### Electromagnetism

#### Magnetic Effect of Current on Electromagnetism The term "Magnetic Effect of Current" means that a current flowing in a wire produces a magnetic field around it. The magnetic effect of current was discovered by Oersted in 1820. Oersted found that a wire carrying a current was able to deflect a magnetic needle. Now, a magnetic needle can only be deflected by a magnetic field. Thus it was concluded that a current flowing in a wire always gives rise to a magnetic field round it. The magnetic effect of the current is called electromagnetism which means that electricity produces magnetism.

#### Tenets of Electromagnetism

##### Magnetic Field Pattern Due to Straight Current-Carrying Conductor

The magnetic lines of force round a straight conductor carrying current are concentric circles whose centers lie on the wire.

The magnitude of magnetic field produced by a straight current-carrying wire at a given point is:

1. Directly proportional to the current passing in the wire, and
2. Inversely proportional to the distance of that point from the wire.

So, the greater the current in the wire, the stronger the magnetic field produced. The greater the distance of a point from the current-carrying wire, the weaker the magnetic field produced at the point.

##### Magnetic Field Pattern Due to a Circular Coil Carrying Current

We know that when a current is passed through a straight wire, a magnetic field is produced around it. It has been found that the magnetic effect of current increases if, instead of using a straight wire, the wire is converted into a circular coil. A circular coil consists of twenty or more turns of insulated copper wire closely wound together. When a current is passed through a circular coil, a magnetic field is produced around it. The lines of force are circular near the wire, but they become straight and parallel towards the middle point of the coil. In fact, each small segment of the coil is surrounded by such magnetic lines of force. At the center of the coil, the lines of force aid each other and the strength of the magnetic field increases.

The magnitude of magnetic field produced by a current carrying wire at its center is:

1. Directly proportional to the current passing through the circular wire, and
2. Inversely proportional to the radius of the circular wire.

A current-carrying circular wire (or coil) behaves as a thin disc magnet, whose one face is a North Pole and the other face is a South Pole.

The strength of magnetic field produced by a current-carrying circular coil can be increased:

1. By increasing the number of turns of wire in the coil.
2. By increasing the current flowing through the coil.
3. By decreasing the radius of the coil.

#### Solenoids

The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bra magnet. The lines of magnetic force pass through the solenoid and return to the other end. If a current carrying solenoid is suspended freely, it comes to rest pointing North and South like a suspended magnetic needle. One end of the solenoid acts like a North Pole and the other end a South Pole. Since the current in each circular turn of the solenoid flows in the same direction, the magnetic field produced by each turn of the solenoid adds up, giving a strong resultant magnetic field inside the solenoid. A solenoid is used for making electromagnets.

The strength of magnetic field produced by a current carrying solenoid is:

1. Directly proportional to the number of turns in the solenoid.
2. Directly proportional to the strength of current in the solenoid.
3. Dependent on the nature of "core material" used in making the solenoid. The use of soft iron rod as core in a solenoid produces the strongest magnetism.

#### Electromagnet

An electric current can be used for making temporary magnets know as electromagnets.

An electromagnet works on the magnetic effect of current. It has been found that if a soft iron rod called core is placed inside a solenoid, then the strength of the magnetic field becomes very large because the iron ore is magnetized by induction. This combination of a solenoid and a soft iron core is called an electromagnet. Thus, an electromagnet consists of a long coil of insulated copper wire wound on a soft iron core.

The electro magnet acts as a magnet only so long as the current is flowing in the solenoid.

The moment the current is switched off the solenoid is demagnetized. The core of the electromagnet must be of soft iron because soft iron loses all of its magnetism when current in the coil is switched off. Steel is not used in electromagnets, because it does not lose all its magnetism when the current is stopped and becomes a permanent magnet.

Electromagnets can be made of different shapes and sizes depending on the purpose for which they are to be used.

##### Factors Affecting the Strength of an Electromagnet

The strength of an electromagnet is:

1. Directly proportional to the number of turns in the coil.
2. Directly proportional to the current flowing in the coil.
3. Inversely proportional to the length of air gap between the poles.

In general, an electromagnet is often considered better than a permanent magnet because it can produce very strong magnetic fields and its strength can be controlled by varying the number of turns in its coil or by changing the current flowing through the coil.