ARRYING CONDUCTOR

ARRYING CONDUCTOR We 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 ( – ) suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor. The force due to a magnetic field acting on a current-carrying conductor can be demonstrated through the following activity.

Choose the correct option. The magnetic field inside a long straight solenoid-carrying current is zero. decreases as we move towards its end. increases as we move towards its end.

is the same at all points. Take a small aluminium rod AB (of about cm). Using two connecting wires suspend it horizontally from a stand, as shown in Fig. .

. Place a strong horse-shoe 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 south pole vertically above the aluminium rod (Fig. .

). 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? 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 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 current and the direction of the magnetic field.

Experiments have shown that the displacement of the rod is largest (or the magnitude of the force is the highest) when the direction of 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. In Activity . , 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. . ).

If the first finger points in the direction of magnetic field and the second finger in the direction of 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. Example . An electron enters a magnetic field at right angles to it, as shown in Fig.

. . The direction of force acting on the electron will be to the right. to the left.

out of the page. into the page. Solution Answer is option (d). The direction of force is perpendicular to the direction of 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. Fleming’s left-hand rule 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.) mass speed velocity momentum In our homes, we receive supply of electric power through a main supply (also called mains), either supported through overhead electric poles or by underground cables.

One of the wires in this supply, usually with red insulation cover, is called live wire (or positive). Another wire, with black insulation, is called neutral wire (or negative). In our country, the potential difference between the two is V. At the meter-board in the house, these wires pass into an electricity meter through a main fuse.

Through the main switch they are connected to the line wires in the house. These wires supply electricity to separate circuits within the house. Often, two separate circuits are used, one of A current rating for appliances with higher power ratings such as geysers, air coolers, etc. The other circuit is of A current rating for bulbs, fans, etc.

The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc. The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current. Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.

In Activity . , 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? A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is towards south towards east downward upward 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.