20.1.4 Magnetic Flux Density

Magnetic Flux Density Definition

• 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 5mT

Worked example: Calculating flux density

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

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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