# Magnetic Field due to a Solenoid

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### Magnetic Field due to a Solenoid

• 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 bar magnet
• The magnetic field lines inside the solenoid are in the form of parallel straight lines.

• The strength of magnetic field is the same at all the points inside the solenoid.
• If the strength of magnetic field is just the same in a region, it is said to be uniform magnetic field.
• The magnetic field is uniform inside a current-carrying solenoid

### Preparing A Electro-Magnet

• The current in each turn of a current-carrying solenoid flows in the same direction due to which the magnetic field produced by each turn of the solenoid adds up, giving a strong magnetic field inside the solenoid.
• The strong magnetic field produced inside a current-carrying solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the solenoid.
• The magnet thus formed is called an electromagnet.

### Factors on Which the strength of magnetic field produced by a current carrying solenoid depends

• The number of turns in the solenoid. Larger the number of turns in the solenoid, greater will be the magnetism produced.
• The strength of current in the solenoid. Larger the current passed through solenoid, stronger will be the magnetic field produced.
• The nature of “core material” used in making solenoid. The use of soft iron rod as
core in a solenoid produces the strongest magnetism

#### Electromagnet

• An electromagnet works on the magnetic effect of current.
• An electromagnet is a magnet consisting of a long coil of insulated copper wire wrapped around an iron core that is magnetized only when electric current is passed through the coil.
• The core of an electromagnet must be of soft iron because soft iron loses all of its magnetism when current in the coil is switched off.

#### Reason why steel is not used as Electro Magnet

• if steel is used for making the core of an electromagnet, the steel does not lose all its magnetism when the current is stopped and it becomes a permanent magnet.
• This is why steel is not used for making electromagnets.

### Factors Affecting the Strength of an Electromagnet

• The number of turns in the coil. If we increase the number of turns in the coil, the strength of
electromagnet increases.
•  The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.
• The length of air gap between its poles. If we reduce the length of air gap between the poles of an electromagnet, then its strength increases. For example, the air gap between the poles of a straight, bar type electromagnet is quite large, so a bar type electromagnet is not very strong. On the other hand, the air gap between the poles of a U-shaped electromagnet is small, so it is a very strong electromagnet

### FORCE ON CURRENT-CARRYING CONDUCTOR PLACED IN A MAGNETIC FIELD

• a magnet exerts a mechanical force on a current-carrying wire, and if the wire is free to move, this force can produce a motion in the wire
• In 1821, Faraday discovered that : When a current-carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which can make the conductor move.
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