Fleming's Left-Hand Rule (Oxford AQA IGCSE Physics)

Fleming's Left-Hand Rule

• The direction of the force (also known as the thrust) on a current-carrying wire depends on the direction of the current and the direction of the magnetic field

• All three will be perpendicular to each other

  • This means that sometimes the force could be into and out of the page (in 3D)

• The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:

Fleming's left-hand rule

• Fleming’s left-hand rule can be used to show that the current-carrying wire in the magnetic field shown below will experience a downward force

Current into the plane of the page

Step 1: Determine the direction of the magnetic field

• Start by pointing your First Finger in the direction of the (magnetic) Field

Step 2: Determine the direction of the current

• Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current

Step 3: Determine the direction of the force

• The THumb will now be pointing down, in the direction of the THrust (the force)

• Therefore, this will be the direction in which the wire will move

Different orientation of the left-hand rule

• It is important to note that the direction of the force is reversed if either:

  • The direction of the current is reversed

  • The direction of the magnetic field is reversed

Electric Motors

• The motor effect can be used to create a simple electric motor

  • The force on a direct current-carrying coil (loop of wire) is used to make it rotate in a single direction

  • While the generator uses force on a coil to produce an alternating current, the motor uses a direct current to produce a force which rotates the coil

• The simple dc motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field

• The coil of wire forms a complete circuit with a cell

  • The coil is attached to a split ring (a circular tube of metal split in two)

  • This split ring is connected in a circuit with the cell via conducting carbon brushes

Forces on the horizontal coil in a motor

• Forces act in opposite directions on each side of the coil, causing it to rotate:

  • In the image above, current travels towards the cell through the blue side of the coil, so the force acts upwards (using Fleming's left-hand rule)

  • On the black side, current flows away from the cell so the force acts downwards

• After rotating 180°, the split ring reconnects with the carbon brushes and current flows through the coil again

  • Now the blue side is on the right and the black side is on the left

• Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped

  • The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right

  • The coil continues to rotate in the same direction, forming a continuously spinning motor

Forces on the coil when rotated 180°