Fleming's left-hand rule (HT only) — AQA GCSE Physics
Test yourself on Fleming's left-hand rule (HT only) with AQA GCSE practice questions.
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Fleming's left-hand rule (HT only) explained
This is Higher tier only. A current carrying wire produces its own magnetic field, and when it is placed in the field of a magnet the two fields interact: they reinforce on one side of the wire and partly cancel on the other, so the wire is pushed from the strong side towards the weak side. By Newton's third law the magnet is pushed back with an equal and opposite force, which is why a magnet resting on a balance shows a change in reading when the current is switched on. The force is greatest when the wire is at right angles to the field, and falls to zero when the wire lies along the field.
This is called the motor effect.
This is Higher tier only. The interaction between a current and a magnetic field, and the force it produces, is called the motor effect. The classic demonstrations are a wire resting on rails between the poles of a pair of magnadur magnets, which jumps when the switch is closed, and a current balance, in which the section of wire in the field moves down when the current flows. In a motor the two sides of the coil carry current in opposite directions, so the forces on them are opposite and the coil turns. In a moving-coil loudspeaker the field is radial, so the coil is pushed in and out along its axis rather than turning. Reverse either the current or the field and the force reverses; reverse both and it is unchanged.
Students should be able to show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.
This is Higher tier only. Fleming's left hand rule fixes how the three directions relate. Hold the thumb, first finger and second finger of the left hand mutually at right angles: the First finger points along the Field, from north to south; the seCond finger points along the Current, taken as conventional current from positive to negative; and the thuMb then gives the direction of the Motion, that is the force on the conductor. Set the two you know and read off the third. The rule works backwards too, so if a question gives the force and the field you can find which way the current must flow. It applies only when the current is at right angles to the field.
Students should be able to recall the factors that affect the size of the force on the conductor.
This is Higher tier only. The size of the force follows from F = B I l, so it depends on three things: the magnetic flux density B, which is how strong the magnet's field is, the current I in the conductor, and the length l of the conductor that is inside the field. Double any one of them and the force doubles. That is why a current balance question asking how to increase the force on the wire is answered by using a stronger magnet or by increasing the current. Orientation is a condition rather than a factor: the relationship holds when the conductor is at right angles to the field, and the force falls to zero when the wire lies along the field.
For a conductor at right angles to a magnetic field and carrying a current: f orce = magnetic f lux density × current × length F = B I l force, F, in newtons, N magnetic flux density, B, in tesla, T current, I, in amperes, A (amp is acceptable for ampere) length, l, in metres, m
This is Higher tier only. For a conductor at right angles to a magnetic field, force = magnetic flux density × current × length, written F = B I l, with F in newtons (N), B in tesla (T), I in amperes (A) and l in metres (m). Rearranged it gives B = F / (I l), I = F / (B l) and l = F / (B I). Most marks are lost on units, so convert centimetres to metres and milliamperes to amperes before substituting, and use only the length of conductor that lies inside the field. The equation carries assumptions that examiners ask you to state: the conductor is straight and at right angles to the field, the field is uniform and the current is constant.
Your focus
- Explain why a current carrying wire in a magnetic field experiences a force.
- State the effect on the magnet producing the field and justify it.
- Predict the force on a conductor placed parallel to the magnetic field.
Show all 15 objectives
- Name the effect and state the conditions needed to observe it.
- Describe a demonstration of the motor effect and state what is seen.
- Predict the change in direction of the force when the current, the field, or both are reversed.
- Use Fleming's left hand rule to find the direction of the force from a given current and field.
- Determine the direction of the current when the force and the field are given.
- Explain which hand is used and what each finger represents.
- List the three quantities that determine the size of the force on a current carrying conductor.
- Suggest a practical change to an apparatus that increases the force, and explain why it works.
- Explain why reversing the current changes the direction of the force but not its size.
- Calculate the force on a current carrying conductor using F = B I l with all quantities in SI units.
- Rearrange F = B I l to find the magnetic flux density or the current.
- State an assumption that must hold for the equation to be applied to a given situation.
Fleming's left-hand rule (HT only) exam tips
Marking Points
- one mark for stating that a force acts on a current carrying conductor placed in a magnetic field
- one mark for stating that the magnet experiences an equal and opposite force
- one mark for stating that the force is greatest when the conductor is at right angles to the field
- one mark for stating that no force acts when the current is parallel to the field
- one mark for naming the motor effect as the force on a current carrying conductor in a magnetic field
- one mark for describing a demonstration in which the wire moves when the switch is closed
- one mark for stating that reversing either the current or the field reverses the direction of the force
- one mark for stating that reversing both the current and the field leaves the direction unchanged
- one mark for using the left hand, with first finger for field, second finger for current and thumb for the force
- one mark for taking the field from north to south and the current as conventional current
- one mark for holding the three directions mutually at right angles
- one mark for stating the resulting direction of the force, or working back to the current or field when the force is given
- one mark for naming the strength of the magnetic field, or the magnetic flux density, as a factor
- one mark for naming the size of the current in the conductor as a factor
- one mark for naming the length of conductor inside the magnetic field as a factor
- one mark for suggesting a specific practical change, such as using a stronger magnet or increasing the current
- one mark for the equation F = B I l, quoted or correctly rearranged for the quantity required
- one mark for a correct substitution with every quantity in SI units, lengths converted to metres
- one mark for the answer given with the correct unit, newtons
- one mark for stating an assumption, such as the conductor being at right angles to the field or the field being uniform
Examiner Tips
- 💡Mention both objects; the mark scheme credits the force on the magnet as well as on the wire.
- 💡State the right angle condition whenever you describe the size of the force.
- 💡This content is Higher tier only, so expect it in explain questions of two or three marks.
- 💡Check what the question gives you: a current suggests the motor effect, a movement the generator effect.
- 💡Name the effect explicitly if the question asks what it is called.
- 💡This content is Higher tier only.
- 💡Write F, B and I against your fingers in the margin before you start.
- 💡Keep the three fingers genuinely perpendicular; a sloppy hand gives the wrong answer.
- 💡This content is Higher tier only, and is usually tested on a diagram.
- 💡'Suggest one change' means one clear change; a list can be marked on the first answer only.
- 💡Use the equation F = B I l to generate the factors rather than trying to recall a list.
- 💡This content is Higher tier only.
- 💡Write the equation, then the substitution, then the answer with its unit, on separate lines.
- 💡Check the right angle condition before using the equation at all.
- 💡This content is Higher tier only, and assumption questions are worth a mark on their own.
Common Mistakes
- saying only the wire experiences a force and the magnet does not
- expecting a force when the wire is lined up along the field direction
- explaining the effect by saying the magnet attracts the wire, as though the wire were magnetic material
- forgetting that there is no force at all until the current is switched on
- using the name motor effect for the generator effect, where movement produces a potential difference
- saying the wire moves because it is attracted to the magnet
- assuming the effect needs an alternating current
- expecting the force to reverse when both the current and the field are reversed
- using the right hand, which belongs to the generator effect
- taking the current as the direction electrons move rather than as conventional current
- pointing the first finger from south to north
- applying the rule when the current is parallel to the field, where there is no force at all
- answering 'increase the voltage' without linking it to an increase in current
- naming the total length of the wire rather than the length of it that is inside the field
- listing reversing the current as a way to increase the force, when it only changes the direction
- offering the mass of the wire or the resistance of the circuit as factors
- substituting a length in centimetres without converting it to metres
- using the whole length of the wire when only part of it lies in the field
- confusing magnetic flux density in tesla with force in newtons
- rearranging incorrectly and dividing by B when the force is what is required