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Magnetic Field of a Solenoid Carrying Current

A solenoid is a long cylindrical coil. It is made by winding many turns of wire on a cylinder. It looks like helical spring. It is commonly employed, to produce a uniform magnetic field. The Figure shows the magnetic field produced by a solenoid. The magnetic field lines of a solenoid are almost identical to that of a bar magnet. Thus, a current carrying solenoid behaves like a bar magnet. The North and South poles of the solenoid are shown in the diagram. The polarity of the face of the solenoid is determined by using the clock rule stated above.
The fact that a current carrying solenoid behaves like a bar magnet can be easily demonstrated. It will attract small pieces of iron only when a current flows through it. Suspend a solenoid by means of a thread and pass a current through it. It will rotate for a few moments and then come to rest pointing in the North-South direction exactly like a bar magnet. 
Notice that the lines of force inside the coil are very nearly straight and the number of lines per square meter of cross-section is fairly constant. Hence, the magnetic field is uniform over a considerable space at the center of the solenoid. The strength of the field can be increased by two ways
  1. by increasing the number of turns per meter of the solenoid, and
  2. by increasing the current through it. 
Electromagnet - Iron Cored Solenoid
Apart from the above mentioned two ways of increasing the strength of the magnetic field of a solenoid, there exists a third method which is by far the most useful method commonly employed to produce very strong magnetic fields. It was found by William Sturgeon, in 1825, that a soft iron core placed inside a solenoid can increase its strength by hundreds and even thousands of times. This device is usually called an electromagnet. The given Figure shows the lines of force of the magnetic field produced by a current carrying solenoid filled with a core of soft iron. Notice the crowding together of the lines of force. This crowding of lines of force indicates that the strength of the field has increased.   
The combination of a solenoid and a soft iron core is called an electromagnet.
Strength of the electromagnet
The strength of the electromagnet depends upon:
  1. the number of turns per unit length of the solenoid
  2. the current through the solenoid and
  3. the length of air gap between its poles.
Think; In order to provide a strong field in a small region, an electromagnet is made in the U – shape. Why?
Here the length of air gap between its poles is less.

Magnetism in Human Beings

  • What produces magnetism in the human body?
  • Extremely weak electric currents are produced in the human body by the movement of charged particles called ions. (ionic currents)
The weak ionic currents flow along the nerve cells, they produce magnetic field in our body.
When will the magnetism create in the human body?
When we try to touch something with our hand, our nerves carry electric impulse to the appropriate muscles. These electric impulses create a temporary magnetism in the body.
Comparison of magnetism produced in the human body with earth’s magnetism. Magnetism produced in the human body is very, very weak as compared to the earth’s magnetism.


Note; Human body organs where magnetism produced is significant are heart and brain.


Name one medical technique which is based on magnetism produced in human body. For what purpose is this technique used?
Magnetic Resonance Imaging (MRI) which is used to obtain images of the internal parts of our body.

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