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Electromagnetic Effects: Question 2

Syllabus 4.5.4

Multiple choice Extended 1 mark

A straight horizontal wire passes between the poles of a magnet, at right angles to the magnetic field. The magnetic field points vertically upward through the wire. The current in the wire flows into the page, away from you.

Using Fleming's left-hand rule, what is the direction of the force (the motor effect) on the wire?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall Fleming’s left-hand rule

For a current-carrying conductor in a magnetic field, Fleming’s left-hand rule gives the direction of the resulting force:

  • First finger →\to Field
  • seCond finger →\to Current
  • thUmb →\to Ultimate direction of motion (the force)

The three directions (field, current, force) are always mutually at right angles to one another.

Step 2: Set up the left hand for this situation

Point the First finger vertically upward, in the direction of the field. Point the seCond finger into the page, away from you, in the direction of the current. These two fingers, held at right angles, fix the orientation of the left hand.

Step 3: Read off the direction of the thumb

With the First finger up and the seCond finger pointing into the page, the thUmb of the left hand points out to the right. This is the direction of the force on the wire.

Why the other options are wrong

OptionDirectionError
BTo the leftLeft hand set up back-to-front (equivalent to using the right hand)
CAlong the field, outwardForce is at right angles to the field, not parallel to it
DAlong the field, inwardForce is at right angles to the field, not parallel (and in the wrong sense)

Final answers

  • The force on the wire acts to the right, option A.