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

Revision Note

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

Flemings Left Hand Rule: the thumb points up, representing the direction of force, the first finger points forward, representing the external magnetic field and the second finger points to the side, at right angles to the thumb and to the first finger, representing the direction of current
Fleming's left-hand rule can be used to determine the directions of the force, magnetic field and current
  • 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

Current points into the plane of the page, the external field points from left to right, IGCSE & GCSE Physics revision notes
Align your fingers with the diagram to use Fleming's left-hand rule

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

The thumb points down, the first finger points right and the second finger points into the page
You may have to move your hand around for different questions - this is fine as long as you remember which finger represents which quantity
  • 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

Examiner Tip

Remember that the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.

If you get a question about Fleming's left-hand rule in your exam, then you must actually use your hand to work it out. Don't worry, the invigilators will know what you are doing.

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

A current-carrying coil in a uniform field experiences an upwards force on the left side (coloured blue) and a downwards force on the right side (coloured black)
Forces acting in opposite directions on each side of the coil, causing it to rotate. The split ring connects the coil to the flow of the current
  • 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°

The black side is now on the left and blue is on the right, but the force on the left is still upwards and the right still experiences a downwards force
Even though the coil has flipped, the current still flows anticlockwise and the forces still cause rotation in the same direction

Worked Example

An electric motor is set up as shown below.

There are two magnetic poles, with south on the left and north on the right. The coil lies in between the poles and is connected to  a battery via a completed circuit and brushes. The battery is in front of the coil. The positive terminal of the battery is on the left and the negative terminal of the battery is on the right.

Determine whether the coil will be rotating clockwise or anticlockwise.

Answer:

Step 1: Draw arrows to show the direction of the magnetic field lines

  • These will go from the north pole of the magnet to the south pole of the magnet

The north pole is on the right and the south pole is on the left, so the field lines point to the left

Step 2: Draw arrows to show the direction the current is flowing in the coils

  • Current will flow from the positive terminal of the battery to the negative terminal

The current flows into the page on the side of the coil closest to the positive terminal and out of the page on the negative side of the coil

Step 3: Use Fleming’s left-hand rule to determine the direction of the force on each side of the coil

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

    • The magnetic field points from north to south

    • So the first finger points from right to left

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

    • Current flows from positive to negative

    • Current flows into the page on the positive side of the coil, and out of the page on the negative side of the coil

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

    • The force will act upwards on the positive side of the coil and downwards on the negative side of the coil

An arrow representing force points upwards on the left side of the coil and another force arrow points downwards on the right side of the coil

Step 4: Use the force arrows to determine the direction of rotation

  • The coil will be turning clockwise

Examiner Tip

You will get the same answer using either side of the coil as long as you have used the left-hand rule correctly. It is a good idea to test both sides to make sure your answer is the same.

The main thing you need to be aware of is that a coil in an external field tends to rotate, and you will have to determine the direction of this rotation using Fleming's left-hand rule.

The brushes and split rings explain how the current always points in the same direction relative to the magnet, but don't get caught up in the details of these when answering exam questions.

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