Fleming's left-hand rule (HT only) — AQA GCSE Combined Science
Test yourself on Fleming's left-hand rule (HT only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Fleming's left-hand rule (HT only) explained
When a current-carrying conductor is placed in a magnetic field, the field of the magnet and the field around the conductor interact.
Read the full explanation
The result is a force on the conductor, and by Newton's third law the conductor exerts an equal and opposite force on the magnet. This is the motor effect (applicable to both tiers). The force is greatest when the conductor is at 90° to the magnetic field lines and zero when parallel. For Higher Tier students, the direction of the force is found using Fleming's left-hand rule: the thumb gives the force (motion), the first finger gives the magnetic field (north to south), and the second finger gives conventional current. Reversing current or field reverses the force.
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.
Fleming's left-hand rule is a memory aid for the motor effect. Hold the left hand with the thumb, first finger and second finger mutually at right angles. Point the first finger along the external magnetic field, from north to south. Point the second finger in the direction of conventional current, from positive to negative. The thumb then points in the direction of the force on the conductor. For example, if the field points left to right and the current points vertically upwards, the thumb reveals the force direction is into the page. The rule shows relative orientation only. Reversing either the current or the field reverses the force, while reversing both leaves it unchanged.
Students should be able to recall the factors that affect the size of the force on the conductor.
The force on a current-carrying conductor in an external magnetic field depends on several factors. Increasing the current increases the force. Increasing the magnetic flux density of the external field increases the force. Increasing the length of conductor within the field increases the force. The angle between the conductor and the field also matters: the force is greatest when the conductor is perpendicular to the field and zero when parallel. For a conductor at right angles to the field, these combine as F = B I l, where F is in newtons, B in tesla, I in amperes and l in metres. For example, doubling the current while keeping B and l fixed doubles the force. The force acts perpendicular to both the field and the conductor.
For a conductor at right angles to a magnetic field and carrying a current:
When a current-carrying conductor sits at right angles to a magnetic field, the fields interact to produce a force on the conductor. The force is perpendicular to both the current and the magnetic field. The magnitude of this force is calculated using the equation F = B I l, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A), and l is the length in metres (m). For example, a 2.0 m wire carrying 3.0 A in a 0.5 T field experiences a force of 3.0 N. Fleming's left-hand rule predicts the force direction: point the first finger along the field, the second finger along the current, and the thumb shows the force. If the conductor is parallel to the field, no force acts.
force = magnetic flux density × current × length
The force on a straight current-carrying conductor at right angles to a uniform magnetic field is calculated using F = B × I × l, where F is force in newtons (N), B is magnetic flux density in tesla (T), I is current in amperes (A) and l is the length of conductor in the field in metres (m). The equation applies when the conductor is perpendicular to the field. If the conductor is at an angle, only the component of length perpendicular to the field contributes, but at GCSE you use the full length when at right angles. For example, a 0.20 m wire carrying 3.0 A in a 0.50 T field experiences F = 0.50 × 3.0 × 0.20 = 0.30 N. Rearranging allows calculation of B, I or l.
F = B I l
This Higher Tier (HT) only equation calculates the force on a straight current-carrying conductor at right angles to a uniform magnetic field. F is force in newtons, B is magnetic flux density in tesla, I is current in amperes and l is the length of conductor inside the field in metres. The force is greatest when perpendicular to the field and zero when parallel. For example, a 0.20 m wire carrying 3.0 A at 90° to a 0.50 T field experiences F = 0.50 × 3.0 × 0.20 = 0.30 N. The direction of this force is found using Fleming's left-hand rule: first finger along the field, second finger along the current, and thumb showing the force.
force, F, in newtons, N
This Higher Tier (HT) only statement identifies the quantity F in the equation F = B I l as force, measured in newtons (N). Force is a vector, so its direction matters as well as its size; in this context it acts on the current-carrying conductor and is perpendicular to both the magnetic field and the current. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s². In calculations, F is found by multiplying magnetic flux density B in tesla, current I in amperes and length l in metres. For example, 0.50 T × 3.0 A × 0.20 m gives 0.30 N. Fleming's left-hand rule gives the direction of this force, with the thumb indicating the force direction.
magnetic flux density, B, in tesla, T
Magnetic flux density, B, measures the strength of a magnetic field. It is a vector quantity, meaning direction matters as well as magnitude. The unit is the tesla, T, where 1 T equals 1 newton per ampere per metre (1 N A⁻¹ m⁻¹). In the motor effect, a wire of length L carrying current I at right angles to a uniform magnetic field experiences a force F = B I L, so B = F ÷ (I L). For example, if a 0.20 m wire carrying 3.0 A feels a force of 0.60 N perpendicular to a magnetic field, then B = 0.60 N ÷ (3.0 A × 0.20 m) = 1.0 T. The magnetic flux density indicates how close the magnetic field lines are; closer lines mean a higher flux density and a stronger magnetic field.
current, I, in amperes, A (amp is acceptable for ampere)
Electric current, I, is the rate of flow of electric charge, measured in amperes, A, where one ampere is one coulomb of charge passing a point each second (1 A = 1 C s⁻¹). The term 'amp' is completely acceptable for ampere in both speech and written exam answers. In the motor effect, current in a conductor placed in a magnetic field produces a force, and Fleming's left-hand rule relates the mutually perpendicular directions of current, field and force. For example, a current of 2.0 A in a 0.50 m wire perpendicular to a 0.30 T field gives F = B I L = 0.30 T × 2.0 A × 0.50 m = 0.30 N. Current is measured with an ammeter connected in series, and conventional current flows from positive to negative outside the source.
length, l, in metres, m
In motor-effect calculations, the length l is the length of the conductor that lies inside the magnetic field and is perpendicular to the field lines. It is measured in metres, m, so always convert centimetres or millimetres before substituting into F = B I l. For example, a 5.0 cm wire inside a 0.20 T field carrying 3.0 A gives l = 0.050 m, so F = 0.20 T × 3.0 A × 0.050 m = 0.030 N. The equation F = B I l applies when the wire is at right angles to the magnetic field. The symbol l is italic because it is a quantity, while the unit symbol m is not italic. Recording l with its unit keeps the calculation consistent with B in tesla and I in amperes.
Your focus
- Describe the motor effect and the conditions under which it occurs.
- Use Fleming's left-hand rule to determine the direction of the force on a current-carrying conductor.
- Explain how reversing current or magnetic field affects the direction of the force.
Show all 30 objectives
- Label the first finger, second finger and thumb of Fleming's left-hand rule with field, current and force respectively.
- Apply the rule to a given arrangement to determine the direction of the force on a current-carrying conductor.
- Predict how reversing the current, the field or both changes the direction of the force.
- List the factors that affect the size of the force on a current-carrying conductor in a magnetic field.
- Describe how changing current, magnetic flux density, length or angle changes the force.
- Use F = B I l to calculate the force when the conductor is perpendicular to the magnetic field.
- State the direction of the force on a current-carrying conductor at right angles to a magnetic field.
- Recall and apply the equation F = B I l to calculate the force on a conductor.
- Explain the effect of reversing the current or the magnetic field on the direction of the force.
- Recall and use the equation force = magnetic flux density × current × length.
- Calculate the force on a current-carrying conductor in a magnetic field using correct units.
- Rearrange the equation to determine magnetic flux density, current or length.
- Recall and use the equation F = B I l to calculate the force on a current-carrying conductor.
- Identify the meaning and SI unit of each symbol in the equation.
- Apply the condition that the conductor must be at right angles to the magnetic field for the equation to give the maximum force.
- Identify F as force and state its SI unit, the newton (N).
- Explain that force is a vector and describe how its direction is found using Fleming's left-hand rule.
- Use the unit N correctly when reporting values calculated from F = B I l.
- State that magnetic flux density B is measured in tesla, T.
- Calculate magnetic flux density using B = F ÷ (I L) for a conductor perpendicular to a uniform field.
- Describe how the direction of the force depends on the directions of the current and the magnetic field.
- State that current I is measured in amperes, A, and that amp is acceptable for ampere.
- Describe current as the rate of flow of charge and measure it with an ammeter in series.
- Apply the relationship F = B I L to explain how current affects the force on a conductor in a magnetic field.
- Identify the length l as the conductor length inside the magnetic field, measured in metres.
- Convert a length from centimetres or millimetres into metres correctly.
- Apply F = B I l with consistent SI units to find the force on a current-carrying conductor.
Fleming's left-hand rule (HT only) exam tips
Marking Points
- A current-carrying conductor in a magnetic field experiences a force; this is the motor effect.
- The force arises because the magnetic field of the current interacts with the magnetic field of the magnet.
- The conductor and the magnet exert equal and opposite forces on each other, consistent with Newton's third law.
- The force is maximum when the conductor is at 90° to the magnetic field and zero when it is parallel to the field.
- (Higher Tier) Fleming's left-hand rule predicts the direction of the force: thumb = force, first finger = magnetic field, second finger = conventional current.
- Reversing the current or reversing the magnetic field reverses the direction of the force.
- States that the first finger represents the direction of the external magnetic field, conventionally from north to south.
- States that the second finger represents the direction of conventional current in the conductor.
- States that the thumb represents the direction of the force acting on the conductor.
- Explains that the three directions are mutually perpendicular, so the rule gives relative orientation rather than magnitude.
- Applies the rule to a described arrangement, for example identifying the force direction when field and current directions are given.
- Recognises that reversing the current or the magnetic field reverses the force, whereas reversing both leaves the force direction unchanged.
- Recalls that increasing the current in the conductor increases the force.
- Recalls that increasing the magnetic flux density of the external field increases the force.
- Recalls that increasing the length of conductor within the magnetic field increases the force.
- Explains that the angle between the conductor and the field affects the force, with maximum force when perpendicular and zero force when parallel.
- Uses the relationship F = B I l for a conductor perpendicular to the field to calculate or compare forces.
- Applies proportional reasoning, for example stating that doubling one factor while others remain constant doubles the force.
- States that the force acts at right angles to both the magnetic field and the current direction.
- Identifies the equation F = B I l to calculate the force on a conductor at right angles to a magnetic field.
- Defines the terms in the equation: F is force in newtons, B is magnetic flux density in tesla, I is current in amperes, and l is length in metres.
- Describes the left-hand rule: first finger = magnetic field (N to S), second finger = current, thumb = force/motion.
- Recognises that if the conductor is parallel to the field, the force is zero.
- Recall and apply the equation force = magnetic flux density × current × length.
- Use the correct units: force in N, magnetic flux density in T, current in A, length in m.
- Rearrange the equation to find magnetic flux density, current or length when required.
- Substitute numerical values correctly and evaluate the force, including appropriate significant figures or decimal places.
- Recognise that the equation applies when the conductor is at right angles to the magnetic field.
- State that F is the force on the conductor in newtons (N) and that B is the magnetic flux density in tesla (T).
- Identify I as the current in amperes (A) and l as the length of the conductor within the magnetic field in metres (m).
- Substitute values into F = B I l and calculate correctly, including the unit N with the answer.
- Recognise that the equation applies when the conductor is at right angles to the magnetic field, and that the force is zero when the conductor is parallel to the field.
- Use Fleming's left-hand rule to determine the direction of the force once the magnitude has been calculated.
- State that F represents force and that its SI unit is the newton (N).
- Recognise that force is a vector quantity, so both magnitude and direction are needed.
- Link the unit N to the equation F = B I l, where tesla × ampere × metre gives newtons.
- Use Fleming's left-hand rule to establish the direction of the force on the conductor.
- Quote calculated force values with the unit N and an appropriate number of significant figures.
- State that magnetic flux density B is measured in tesla, T, and describe it as the strength of a magnetic field.
- Use the motor-effect relationship F = B I L to calculate B when force, current and length perpendicular to the field are known.
- Recognise that 1 T is equivalent to 1 N A⁻¹ m⁻¹, linking the unit to the defining equation.
- Explain that B is a vector quantity, so reversing the magnetic field direction reverses the force direction.
- Interpret typical field strengths, for example the Earth's field is around 5 × 10⁻⁵ T while an MRI scanner field can exceed 1 T.
- State that current I is measured in amperes, A, and that 'amp' is an acceptable written and spoken form of ampere.
- Define current as the rate of flow of charge, I = Q ÷ t, with charge in coulombs and time in seconds.
- Use I in the motor-effect equation F = B I L, identifying current as the flow through the conductor perpendicular to the magnetic field.
- Describe how to measure current with an ammeter connected in series, and explain that current is the same everywhere in a single closed loop.
- Apply Fleming's left-hand rule to predict the direction of force from the directions of current and magnetic field.
- Recognise that increasing current increases the force on the conductor in a magnetic field, for a fixed field and length.
- State that l is the length of conductor within the magnetic field, measured in metres (m).
- Convert lengths given in cm or mm into metres before using F = B I l.
- Recognise that the equation F = B I l is used when the conductor is perpendicular to the magnetic field.
- Keep the unit symbol m separate from the quantity symbol l, and give the final force in newtons.
- Substitute values correctly into F = B I l, for example 0.20 T × 3.0 A × 0.050 m = 0.030 N.
Examiner Tips
- 💡State that the conductor must be at an angle to the magnetic field, with maximum force at 90°, to explain why the motor effect occurs.
- 💡When using Fleming's left-hand rule, identify each finger's meaning before applying it to the diagram.
- 💡Remember that reversing either the current or the magnetic field reverses the force, but reversing both leaves the force direction unchanged.
- 💡Sketch a simple hand diagram and label first finger field, second finger current and thumb force before substituting values.
- 💡Check the current direction carefully: conventional current flows from positive to negative, opposite to electron flow.
- 💡When a question reverses a direction, state the effect on the force explicitly rather than redrawing the whole arrangement.
- 💡Write the relationship as F = B I l and check that each quantity is in its standard unit before substituting.
- 💡For comparison questions, change one factor at a time and state the proportional effect on the force.
- 💡If the conductor is not perpendicular to the field, explain that the component of length perpendicular to the field determines the force.
- 💡Always check that length is in metres and current is in amperes before substituting into F = B I l.
- 💡Sketch a simple diagram with the field, current and force arrows labelled at right angles to each other to secure method marks.
- 💡When asked to describe the rule, name each finger and what it represents in a clear sequence.
- 💡Write the equation, then substitute values with units before calculating to reduce errors.
- 💡Check that the conductor is at right angles to the field; if not, the simple equation may not apply directly.
- 💡Give the unit with your final answer and round sensibly, matching the precision of the data.
- 💡Write the equation, then substitute values with units before calculating to reduce arithmetic slips.
- 💡Check that the conductor is perpendicular to the field; if it is parallel, the force is zero and the equation does not apply directly.
- 💡As this is Higher Tier only content, be prepared to combine this calculation with Fleming's left-hand rule to find both magnitude and direction.
- 💡Always include the unit N with a calculated force value.
- 💡When asked for direction, refer explicitly to Fleming's left-hand rule and name the relevant fingers.
- 💡Remember that questions involving F = B I l and Fleming's left-hand rule will only appear on Higher Tier papers.
- 💡Always write the unit T after a calculated flux density and check that the rearranged equation gives the correct subject before substituting numbers.
- 💡When a question gives force, current and length, identify them clearly and show the rearrangement B = F ÷ (I L) before calculating.
- 💡Use Fleming's left-hand rule to confirm the direction of force, current or field, and state that the three directions are mutually perpendicular.
- 💡Write the unit A or amp after every current value and check that the ammeter is described as connected in series.
- 💡When using F = B I L, confirm that the current direction is perpendicular to the magnetic field before substituting values.
- 💡Use the left-hand rule with the first finger for field, second finger for current and thumb for force, and state the direction clearly.
- 💡Underline the length value and its unit in the question, then convert to metres before calculating.
- 💡Write the equation, substitute values with units, and state the force unit in your answer.
- 💡Ensure you only use the length of the wire that is actually within the magnetic field when calculating force.
Common Mistakes
- Using the right hand for Fleming's left-hand rule; correction: use the left hand, with the thumb, first finger and second finger mutually perpendicular.
- Confusing the direction of conventional current with electron flow; correction: the second finger represents conventional current, from positive to negative.
- Thinking the force acts only on the magnet or only on the conductor; correction: the conductor and magnet exert equal and opposite forces on each other.
- Using the right hand instead of the left hand: the error gives a reversed force direction; correction is to reserve the left hand for motor-effect force questions and the right hand for generator-effect questions.
- Confusing which finger represents current and which represents field: the error swaps two directions; correction is to remember first finger field, second finger current, thumb thrust or force.
- Treating the rule as giving the size of the force: the error ignores that magnitude depends on current, field strength, length and angle; correction is to use the rule only for direction and use F = B I l for magnitude when the conductor is perpendicular to the field.
- Thinking that a longer conductor always increases the force regardless of position: the error ignores that only the length within the field and the angle matter; correction is to consider the effective length perpendicular to the field.
- Believing that the force is largest when the conductor is parallel to the field: the error reverses the geometry; correction is that the force is zero when parallel and maximum when perpendicular.
- Confusing magnetic flux density B with force F or with current I: the error mixes symbols and units; correction is to associate B with tesla, I with ampere and F with newton.
- Omitting the equation F = B I l when asked to calculate the force: the error prevents finding the magnitude; correction is to recall and apply the formula with correct units.
- Using the right hand instead of the left hand: the left hand is used for the motor effect, while the right hand is used for the generator effect.
- Pointing the first finger along the current and the second finger along the field: the first finger is the field and the second finger is the current.
- Using length in centimetres instead of metres: convert cm to m by dividing by 100 before substituting.
- Confusing magnetic flux density B with magnetic field strength or using the wrong unit, such as N instead of T.
- Forgetting to rearrange the equation correctly when finding B, I or l, leading to multiplying instead of dividing.
- Using the total length of the wire instead of only the length inside the magnetic field; correct by identifying the portion of conductor between the magnetic poles.
- Mixing up B and I in the equation; correct by checking units, since B is in tesla and I is in amperes.
- Forgetting to convert centimetres to metres before substituting; correct by dividing the length in cm by 100 to obtain metres.
- Treating force as a scalar and giving only a number without direction; correct by stating the direction using Fleming's left-hand rule.
- Writing the unit as N m or N/m; correct by remembering that force is measured simply in newtons, N.
- Confusing force with magnetic flux density; correct by noting that B is measured in tesla while F is measured in newtons.
- Confusing the quantity symbol with the unit; correction: the quantity symbol is italic B, while the unit is the tesla, T.
- Using B = F ÷ (I L) when the wire is parallel to the field; correction: the equation applies when the wire is perpendicular to the magnetic field, and force is zero when parallel.
- Writing the unit as 'teslas' with an incorrect symbol or omitting the capital T; correction: the unit symbol is T.
- Connecting an ammeter in parallel with a component; correction: an ammeter must be connected in series so the current to be measured passes through it.
- Confusing current with potential difference or charge; correction: current is the rate of charge flow in amps, potential difference is energy per unit charge in volts, and charge is in coulombs.
- Using the symbol A for both the ampere and area; correction: in equations, use I for current and A for area, and include units to avoid ambiguity.
- Substituting a length in centimetres directly into F = B I l; correct this by dividing by 100 to convert to metres first.
- Using the whole wire length when only part of it lies in the magnetic field; correct this by identifying only the length inside the field.
- Treating l as a unit rather than a quantity; correct this by writing l = 0.050 m, where l is the quantity and m is its unit.