current Carrying Conductor
Now we will discuss about the pattern of magnetic field produced by a straight current carrying conductors. Take a straight copper wire and place it in the north south direction as shown in the figure.
When we pass the current in the upward direction, the magnetic field pattern that will be formed is the concentric circle with centre on the wire. The magnetic field lines will be in anticlockwise direction. If we reverse the direction of the current, the direction of the magnetic field also changes,
The magnitude of the magnetic field produced by a straight wire at a given point is:
So magnetic field will be stronger if more current is flown through the wire and strength will decrease as we move away from the wire. In other words intensity of magnetic field will be stronger near the wire and weakest at the distant point-from the wire. The direction of the magnetic field can be obtained by using Maxwell right hand thumb rule. According to this rule: if we hold a straight wire carrying current in our right hand in such away that thumbs points in the direction of current, the direction of magnetic field will be in the direction of fingers holding the wire which encircles the wire.
Magnetic Field Due to a Circular Loop
Let us take a straight wire bent in the form of circular loop as shown in the figure below. When the current is passed through the loop the magnetic field is produced around it.
The pattern of the magnetic field is, it is circular near the current carrying loop. As we move away from the wire, the concentric circles representing magnetic field lines become bigger and bigger. It gets straighten as we move towards the centre of the loop. At the centre of the loop it becomes the straight line. The magnitude of the magnetic field produced is directly proportional to the strength of the current flowing through the loop and inversely proportional to the radius of the circular loop. The strength of the magnetic field can also be increased by increasing the number of turns of the insulated copper wire.
Magnetic Field Pattern Due to a Solenoid
A solenoid is a circular coils of many turns of insulated copper wire, wrapped closely in the shape of a cylinder.
The magnetic field pattern produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet. Inside the solenoid, the magnetic field pattern is in the form of the parallel straight lines, which indicates that the magnetic field strength is same at all points, inside the solenoid.
The two end of the solenoid acts like the pole of the bar magnet, i.e. North Pole and South Pole. The strength of magnetic field depends on:
Normally soft iron rod is used as the core material of the solenoid as it produce strongest magnetism.
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