MAGNETIC EFFECTS OF ELECTRIC CURRENT

FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

An electric current through a conductor produces a magnetic field. This field exerts a force on a magnet placed near the conductor.

French scientist Andre Marie Ampere (1775–1836) suggested that the magnet also exerts an equal and opposite force on the current-carrying conductor. This can be demonstrated through an activity.

A current-carrying rod, AB, experiences a force perpendicular to its length and the magnetic field.

The displacement of the rod (AB) suggests that

· A force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field.

· Direction of force is also reversed when the direction of current through the conductor is reversed.

Change the direction of field to vertically downwards by interchanging the two poles of the magnet. Again, 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 magnetic field.

Displacement of the rod is largest (or magnitude of the force is highest) when the direction of current is at right angles to the direction of the magnetic field. Here, direction of the force can be found through Fleming’s left-hand rule. According to this, stretch the thumb, forefinger and middle finger of left hand such that they are mutually perpendicular. First finger points in the direction of magnetic field, second finger points in the direction of current and the thumb points in the direction of motion or the force acting on the conductor.

Electric motor, electric generator, loudspeakers, microphones & measuring instruments are the devices that use current-carrying conductors & magnetic fields.

Problem: An electron enters a magnetic field at right angles to it, as shown below. The direction of force acting on the electron will be

(a) to the right. (b) to the left.
(c) out of the page. (d) into the page.

Solution:

Answer is (d). The direction of force is perpendicular to the direction of magnetic field and current as given by Fleming’s left-hand rule. The direction of current is taken opposite to the direction of motion of electrons. The force is therefore directed into the page.

Magnetism in medicine

Electric impulses carrying through the nerves can produce a temporary magnetic field. These fields are very weak (one-billionth of the earth’s magnetic field). Heart and brain can produce significant magnetic field. The magnetic field inside the body is used to obtain the images of body parts. This technique is called Magnetic Resonance Imaging (MRI). These images help in medical diagnosis.