Contents
- 1 Why does Electric Current Produce a Magnetic Field ?
- 1.1 Experiment to Demonstrate the Magnetic Effect of Current
- 1.2 Magnetic Field Patterns Produced by Current-Carrying Conductors Having Different Shapes
- 1.3 1. Magnetic Field Pattern due to Straight Current-Carrying Conductor (Straight Current-Carrying Wire)
- 1.4 Direction of Magnetic Field Produced by Straight Current-Carrying Conductor (Straight Current-Carrying Wire)
- 1.5 2. Magnetic Field Pattern due to a Circular Loop (or Circular Wire) Carrying Current
- 1.6 Clock Face Rule
- 1.7 3. Magnetic Field due to a Solenoid
- 1.8 Electromagnet
Physics Topics such as mechanics, thermodynamics, and electromagnetism are fundamental to many other scientific fields.
Why does Electric Current Produce a Magnetic Field ?
The magnetic effect of current was discovered by Oersted in 1820. Oersted found that a wire carrying a current was able to deflect a compass needle. Now, the compass needle is a tiny magnet which can be deflected only by a magnetic field. Since a current carrying wire was able to deflect a compass needle, it was concluded that a current flowing in a wire always gives rise to a magnetic field around it.
The importance of magnetic effect of current lies in the fact that it gives rise to mechanical forces. The electric motor, electric generator, telephone and radio, all utilize the magnetic effect of current. The magnetic effect of current is also called electromagnetism which means electricity produces magnetism.
Experiment to Demonstrate the Magnetic Effect of Current
We will now describe Oersted’s experiment to show that a current carrying wire produces a magnetic field around it. We take a thick insulated copper wire and fix it in such a way that the portion AB of the wire is in the north-south direction as shown in Figure (a). A plotting compass M is placed under the wire AB. The two ends of the wire are connected to a battery through a switch. When no current is flowing in the wire AB, the compass needle is parallel to the wire AB and points in the usual north-south direction [Figure (a)].
Let us pass the electric current through wire AB by pressing the switch. On passing the current we find that compass needle is deflected from its north-south position as shown in Figure (b). And when the current is switched off, the compass needle returns to its original position. We know that a freely pivoted compass needle always sets itself in the north-south direction.
It deflects from its usual north-south direction only when it is acted upon by another magnetic field. So, the deflection of compass needle by the current¬carrying wire in the above experiment shows that an electric current produces a magnetic field around it. It is this magnetic field which deflects the compass needle placed near the current-carrying wire.
If we reverse the direction of electric current flowing in the wire AB by reversing the battery connections, we will find that the compass needle is deflected in the opposite direction. This shows that when we reverse the direction of electric current flowing in the wire, then the direction of magnetic field produced by it is also reversed.
A concealed current-carrying conductor can be located due to the magnetic effect of current by using a plotting compass. For example, if a plotting compass is moved on a wall, its needle will show deflection at the place where current-carrying wire is concealed.
Magnetic Field Patterns Produced by Current-Carrying Conductors Having Different Shapes
The pattern of magnetic field (or shape of magnetic field lines) produced by a current-carrying conductor depends on its shape. Different magnetic field patterns are produced by current-carrying conductors having different shapes. We will now study the magnetic field patterns produced by :
- a straight conductor (or straight wire) carrying current,
- a circular loop (or circular wire) carrying current, and
- a solenoid (long coil of wire) carrying current.
We will discuss all these cases, one by one. Let us start with the straight current-carrying conductor.
1. Magnetic Field Pattern due to Straight Current-Carrying Conductor (Straight Current-Carrying Wire)
The magnetic field lines around a straight conductor (straight wire) carrying current are concentric circles whose centres lie on the wire (Figure). In Figure, we have a straight vertical wire (or conductor) AB which passes through a horizontal cardboard sheet C. The ends of the wire AB are connected to a battery through a switch. When current is passed through wire AB, it produces a magnetic field around it.
This magnetic field has magnetic field lines around the wire AB which can be shown by sprinkling iron filings on the cardboard C. The iron filings get magnetised. And on tapping the cardboard sheet, the iron filings arrange themselves in circles around the wire showing that the magnetic field lines are circular in nature.
A small plotting compass M placed on the cardboard indicates the direction of the magnetic field. When current in the wire flows in the upward direction (as shown in Figure), then the lines of magnetic field are in the anticlockwise direction. If the direction of current in the wire is reversed, the direction of magnetic field lines also gets reversed.
It has been shown by experiments that the magnitude of magnetic field produced by a straight current-carrying wire at a given point is : (i) directly proportional to the current passing in the wire, and (ii) inversely proportional to the distance of that point from the wire. So, greater the current in the wire, stronger will be the magnetic field produced. And greater the distance of a point from the current-carrying wire, weaker will be the magnetic field produced at that point. In fact, as we move away from a current-carrying straight wire, the concentric circles around it representing magnetic field lines, become larger and larger indicating the decreasing strength of the magnetic field.
Direction of Magnetic Field Produced by Straight Current-Carrying Conductor (Straight Current-Carrying Wire)
If the direction of current is known, then the direction of magnetic field produced by a straight wire carrying current can be obtained by using Maxwell’s right-hand thumb rule. According to Maxwell’s right- hand thumb rule : Imagine that you are grasping (or holding) the current-carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire.
Figure shows a straight current-carrying wire AB in which the current is flowing vertically upwards from A to B. To find out the direction of magnetic field lines produced by this current, we imagine the wire AB to be held in our right hand as shown in Figure so that our thumb points in the direction of current towards B. Now, the direction in which our fingers are folded gives the direction of magnetic field lines. In this case our fingers are folded in the anticlockwise direction, so the direction of magnetic field (or magnetic field lines) is also in the anticlockwise direction (as shown by the circle drawn at the top of the wire).
Maxwell’s right-hand thumb rule is also known as Maxwell’s corkscrew rule (Corkscrew is a device for pulling corks from bottles, and consists of a spiral metal rod and a handle). According to Maxwell’s corkscrew rule : Imagine driving a corkscrew in the direction of current, then the direction in which we turn its handle is the direction of magnetic field (or magnetic field lines). The corkscrew rule is illustrated in Figure. In Figure, the direction of current is vertically downwards.
Now, if we imagine driving the corkscrew downwards in the direction of current, then the handle of corkscrew is to be turned in the clockwise direction. So, the direction of magnetic field (or magnetic field lines) will also be in the clockwise direction. This example is opposite to the one we considered in right-hand thumb rule given above. Thus, when electric current flows vertically upwards the direction of magnetic field produced is anticlockwise. On the other hand, when electric current flows vertically downwards then the direction of magnetic field is clockwise.
2. Magnetic Field Pattern due to a Circular Loop (or Circular Wire) Carrying Current
We know that when current is passed through a straight wire, a magnetic field is produced around it. It has been found that the magnetic effect of current increases if instead of using a straight wire, the wire is converted into a circular loop (as shown in Figure). In Figure , a circular loop (or circular wire) is fixed to a thin cardboard sheet T. When a current is passed through the circular loop of wire, a magnetic field is produced around it.
The pattern of magnetic field due to a current-carrying circular loop (or circular wire) is shown in Figure. The magnetic field lines are circular near the current-carrying loop. As we move away, the concentric circles representing magnetic field lines become bigger and bigger. At the centre of the circular loop, the magnetic field lines are straight (see point M in Figure).
By applying right- hand thumb rule, it can be seen that each segment of circular loop carrying current produces magnetic field lines in the same direction within the loop. At the centre of the circular loop, all the magnetic field lines are in the same direction and aid each other, due to which the strength of magnetic field increases.
The magnitude of magnetic field produced by a current-carrying circular loop (or circular wire) at its centre is :
- directly proportional to the current passing through the circular loop (or circular wire), and
- inversely proportional to the radius of circular loop (or circular wire).
In this discussion we have considered the magnetic field produced by a circular loop (or circular wire) which consists of only ‘one turn’ of the wire. The strength of magnetic field can be increased by taking a circular coil consisting of a number of turns of insulated copper wire closely wound together.
Thus, if there is a circular coil having n turns, the magnetic field produced by this current-carrying circular coil will be n times as large as that produced by a circular loop of a single turn of wire. This is because the current in each circular turn of coil flows in the same direction and magnetic field produced by each turn of circular coil then just adds up. We can now say that :
The strength of magnetic field produced by a circular coil carrying current is directly proportional to both, number of turns (n) and current (I); but inversely proportional to its radius (r). Thus, the strength of magnetic field produced by a current-carrying circular coil can be increased :
- by increasing the number of turns of wire in the coil,
- by increasing the current flowing through the coil, and
- by decreasing the radius of the coil.
Clock Face Rule
A current-carrying circular wire (or loop) behaves like a thin disc magnet whose one face is a north pole and the other face is a south pole. The polarity (north or south) of the two faces of a current-carrying circular coil (or loop) can be determined by using the clock face rule given below.
According to Clock face rule, look at one face of a circular wire (or coil) through which a current is passing :
- if the current around the face of circular wire (or coil) flows in the Clockwise direction, then that face of the circular wire (or coil) will be South pole (S-pole).
- if the current around the face of circular wire (or coil) flows in the Anticlockwise direction, then that face of circular wire (or coil) will be a North pole (N-pole)
For example, in Figure (a), the current in a face of the circular wire is flowing in the Clockwise direction, so this face of current-carrying circular wire will behave as a South magnetic pole (or S-pole). On the other hand, in Figure (b) the current in the face of the circular wire is flowing in the Anticlockwise direction, so this face of current-carrying circular wire will behave as a North magnetic pole (or N-pole).
Please note that if the direction of current in the front face of a circular wire is clockwise, then the direction of current in the back face of this circular wire will be anticlockwise (and vice versa). This means that the front face of this current-carrying circular wire will be a south pole but its back face will be a north pole. For example, the direction of current in the front face of current-carrying circular wire shown in Figure is clockwise, so the front face of this current-carrying circular wire will be a south magnetic pole (S-pole).
If, however, we view the current-carrying circular wire given in Figure from back side, we will find that the direction of current flowing in the back face of this circular wire is anticlockwise. Due to this, the back face of this current-carrying circular wire will be a North magnetic pole (N-pole). The Clock face rule is also used in determining the polarities of the two faces (or two ends) of a current-carrying solenoid as well as an electromagnet.
3. Magnetic Field due to a Solenoid
The solenoid is a long coil containing a large number of close turns of insulated copper wire. Figure shows a solenoid SN whose two ends are connected to a battery B through a switch X. When an electric current is passed through the solenoid, it produces a magnetic field around it. The magnetic field produced by a current carrying solenoid is shown in Figure.
The magnetic field produced by a current-carrying solenoid is similar to the magnetic field produced by a bar magnet. Please note that the lines of magnetic field pass through the solenoid and return to the other end as shown in Figure.
The magnetic field lines inside the solenoid are in the form of parallel straight lines. This indicates that the strength of magnetic field is the same at all the points inside the solenoid. If the strength of magnetic field is just the same in a region, it is said to be uniform magnetic field. Thus, the magnetic field is uniform inside a current-carrying solenoid. The uniform magnetic field inside the current-carrying solenoid has been represented by drawing parallel straight field lines (see Figure 16). Even the earth’s magnetic field at a given place is uniform which consists of parallel straight field lines (which run roughly from south geographical pole to north geographical pole).
One end of the current-carrying solenoid acts like a north-pole (N-pole) and the other end a south pole (S-pole). So, if a current-carrying solenoid is suspended freely, it will come to rest pointing in the north and south directions (just like a freely suspended bar magnet). We can determine the north and south poles of a current-carrying solenoid by using a bar magnet.
This can be done as follows : We bring the north pole of a bar magnet near both the ends of a current-carrying solenoid. The end of solenoid which will be repelled by the north pole of bar magnet will be its north pole, and the end of solenoid which will be attracted by the north pole of bar magnet will be its south pole.
The current in each turn of a current-carrying solenoid flows in the same direction due to which the magnetic field produced by each turn of the solenoid adds up, giving a strong magnetic field inside the solenoid. The strong magnetic field produced inside a current-carrying solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the solenoid. The magnet thus formed is called an electromagnet. So, a solenoid is used for making electromagnets.
The strength of magnetic field produced by a current carrying solenoid depends on :
- The number of turns in the solenoid. Larger the number of turns in the solenoid, greater will be the magnetism produced.
- The strength of current in the solenoid. Larger the current passed through solenoid, stronger will be the magnetic field produced.
- The nature of “core material” used in making solenoid. The use of soft iron rod as core in a solenoid produces the strongest magnetism.
Electromagnet
An electric current can be used for making temporary magnets known as electromagnets. An electromagnet works on the magnetic effect of current. Let us discuss it in detail. We have just studied that when current is passed through a long coil called solenoid, a magnetic field is produced. It has been found that if a soft iron rod called core is placed inside a solenoid, then the strength of magnetic field becomes very large because the iron core gets magnetised by induction.
This combination of a solenoid and a soft iron core is called an electromagnet. Thus, An electromagnet is a magnet consisting of a long coil of insulated copper wire wrapped around a soft iron core that is magnetised only when electric current is passed through the coil. A simple electromagnet is shown in Figure. To make an electromagnet all that we have to do is to take a rod NS of soft iron and wind a coil C of insulated copper wire round it. When the two ends of the copper coil are connected to a battery, an electromagnet is formed (see Figure).
It should be noted that the solenoid containing soft iron core in it acts as a magnet only as long as the current is flowing in the solenoid. If we switch off the current in the solenoid, it no more behaves as a magnet. All the magnetism of the soft iron core disappears as soon as the current in the coil is switched off. A very important point to be noted is that it is the iron piece inside the coil which becomes a strong electromagnet on passing the current.
The core of an electromagnet must be of soft iron because soft iron loses all of its magnetism when current in the coil is switched off. On the other hand, if steel is used for making the core of an electromagnet, the steel does not lose all its magnetism when the current is stopped and it becomes a permanent magnet. This is why steel is not used for making electromagnets. Electromagnets can be made in different shapes and sizes depending on the purpose for which they are to be used.
Factors Affecting the Strength of an Electromagnet. The strength of an electromagnet depends on :
(i) The number of turns in the coil. If we increase the number of turns in the coil, the strength of electromagnet increases.
(ii) The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.
(iii) The length of air gap between its poles. If we reduce the length of air gap between the poles of an electromagnet, then its strength increases. For example, the air gap between the poles of a straight, bar type electromagnet is quite large, so a bar type electromagnet is not very strong. On the other hand, the air gap between the poles of a U-shaped electromagnet is small, so it is a very strong electromagnet (see Figure).
It should be noted that in many respects an electromagnet is better than a permanent magnet because it can produce very strong magnetic fields and its strength can be controlled by varying the number of turns in its coil or by changing the current flowing through the coil.
We can determine the polarity of electromagnet shown in Figure by using the clock face rule. If we view the electromagnet from its left end, we will see that the direction of current flowing in the coil is anticlockwise. So, the left end of this electromagnet will be North pole (N-pole). And if we view the electromagnet given in Figure from its right end, we will see that the direction of current in its coil is clockwise. So, the right side end of this electromagnet is a South pole (S-pole). Some uses of electromagnets are shown in Figures.
Differences Between a Bar Magnet (or Permanent Magnet) and an Electromagnet
Bar magnet (or Permanent magnet) |
Electromagnet |
1. The bar magnet is a permanent magnet | 1. An electromagnet is a temporary magnet. Its magnetism is only for the duration of current passing through it. So, the magnetism of an electromagnet can be switched on or switched off as desired. |
2. A permanent magnet produces a comparatively weak force of attraction. | 2. An electromagnet can produce very strong magnetic force. |
3. The strength of a permanent magnet cannot be changed. | 3. The strength of an electromagnet can be changed by changing the number of turns in its coil or by changing the current passing through it. |
4. The (north-south) polarity of a permanent magnet is fixed and cannot be changed. | 4. The polarity of an electromagnet can be changed by – changing the direction of current in its coil. |
Permanent magnets are usually made of alloys such as : Carbon steel, Chromium steel, Cobalt steel, Tungsten steel, and Alnico (Alnico is an alloy of aluminium, nickel, cobalt and iron). Permanent magnets of these alloys are much more strong than those made of ordinary steel. Such strong permanent magnets are used in microphones, loudspeakers, electric clocks, ammeters, voltmeters, speedometers, and many other devices. Let us solve one problem now.
Example Problem.
The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:
A uniform magnetic field in a region is represented by drawing equidistant, parallel straight lines, all pointing in the same direction. For example, the uniform magnetic field which exists inside a current-carrying solenoid can be represented by parallel straight lines pointing from its S-pole to N-pole (as shown in Figure alongside).