Your Guide to Physics

5 Electricity and Magnetism

by Robina

Posted on 2019-04-04

5.4 Magnetic Fields




Magnetic fields are found around moving charge, the magnetic field flux density (B) is the measurement of magnetic field strength it is measured in \frac{N.s}{C.m}  in other words in tesla (T).

Magnets come in two poles, north and south pole, till now single pole was never found in nature poles always come in pairs.

Even if you keep on cutting the magnet into smaller and smaller pieces you will end up with tiny magnets of two poles. Since the atom, itself from which the magnet is made up of you can consider it as if a small bar magnet due to the spin-orbital interaction of the electron.

One property of magnets is that like poles repel whereas opposite poles attract.


►Magnetic Field Lines Around Bar Magnet:



Magnetic field notes:

  • Points from north to south outside the magnet.

  • Points from south to north inside the magnet.

  • They are closed loops.

  • As you go away in distance from the magnet the distance between the loops increase which shows that the field decrease in strength.

  • As shown in the figure if you place a compass the north of the needle will always be pointing in direction of field and tangent to the loop.

►Earth Giant Magnet:



  • Around Earth, there is a magnetic field that point from the geographic south to the geographic north since you can consider that as if inside earth there is a large bar magnet where the magnetic north is in the geographic south and the magnetic south is toward the geographic north. That's why when we hold a compass the north of the compass always points to the geographic north.

  • The reason behind the presence of that field is that inside earth's core the temperature is not equally distributed which will lead to convection current which will lead to the circulation of the ions which will lead to the creation of magnetic field.

►Magnetic field around a wire carrying current:



Recal that current is the rate at which charges flow ⇒ A magnetic field will be created around the wire, the magnetic field have direction and magnetude;

  1. Direction:

  • determined by the right hand rule if its a conventional current.

  • determined by the left hand rule if its negative curren.

  1. Magnetude: The strength of the magnetic field created aroundd the wire is proportional to the current and inversly proportional to the distance from the wire, given by the below equation;

B= \mu_{0} \frac{I}{r}


  • B= magnetic field flux density (T)

  • \mu_{0}= permiability of free space. (T.m.A-1)

  • I= Current (A)

  • r=distance (m)








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