Magnetic Field Lines – Learn


Magnetic field line diagrams are used to illustrate the direction and strength of a magnetic field. The direction of a magnetic field at any point can be determined by placing a magnet in that position.

  • The direction of magnetic field lines is from the north pole toward the south pole.
  • The direction of the field indicates the force acting on a north magnetic pole.
  • The density of the field lines (or how close together they are) indicates the strength of the magnetic field.
  • The strength, or vector magnitude, of the magnetic field is denoted by the variable B and the unit is the Tesla (T)
  • The magnetic field (B) is a vector quantity.

In this section, we must understand the nature (shape, direction and strength) of a magnetic field produced by:

  • magnets
  • current-carrying wires
  • solenoids

Further to this, we must be able to describe the effect on magnetic materials that are placed in these fields.


Fields around magnets

Magnetic field lines leave the north end of a magnet and tend toward the south end of the same magnet. The shape of the field will depend on the type of magnet (bar, horseshoe, disc, ring).

               

When two or magnets are placed near each other, their fields will interact and a resulting field can be observed. Common interactions you should be familiar with include those created by bar magnets. The resultant direction of the magnetic field at a particular point is the vector addition of each individual magnetic field acting at that point.

When two magnets are placed close together two situations may arise. If the poles are unlike (north-south) then attraction will occur between them and a magnetic field will be created that extends between the two poles. If like poles (north-north or south-south) are near each other they will repel each other. In this situation there is a neutral point (no magnetic field) between the two poles. The diagram below shows a bar magnet and its associated magnetic field lines:

                  


Fields around current-carrying conductors

Current-carrying conductors (wires) create circular loops of magnetic field around the wire. This field can be observed with iron filings and a compass at various points around the wire as shown below:

             

  • The magnetic field is perpendicular to the current-carrying wire.
  • The direction of the field depends on the current direction.
  • The strength of the magnetic field increases with an increasing current.
  • The strength of the magnetic field gets weaker as the distance from the current-carrying wire increases.

Right-hand grip rule

The direction of the magnetic field around a current-carrying conductor can be determined using the right-hand grip rule: wrap your hand around the wire so that your thumb points in the direction of the conventional current (positive to negative) moving along the wire. The field is perpendicular to the wire and in the direction that your fingers are wrapped around the wire. The right-hand grip rule is illustrated below:


Wires and fields into and out of the page

Problems that involve current-carrying conductors that are into or out of the page use the following notation:

  • a cross (×) for currents travelling into the page 
  • a dot (⋅) for currents travelling out of the page

The right hand grip rule still applies to these situations as illustrated below:

Problems that involve magnetic fields that are into or out of the page use the following notation:

  • a cross (×) for fields travelling into the page 
  • a dot (⋅) for fields travelling out of the page

The density of these points indicates the strength of the field.


Solenoids

If a current-carrying wire is coiled into a loop the field is concentrated in the centre of the loop and weaker on the outside:

If the current-carrying wire is made into many loops to form a coil this creates a very strong field inside the coil and a weak field outside of the coil. A current-carrying wire made into a coil is called a solenoid. A solenoid creates a magnetic field that is the same shape as a bar magnet; hence it has a north and south end. The direction of the field is determined by another right-hand grip rule:

  • The hand wraps around the coil with the fingers pointing in the direction that the current is flowing.
  • The thumb points to the north end of the solenoid.

            

The strength of the field around a solenoid is increased by:

  • increasing the number of coils
  • increasing the current through the solenoid
  • placing a ferromagnetic material in the coil

Example 1:

How could you determine if an object was a magnet or just made of a magnetic material?

Answer:

Expose the object to both ends of a bar magnet. If the object was only made of a magnetic material it will be attracted to both ends of the bar magnet. If it is a magnet it will be attracted to one end and repelled by the other.


Example 2: 

Describe the magnetic field created by the current-carrying conductors below:

a) 

b) 

Answers:

a) into the page above the wire and coming out of the page below the wire (right-hand grip rule – wrap right hand around wire, thumb pointing to the left)

b) clockwise (right-hand grip rule – wrap right hand around wire, thumb pointing into the page)


Example 3:

Which end of the following solenoid would be the north pole, right or left?

Answer:

Left hand side (right-hand grip rule – wrap hand around solenoid, fingers following the direction of the the current)

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