K326 / K327 / 2027
Magnetism and electromagnetism overview

Lesson 7 of 7

Why a current-carrying coil turns

  • I can explain the turning effect on a current-carrying coil in a magnetic field.Syllabus

    Syllabus K326 / K327, 15(i). Explain the turning effect on a current-carrying coil in a magnetic field.

Opposite sides of a current-carrying coil carry current in opposite directions. In a magnetic field, the forces on those sides can form a pair that turns the coil.

A moment is the turning effect of a force about a pivot or axis. It depends on the force and the perpendicular distance from the axis to the force's line of action. Use Fleming's left-hand rule to find each magnetic force.

Follow the current around one coil

The two active sides are parts of one conducting loop. Current runs along one side and back along the other, so their directions are opposite. View the coil along its axle, with the magnetic field directed right.

Two active sides belong to one connected coil

First trace the current. Then look from the near end along the axle to work out the two forces.

Complete current path through one rectangular coilAn oblique drawing shows the two long active sides of one rectangular coil, A on the left and B on the right. A far connection joins their far ends. The near connection is interrupted by the two leads to a d.c. source. The long positive cell plate is on the right and the short negative plate on the left. Conventional current leaves positive, reaches B's near end, travels away along B to its far end, crosses the far connection towards A, and travels along A towards its near end before returning to negative. Thus an observer looking from the near ends sees current towards them in A and away in B. Brown arrows show current, not forces. No commutator structure is required for this turning-coil explanation.NearendFar connectionAB-+D.c. supplyTowardsAway

A carries current from far to near. B carries the same current from near to far. The opposite directions are two parts of one circuit.

End view: the forces make a clockwise turning pair

Field right, left current out and right current in give clockwise turningThe observer looks from the near end along the rotation axis, which is marked O at the centre. The north pole is left and south pole right, so the field runs right. Active side A is left of O and carries current out of this view, towards the observer, shown by a dot. Its magnetic force is up. Active side B is right of O and carries current into the view, shown by a cross. Its force is down. The equally long force arrows have distinct vertical lines of action separated horizontally, so their turning effects about O are both clockwise. Their resultant force is zero in the symmetric model, but the turning effect is not zero. The dashed horizontal line locates the coil's plane in this end view; the complete electrical connections are in the preceding diagram.NSFieldABForce upForce downOClockwise about ODot: towards you. Cross: away from you.

Green arrow: magnetic field. Brown dot/cross: current. Purple arrows: forces. Blue curve: turning sense. The force arrows use one qualitative scale.

The connection view identifies both active sides as parts of one coil. In the end view, the left side carries current out of the page and is forced up; the right side carries current into the page and is forced down.

Explain the rotation

Field right, left current out, right current in

  1. For the left side, rightward field and out-of-page current give upward force.
  2. For the right side, rightward field and into-page current give downward force.
  3. Upward force on the left gives a clockwise moment about the axle. Downward force on the right also gives a clockwise moment.
  4. The turning effects add, so the coil tends to turn clockwise in this view.

In the symmetric model, the forces are equal and opposite, so their resultant force is zero. Their lines of action are separated, however, so their moments do not cancel. Zero resultant force does not establish zero turning effect.

Reversing the current with the field fixed reverses both forces and the turning direction at this orientation. Reversing only the field has the same effect. Reversing both preserves the original turning direction.

As the coil turns, the perpendicular distances from the axle to the force lines change. At an orientation where those lines pass through the axle, there is no turning moment from them. Opposite forces produce a turning pair only when their lines of action have the required separation.

Put these in order

A rectangular coil carrying a current sits between the poles of a magnet, with its sides across the field. Put the explanation of why it turns in order.

Show the answer

1. Current flows in opposite directions along the two sides of the coil that lie across the field. A current across a magnetic field feels a force, given by Fleming's left-hand rule.

2. Each of those sides feels a force at right angles to both the current and the field. Opposite currents in the same field give forces in opposite directions.

3. One side is pushed up and the other side is pushed down. Two opposite forces on either side of the axis each have a moment about it, in the same sense.

4. Together the forces give a turning effect, so the coil rotates about its axis.

Check your understanding

Viewed along its axle, a coil has its left active side carrying current out of the page and its right side carrying current into the page. The field is rightward. Which explanation is correct?

  1. Both sides are forced upward because they are in the same field.
  2. The equal opposite forces cancel, so the coil cannot turn.
  3. Left side up, right side down, so the coil turns clockwise in this view.
Show the answer

Left side up, right side down, so the coil turns clockwise in this view.

Apply Fleming's left-hand rule to each side separately. Each force produces a clockwise moment about the axle, so their turning effects add even though their resultant force is zero in this symmetric model.