2.3.6Interactions Over Small and Large Distances

Fleming’s Left-Hand Rule (HT Only)

5 min read 9 parts Lesson 25 of 26 1 quiz

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Section 1 of 9The rule

Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field. Hold the thumb, first finger and second finger of the left hand at right angles to each other.

1First finger

First finger points in the direction of the Field (north to south).

2Second finger

Second finger points in the direction of the Current (conventional, positive to negative).

3Thumb

Thumb points in the direction of the Thrust or Motion, the force.

A memory deviceFBI, Force, B-field, I-current, or 'seCOND finger = current'.

Section 2 of 9Using the rule step by step

Work out the direction of the field: from the north pole to the south pole.

Work out the conventional current direction: away from the positive terminal of the cell.

Set your left hand so that the first finger points along the field and the second finger along the current.

Your thumb points the way the wire is pushed.

Practice with diagrams where the field goes into or out of the page. A dot (●) means the field is coming out of the page towards you; a cross (×) means it goes into the page.

Section 3 of 9Worked directions

Suppose the field points to the right across a page and the current flows up the page. Left-hand rule: first finger pointing right, second finger pointing up, and the thumb points into the page, away from you. So the wire is pushed into the page. If the current is reversed, the force is out of the page.

Section 4 of 9Which hand, which rule?

Fleming's left-hand rule is for the motor effect: current and field are known, find the force. Fleming's right-hand rule is for generators and induction (6.2.1): motion and field are known, find the induced current. The mnemonic is that motors use the left hand. The right-hand grip rule (6.1.5) is a different rule for the field produced by a current.

Section 5 of 9Applications

Applications

Finding which way a motor coil spins and checking the direction of torque.

Predicting the way a loudspeaker cone moves for a given current direction.

Predicting the deflection of a moving charged particle in a field (6.1.10): the thumb gives the force for a positive charge, with the current direction being the direction of positive charge motion.

Section 6 of 9Practice problems for the rule

Always work in three stepsdraw the field arrows, mark the conventional current, then apply the rule.

Consider a coil in a motor: the left side carries current towards you, the right side away from you, and the field points left to right.

On the left side, field to the right and current out of the page give a force upwards; on the right side the current is into the page,

so the force is downwards.

The coil therefore rotates clockwise as seen from the front.

When the coil reaches the vertical position, the commutator reverses the current so the forces keep turning the coil in the same direction.

The rule only gives direction. To find the size of the force use F = BIl (6.1.6).

Common situations to practise

Three layouts crop up constantly.

(1) A straight wire between two poles with the field horizontal and the current along the wire: the force is vertical, so the wire lifts or drops.

(2) A rectangular coil in a motor, where the two long sides feel equal and opposite forces and the coil turns.

(3) A beam of charged particles, where you replace 'current' with the direction a positive charge moves.

In every case the three directions field, current and force are mutually perpendicular, and if two of them are parallel there is no force.

If a problem gives the force and the field and asks for the current direction, simply rotate the hand so the thumb and the first finger match,

then read the direction of the second finger.

Section 7 of 9Worked examples

A wire carries current from left to right between the poles of a magnet. The field is from the top of the page (N) to the bottom (S). In which direction is the force?
In the same set up the magnet is turned round so that the field points up the page. What happens to the force?

Section 8 of 9Key terms

Term	Meaning
Fleming's left-hand rule	Rule giving direction of force on a conductor in a field: First finger field, seCond finger current, thuMb motion.
Conventional current	Current direction from positive to negative.
Thrust	The force on the wire.
Field into page	Shown as crosses (×).

Section 9 of 9Exam technique

Using the right hand.

Using the electron flow direction instead of conventional current.

Not keeping the three digits mutually at right angles.

Confusing thumb with the direction of the current.