Fleming's left-hand rule

The idea

With the thumb, forefinger and middle finger of the left hand mutually perpendicular, if the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of the force (motion) on the conductor.

Three fingers

A rod carries a current in a magnetic field. The field, the current and the force on the rod are at right angles to one another. A scientist named Fleming found a way to remember them with the fingers of one hand.

Which hand does the rule use?

• The left hand
• The right hand
• Either hand gives the same result

Fingers and directions

Press Next step to match the fingers with the field, the current and the force.

A rule for the force

Stretch the thumb, the forefinger and the middle finger of the left hand so that they are at right angles to each other. The forefinger shows the field, the middle finger shows the current, and the thumb shows the force on the conductor.

Fleming's left-hand rule: with the thumb, forefinger and middle finger of the left hand mutually perpendicular, if the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of the force (motion) on the conductor.

Notes

Fleming's left-hand rule: forefinger along the field, middle finger along the current, thumb gives the force on the conductor.

Check yourself

Which matching of fingers is right in Fleming's left-hand rule?

The magnetic field points into the page and the current in a rod flows to the right. By Fleming's left-hand rule, the force on the rod points…

In the same field, into the page, the current in the rod is reversed to flow to the left. The force on the rod now points…

Which rule gives the direction of the force on a current-carrying conductor in a magnetic field?

• Forefinger: current, middle finger: field, thumb: force. The forefinger points along the magnetic field and the middle finger along the current.
• Forefinger: force, middle finger: current, thumb: field. The thumb gives the direction of the force. The forefinger points along the field.
• Forefinger: field, middle finger: current, thumb: force — correct. Yes!
• up the page — correct. Yes!
• down the page. With the forefinger into the page and the middle finger to the right, the thumb points up the page.
• to the right, along the current. The force is perpendicular to both the current and the field, so it does not point along the current.
• up the page. Reversing the current reverses the force, so it now points down the page.
• down the page — correct. Yes!
• to the left, along the current. The force is perpendicular to both the current and the field, so it does not point along the current.
• The right-hand thumb rule. The right-hand thumb rule gives the direction of the field lines around a current-carrying conductor.
• Maxwell's corkscrew rule. Maxwell's corkscrew rule says the same as the right-hand thumb rule: it gives the direction of the field round a current.
• Fleming's left-hand rule — correct. Yes!