• 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:

• 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 straight conductor 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

Step 1: Write out the known quantities

Force on wire, F = 0.04 N

Current, I = 3.0 A

Length of wire = 15 cm = 15 × 10^-2?m

Step 2:?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