AQA A Level Physics复习笔记7.8.3 Magnetic Flux Density

• 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