OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Magnetic fields: (d) Fleming’s left-hand rule
Fleming's left-hand rule predicts the direction of the force on a current-carrying conductor placed in an external magnetic field.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fleming's left-hand rule predicts the direction of the force on a current-carrying conductor placed in an external magnetic field. Hold the left hand with the thumb, first finger and second finger mutually perpendicular. The first finger points in the direction of the magnetic field, the second finger in the direction of conventional current, and the thumb gives the direction of the force, also called the motor effect force. The rule applies when the current is not parallel to the field; if current and field are parallel, the force is zero. The force direction is reversed if either the current direction or the field direction is reversed. This rule is used for motors and for charged particles treated as conventional current.
What earns the marks
- Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field.
- The first finger points in the direction of the magnetic field.
- The second finger points in the direction of conventional current.
- The thumb gives the direction of the force on the conductor.
- The three directions are mutually perpendicular.
- If the current is parallel to the field, the force is zero.
Common mistakes and how to put them right
Using the right hand instead of the left.
Put it right: Fleming's left-hand rule applies to the motor effect force on a current.
Pointing the second finger along electron flow.
Put it right: It must point along conventional current, opposite to electron flow.
Assuming a force always exists.
Put it right: If the current is parallel to the magnetic field, the force is zero.
Forgetting that reversing either the current or the field reverses the force direction.
Exam tips
- Set up your hand physically before answering, keeping the three directions mutually perpendicular.
- Identify which finger represents field, current and force before reading the options.
- If the question mentions electron flow, convert to conventional current direction first.
- Check whether the current is parallel to the field, because then the force is zero.
Try the lesson
The start of the Magnetic fields: (d) Fleming’s left-hand rule lesson, made from checked teaching notes for this specification point.
Listen
Fleming's left-hand rule predicts the direction of the force on a current-carrying conductor placed in an external magnetic field. Hold the left hand with the thumb, first finger and second finger mutually perpendicular. The first finger points in the direction of the magnetic field, the second finger in the direction of conventional current, and the thumb gives the direction of the force, also called the motor effect force. The rule applies when the current is not parallel to the field; if current and field are parallel, the force is zero. The force direction is reversed if either the current direction or the field direction is reversed. This rule is used for motors and for charged particles treated as conventional current. A strong answer earns marks for these points. Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field. The first finger points in the direction of the magnetic field. The second finger points in the direction of conventional current. The thumb gives the direction of the force on the conductor. Exam tip: Set up your hand physically before answering, keeping the three directions mutually perpendicular.
See it
Key words
Say what each word means before you turn the card.
Motor effect
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Direction of force on a conductor
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Conventional current versus electron flow
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Perpendicular field and current
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See every definition
- Motor effect
- The force whose direction is given by the thumb when using Fleming's left-hand rule on a current-carrying conductor in an external magnetic field.
- Direction of force on a conductor
- Given by the thumb of the left hand when the first finger points along the magnetic field and the second finger along conventional current.
- Conventional current versus electron flow
- The second finger must point along conventional current, which is opposite to electron flow.
- Perpendicular field and current
- The thumb, first finger and second finger are held mutually perpendicular; if current and field are parallel, the force is zero.
Label the diagram
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1Box 1
2Box 2
3Box 3
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