• The magnetic flux density?B?is defined as:

The force acting per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field

• Rearranging the equation for magnetic force on a wire, the magnetic flux density is defined by the equation:

• Where:
  • B?= magnetic flux density (T)
  • F?= magnetic force (N)
  • I =?current (A)
  • L?= length of the wire (m)

   

• Note:?this equation is only relevant when the B-field is?perpendicular?to the current
• Magnetic flux density is measured in units of?tesla, which is defined as:

A wire carrying a current of 1A normal to a magnetic field of flux density of 1 T with force per unit length of the conductor of 1 N m-1

• To put this into perspective, the Earth's magnetic flux density is around 0.032 mT and an ordinary fridge magnet is around 5 mT
• The magnetic flux density is sometimes referred to as the?magnetic?field strength

Step 1: Write out the known quantities

• • Force on wire,?F?= 0.04 N
  • Current,?I?= 3.0 A
  • Length of wire,?L?= 15 cm = 15 × 10-2?m

   

Step 2: Write out the magnetic flux density?B?equation

Step 3: Substitute in values

Step 4: Determine the direction of the B field

• • Using?Fleming’s left-hand rule :

F?= to the left

I?= vertically upwards

therefore,?B?= into the page