# CIE A Level Physics复习笔记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 5 mT

#### Worked Example

A 15 cm length of wire is placed vertically and at right angels to a magnetic field.When a current of 3.0 A flows in the wire vertically upwards, a force of 0.04 N acts on it to the left.Determine the flux density of the field and its direction.

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 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