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IMPACT OF FORCE IN MAGNETIC FIELD
Force on a current carrying conductor In a Magnetic Field
IMPACT OF FORCE IN MAGNETIC FIELDWe 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. French scientist Andre Marie Ampere (1775–1836) suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor. The force due to magnetic field acting on a current-carrying conductor can be demonstrated through the following activity.
Activity 13.7: ( force on current carrying conductor )
* Take a small aluminium rod AB (of about 5 cm). Using two connecting wires suspend it horizontally from a stand, as shown in Fig. 13.12.
* Place a strong horseshoe 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 the south pole vertically above the aluminium rod (Fig. 13.12).
* Connect the aluminium rod in series with a battery, a key, and a rheostat.
* 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?
Figure 13.12 ( force on current carrying conductor ): A current-carrying rod, AB, experiences a force perpendicular to its length and the magnetic field. Support for the magnet is not shown here, for simplicity
The displacement of the rod in the above activity 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 the 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 the current and the direction of the magnetic field. Experiments have shown that the displacement of the rod is the largest (or the magnitude of the force is the highest) when the direction of the current is at right angles to the direction of the magnetic field. In such a condition we can use a simple rule to find the direction of the force on the conductor.
Figure 13.13: Fleming’s left-hand rule
In Activity 13.7, 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. According to this rule, stretch the thumb, forefinger, and middle finger of your left hand such that they are mutually perpendicular (Fig. 13.13). If the first finger points in the direction of the magnetic field and the second finger in the direction of the 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, electric generator, loudspeakers, microphones, and measuring instruments. In the next few sections, we shall study about electric motors and generators.
More to know about magnetism in medicine
An electric current always produces a magnetic field. Even weak ion currents that travel along the nerve cells in our body produce magnetic fields. When we touch something, our nerves carry an electric impulse to the muscles we need to use. This impulse produces a temporary magnetic field. These fields are very weak and are about one-billionth of the earth’s magnetic field. Two main organs in the human body where the magnetic field produced is significant, are the heart and the brain. The magnetic field inside the body forms the basis of obtaining the images of different body parts. This is done using a technique called Magnetic Resonance Imaging (MRI). Analysis of these images helps in medical diagnosis. Magnetism has, thus, got important uses in medicine.
Illustration 13.2:
An electron enters a magnetic field at right angles to it, as shown in Fig. 13.14. The direction of force acting on the electron will be
Figure 13.14:
(a) to the right. (b) to the left.
(c) out of the page. (d) into the page.
Sol:
Answer is option (d). The direction of force is perpendicular to the direction of the magnetic field and current as given by Fleming’s left-hand rule. Recall that the direction of current is taken opposite to the direction of motion of electrons. The force is therefore directed into the page
Questions
1. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) mass (b) speed
(c) velocity (d) momentum
2. In Activity 13.7, how do we think the displacement of rod AB will be affected if
(i) current in rod AB is increased;
(ii) a stronger horse-shoe magnet is used, and
(iii) length of the rod AB is increased?
3. A positively-charged particle (alpha-particle) projected towards the west is deflected towards north by a magnetic field. The direction of the magnetic field is
(a) towards south.
(b) towards east.
(c) downward.
(d) upward
Source: This topic is taken from NCERT TEXTBOOK