Magnetic Effects of Electric Current

An electric current-carrying wire behaves like a magnet. Electricity and magnetism are linked to each other.

A compass needle gets deflected when brought near a bar magnet. A compass needle is a small bar magnet. The ends of the compass needle point approximately towards north and south directions. The end pointing towards north is called north pole. The other end that points towards south is called south pole.

Magnetic Field and Field Lines

Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.

The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded.

No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.

Magnetic Field Due to Current Carrying Conductor

Straight Conductor

The magnitude of the magnetic field produced at a given point increases as the current through the wire increases.

The magnetic field produced by a given current in the conductor decreases as the distance from it increases.

Right-Hand Thumb Rule

Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

Circular Loop

If there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

Solenoid

A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.

Force on Current Carrying Conductor in Magnetic Field

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.

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. 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.

Domestic Electric Circuits

In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V.

The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.

Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.