Magnetism - Part III

Magnetic field due to a solenoid
Roll a cardboard sheet and wind an electric wire around it. Make as many turns as possible.  The cylindrical shaped wire arrangement is called a solenoid. The axis of the cylinder XY is the axis of the solenoid.  

When a current is passed through a solenoid, it functions as what is known as an electromagnet. For studying the behaviour of an electromagnet, connect the ends of the solenoid to a battery via a key. Take a magnetic compass needle.  

Vary the number of turns of the solenoid, the current flowing through it, the material on which the solenoid is wound and the direction of the current.  

You will observe the following :

• The greater the number of turns, larger is the deflection; that is larger is the induced magnetic field around the solenoid.

• Greater the strength of the current,  larger is the induced magnetic field.

• The strength of the electric field also depends on the material on which the solenoid is wound.

• The polarity of the induced field depends on the direction in which the current is flowing.

If you place iron filings on a cardboard and put it across the solenoid,  when the current is made to pass, the lines of force will appear similar to a bar magnet. Thus the induced magnetic field of a solenoid behaves similar to a bar magnet.

Electric current due to a changing magnetic field
Up till now, we have seen how moving electric charges or currents produce or induce magnetic fields. The reverse is also true. A moving magnet can also induce currents. In the diagram below, a coil of conducting wire is connected to an ammeter. A strong magnet is pulled in and out of the centre of the coil. You will see deflection in the ammeter. When the magnet is being pulled in, the deflection will be in one direction and when the magnet is being pulled out, the deflection will be in another direction.

Application of induced electric and magnetic fields
Application of induced electric and magnetic fields are many. The most important use of magnets in the home, is with electric motors. All electric motors use electromagnets. These motors run refrigerators, vacuum cleaners, washing machines, compact disc players, blenders, drills, cars etc. Audiotape and videotape players have electromagnets called heads that record and read information on tapes covered with tiny magnetic particles. Magnets in speakers transform the signal into a sound by making the speakers vibrate. An electromagnet called a deflection yoke in TV picture tubes helps form images on a screen.  Huge electro-magnets are used to lift heavy iron objects such as cars; maglev trains run on electromagnets.  Basic electricity generation also depends on the induced electric and magnetic fields.

Units for measuring magnetic field and magnetic lines of force
The units of measuring magnetic field in the M.K.S. System is Tesla (T).  In C.G.S system it is Gauss (g).  1 tesla = 10⁴ gauss.  
The density of magnetic lines of force or flux is measured in M.K.S System is Weber (Wb).  One Weber is the magnetic flux obtained in a circuit of one turn that produces a voltage of 1 volt, when the magnet is moved at a uniform rate in one second. 
In the C.G.S system, the magnetic flux is measured in Maxwell (Mx).  1 Wb = 10⁸ Mx. Also by definition, 1 T = 1 Wb/m².

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