Magnetic Field and its Properties

Magnetic Field and its Properties

• Before beginning the study of steady magnetic fields, let us study the basic properties of the magnetic field. To understand these properties, consider a permanent magnet. It has two poles, north (N) and south (S). The region around a magnet within which the influence of the magnet can be experienced is called magnetic field. The existence of such a field can be experienced with the help of compass needle. Such a field is represented by imaginary lines around the magnet which are called magnetic lines of force. These are introduced by the Scientist Michael Faraday. The direction of such lines is always from N pole to S pole, external to the magnet as shown in the Fig. 7.2.1. These lines of force are also called magnetic lines of flux or magnetic flux lines.

• An important difference between electric flux lines and magnetic flux lines can be observed here. In case of electric flux, the flux lines originate from an isolated positive charge and diverge to terminate at infinity. While for a negative charge, electric flux lines converge on a charge, starting from infinity. But in case of magnetic flux, the poles exist in pairs only.

Key Point : An isolated magnetic pole can not exist.

• Hence every magnetic flux line starting from north pole must end at south pole and complete the path from south to north internal to the magnet.

Key Point : Thus magnetic flux lines exist in the form of closed loop.

• This is true whether the field is due to permanent magnet or due to conductor carrying direct current.

1. Magnetic Field due to Current Carrying Conductor

• When a straight conductor carries a direct current, it produces a magnetic field around it, all along its length. The lines of force in such a case are in the form of concentric circles in the planes at right angles to the conductor. This is shown in the Fig. 7.2.2. The direction of such magnetic flux can be experienced using a compass needle. The direction of concentric circles around, depends on the direction of current through the conductor. As long as direction of current is constant and current is time independent, magnetic lines of force are also  constant, static and time independent, giving a steady magnetic field in the space around the conductor.

• A right hand thumb rule is used to determine the direction of magnetic field around a conductor carrying a direct current. It states that, hold the current carrying conductor in the right hand such that the thumb pointing in the direction of current and parallel to the conductor, then curled fingers point in the direction of the magnetic lines of flux around it. The Fig. 7.2.3 explains the rule.

• Practically the current carrying conductor is represented by a small circle i.e. top view of straight conductor while the direction of current through it is indicated by a 'cross' or a 'dot'. The cross indicates that the current direction is going into the plane of the paper away from the observer. The dot indicates that the current direction is coming out of the plane of the paper coming towards the observer. Using right hand thumb rule, the direction of magnetic flux around such a conductor is either clockwise or anticlockwise as shown in the Fig. 7.2.4.

• Another method of identifying the direction of magnetic flux around a conductor is right handed screw rule. It states that, imagine a right handed screw to be along the conductor carrying current with its axis parallel to the conductor and tip pointing in the direction of the current flow. Then the direction of magnetic field is given by the direction in which the screw must be turned so as to advance in the direction of the current flow. The Fig. 7.2.5 illustrates this rule.

• Thus the magnetic lines of force i.e. magnetic flux lines always form a closed loop and exist in the form of concentric circles, around a current carrying conductor. The total number of magnetic lines of force is called a magnetic flux denoted as ϕ. It is measured in weber (Wb). One weber means 108 lines of force.

2. Magnetic Field Intensity

• The quantitative measure of strongness or weakness of the magnetic field is given by magnetic field intensity or magnetic field strength. The magnetic field intensity at any point in the magnetic field is defined as the force experienced by a unit north pole of one weber strength, when placed at that point. The magnetic flux lines are measured in webers (Wb) while magnetic field intensity is measured in newtons/weber (N/Wb) or amperes per metre (A/m) or ampere-tums/metre (AT/m). It is denoted as . It is a vector quantity. This is similar to the electric field intensity   in electrostatics.

3. Magnetic Flux Density

• The total magnetic lines of force i.e. magnetic flux crossing a unit area in a plane at right angles to the direction of flux is called magnetic flux density. It is denoted as  and is a vector quantity. It is measured in weber per square metre. (Wb/m2) which is also called Tesla (T). This is similar to the electric flux density  in electrostatics.

4. Relation between 

• In electrostatics,  are related to each other through permittivity e of the region. In magnetostatics, the  are related to each other through the property of the region in which current carrying conductor is placed. It is called permeability denoted as p. It is the ability or ease with which the current carrying conductor forces the magnetic flux through the region around it. For a free space, the permeability is denoted as µ0 and its value is 4π×10^-7. As Ɛ is measured in F/m, the permeability p is measured in henries per metre (H/m). For any other region, a relative permeability is specified as µ_r and µ = µ₀µ_r.

• For all nonmagnetic media µ_r = 1 while for magnetic materials µ_r is greater than unity.

Review Questions

1. Define magnetic flux intensity and magnetic flux density. State their units.

2. Define magnetic field and state its properties.