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From the study of subatomic particles to the laws of motion, Physics Topics offer insights into the workings of the world around us.
Demonstration of Magnetic Field Due to Current Carrying Conductor
Extremely weak electric currents are produced in the human body by the movement of charged particles called ions. These are called ionic currents. Now, we have studied that whenever there is an electric current, a magnetic field is produced. So, when the weak ionic currents flow along the nerve cells, they produce magnetic field in our body.
For example, when we try to touch something with our hand, our nerves carry electric impulse to the appropriate muscles. And this electric impulse creates a temporary magnetism in the body. The magnetism produced in the human body is very, very weak as compared to the earth’s magnetism. The two main organs of the human body where the magnetic field produced is quite significant are the heart and the brain.
The magnetism produced inside the human body (by the flow of ionic currents) forms the basis of a technique called Magnetic Resonance Imaging (MRI) which is used to obtain images (or pictures) of the internal parts of our body (see Figure). It is obvious that magnetism has an important use in medical diagnosis because, through MRI scans, it enables the doctors to see inside the body. For example, MRI can detect cancerous tissue inside the body of a person. Please note that the magnetism in human body is actually electromagnetism (which is produced by the flow of ionic currents inside the human body).
Force On Current-Carrying Conductor Placed In A Magnetic Field
We have already described Oersted’s experiment which shows that a current-carrying wire exerts a force on a compass needle and deflects it from its usual north-south position. Since a compass needle is actually a small freely pivoted magnet, we can also say that a current-carrying wire exerts a mechanical force on a magnet, and if the magnet is free to move, this force can produce a motion in the magnet. The reverse of this is also true, that is, a magnet exerts a mechanical force on a current-carrying wire, and if the wire is free to move, this force can produce a motion in the wire (see Figure).
In fact, this result can be obtained by applying Newton’s third law of motion according to which if a current-carrying wire exerts a force on a magnet, then the magnet will exert an equal and opposite force on the current carrying wire. In 1821, Faraday discovered that: When a current-carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which can make the conductor move.
This is known as the motor principle and forms the basis of a large number of electrical devices like electric motor and moving coil galvanometer. We will now describe an experiment to demonstrate the force exerted by a magnet on a current-carrying wire and to show how the direction of force is related to the direction of current and the direction of magnetic field.
Experiment to Demonstrate the Force Acting on a Current-Carrying Conductor Placed in a Magnetic Field : The Kicking Wire Experiment
A thick copper wire AB is suspended vertically from a support T by means of a flexible joint J (Figure). The lower end B of this wire is free to move between the poles of a U-shaped magnet M. The lower end B of the wire just touches the surface of mercury kept in a shallow vessel V so that it can move when a force acts on it.
The positive terminal of a battery is connected to end A of the wire. The circuit is completed by dipping another wire from the negative terminal of the battery into the mercury pool as shown in Figure. We know that mercury is a liquid which is a good conductor of electricity, so the circuit is completed through mercury contained in the vessel V.
On pressing the switch, a current flows in the wire AB in the vertically downward direction. The wire AB is kicked in the forward direction (towards south) and its lower end B reaches the position B’, so that the wire comes to the new position AB’ as shown by dotted line in Figure.
When the lower end B of the hanging wire comes forward to B’, its contact with the mercury surface is broken due to which the circuit breaks and current stops flowing in the wire AB. Since no current flows in the wire, no force acts on the wire in this position and it falls back to its original position.
As soon as the -wire falls back, its lower end again touches the mercury surface, current starts flowing in the wire and it is kicked again. This action is repeated as long as the current is passed in wire AB. It should be noted that the current-carrying wire is kicked forward because a force is exerted on it by the magnetic field of the U-shaped magnet. From this experiment we conclude that when a current-carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which makes it move.
In Figure, the current is flowing in the vertically downward direction and the direction of magnetic field is from left to right directed towards east, thus, the current carrying conductor is at right angles to the magnetic field. Now, we have just seen that the motion of the conductor is in the forward direction (towards south) which is at right angles to both, the direction of current and the direction of magnetic field.
Since the direction of motion of the wire represents the direction of force acting on it, we can say that: The direction of force acting on a current-carrying wire placed in a magnetic field is (i) perpendicular to the direction of current, and (ii) perpendicular to the direction of magnetic field.
In other words, we can say that the current, the magnetic field and the force, are at right angles to one another. It should be noted that the maximum force is exerted on a current-carrying conductor only when it is perpendicular to the direction of magnetic field. No force acts on a current-carrying conductor when it is parallel to the magnetic field.
If we reverse the direction of current in the wire AB so that it flows in the vertically upward direction from B to A, then the wire swings in the backward direction (towards north). This means that the direction of force on the current-carrying wire has been reversed. From this we conclude that the direction of force on a current-carrying conductor placed in a magnetic field can be reversed by reversing the direction of current flowing in the conductor.
Keeping the direction of current unchanged, if we reverse the direction of magnetic field applied in Figure by turning the magnet M so that its poles are reversed, even then the wire swings backwards showing that the direction of force acting on it has been reversed. Thus, the direction of force on a current-carrying conductor placed in a magnetic field can also be reversed by reversing the direction of magnetic field.
If the direction of current in a conductor and the direction of magnetic field (in which it is placed), are known, then the direction of force acting on the current-carrying conductor can be found out by using Fleming’s left-hand rule. This is described below.
Fleming’s Left-Hand Rule for the Direction of Force
When a current carrying wire is placed in a magnetic field, a force is exerted on the wire. Fleming gave a simple rule to determine the direction of force acting on a current carrying wire placed in a magnetic field. This rule is known as Fleming’s left-hand rule and it can be stated as follows.
According to Fleming’s left-hand rule : Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another [as shown in Figure(a)]. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current, then the direction in which thumb points, gives the direction of force acting on the conductor. Since the conductor (say, a wire) moves along the direction in which the force acts on it, we can also say that the direction in which the thumb points gives the direction of motion of the conductor.
Thus, we can write Fleming’s left-hand rule in another way as follows : Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current in the conductor, then the direction in which the thumb points gives the direction of motion of the conductor.
To memorize Fleming’s left-hand rule we should remember that the forefinger represents the (magnetic) field (both, forefinger and field, start with the same letter/), the centre finger represents current (both, centre and current start with letter c), and the thumb represents force or motion (both, thumb and motion contain the letter m). We will make the Fleming’s left hand rule more clear by taking an example.
Suppose we have a vertical current-carrying wire or conductor placed in a magnetic field. Let the direction of magnetic field be from west to east as shown by arrow AB in Figure (b). Again suppose that the direction of current in the wire is vertically downwards as shown by arrow AC. Now, we want to find out the direction of force which will be exerted on this current-carrying wire. We will find out this direction by using Fleming’s left-hand rule as follows :
We stretch our left hand as shown in Figure (a) so that the forefinger, the centre finger and the thumb are perpendicular to one another. Since the direction of magnetic field is from west to east, so we point our forefinger from west to east direction to represent the magnetic field [Figure (a)]. Now, the current is flowing vertically downwards, so we point our centre finger vertically downwards to represent the direction of current. Now, let us look at the direction of our thumb.
The thumb points in the forward direction towards south. This gives us the direction of force acting on the wire (or direction of motion of wire). So, the force acting on the current carrying wire will be in the south direction as shown by the arrow AD in Figure 24(b), and the wire will move in the south direction.
The devices which use current-carrying conductors and magnetic fields include electric motor, electric generator, microphone, loudspeakers, and current detecting and measuring instruments (such as ammeter and galvanometer, etc.)
Before we solve problems involving direction of current, direction of magnetic field and the direction of force by using Fleming’s left-hand rule, we should keep the following points in mind :
(i) By convention, the direction of flow of positive charges is taken to be the direction of flow of current. So, the direction in which the positively charged particles such as protons or alpha particles, etc., move will be the direction of electric current.
(ii) The direction of electric current is, however, taken to be opposite to the direction of flow of negative charges (such as electrons). So, if we are given the direction of flow of electrons, then the direction of electric current will be taken as opposite to the direction of flow of electrons.
(iii) The direction of deflection of a current-carrying conductor (or a stream of positively charged particles or a stream of negatively charged particles like electrons) tells us the direction of force acting on it.
Let us solve some problems now.
Example Problem 1.
A stream of positively charged particles (alpha particles) moving towards west is deflected towards north by a magnetic field. The direction of magnetic field is :
(a) towards south
(b) towards east
(c) downward
(d) upward
Solution:
Here the positively charged alpha particles are moving towards west, so the direction of current is towards west. The deflection is towards north, so the force is towards north. So, we are given that:
- direction of current is towards west, and
- direction of force is towards north.
Let us now hold the forefinger, centre finger and thumb of our left-hand at right angles to one another. Adjust the hand in such a way that our centre finger points towards west (in the direction of current) and thumb points towards north (in the direction of force). Now, if we look at our forefinger, it will be pointing upward. Since the direction of forefinger gives the direction of magnetic field, therefore, the magnetic field is in the upward direction.
So, the correct answer is : (d) upward.
Example Problem 2.
Think you are sitting in a chamber with your back to one wall. An electron beam moving horizontally from back wall towards the front wall is deflected by a strong magnetic field to your right side. What is the direction of magnetic field ?
Solution:
Here the electron beam is moving from our back wall to the front wall, so the direction of current will be in the opposite direction, from front wall towards back wall or towards us. The direction of deflection (or force) is towards our right side. We now know two things :
- direction of current is from front towards us, and
- direction of force is towards our right side.
Let us now hold the forefinger, centre finger and thumb of our left hand at right angles to one another. We now adjust the hand in such a way that our centre finger points towards us (in the direction of current) and thumb points towards right side (in the direction of force).
Now, if we look at our forefinger, it will be pointing vertically downwards. Since the direction of forefinger gives the direction of magnetic field, therefore, the magnetic field is in the vertically downward direction.