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Force on a current-carrying conductor and Fleming’s left-hand rule

Key idea: O Level electromagnetism: force on a current-carrying conductor in a magnetic field, and Fleming’s left-hand rule.

  • SEC G3 Physics 2027
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Learning objectives

  • State the properties of magnets
  • Describe induced magnetism
  • Distinguish temporary and permanent magnets
  • Determine magnetic-field direction with a compass or bar magnet
  • Interpret bar-magnet field patterns
  • Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
  • Interpret the field pattern around a straight current-carrying wire
  • Draw the magnetic field pattern around a straight current-carrying wire
  • Interpret the field pattern around a current-carrying solenoid
  • Draw the magnetic field pattern around a current-carrying solenoid
  • Relate current magnitude and direction to magnetic field
  • Describe electromagnet applications
  • Describe experiments showing the force on a current-carrying conductor in a magnetic field
  • Describe magnetic force on a charged-particle beam
  • Predict force reversal when current or field reverses
  • Use Fleming’s left-hand rule
  • Explain the turning effect on a current-carrying coil
  • Explain how current and turns increase the turning effect
  • Describe split-ring commutator action
  • Describe the effect of winding a motor coil on a soft-iron cylinder
  • Deduce that a changing magnetic field can induce an e.m.f.
  • Deduce that induced e.m.f. opposes the change producing it
  • Deduce factors affecting induced e.m.f. magnitude
  • Describe a simple a.c. generator and slip rings
  • Sketch a simple a.c. generator voltage–time graph
  • Describe a simple iron-cored transformer
  • Apply ideal-transformer equations
  • Explain cable loss and high-voltage transmission

1. Definition

When a current-carrying conductor is placed in a magnetic field, it experiences a force if the current direction is not parallel to the magnetic field direction.

2. Key Ideas

  • No force when current is parallel to the magnetic field.
  • Maximum force when current is perpendicular to the magnetic field.
  • The force direction is perpendicular to both current and field.
  • Use Fleming’s left-hand rule to find the force direction.
  • Reversing current or reversing field reverses the force direction (reversing both keeps it the same).
  • In end-on diagrams, ⊙ means out of the page and ⊗ means into the page.
What you need for this course

You should be able to describe the force on a current-carrying conductor in a magnetic field and use Fleming’s left-hand rule.

3. Detailed Explanations

A. When does the force act?

The force acts when current is not parallel to the field (i.e. there is a component of current across the field).

B. Direction of force (Fleming’s left-hand rule)

Hold your left hand with the three directions at right angles:

  • first finger: magnetic field (N → S)
  • second finger: current (conventional current)
  • thumb: force (motion of the wire)
Current-produced fields and the motor effectThree panels show an anticlockwise field around current out of the page; a longitudinal section through a solenoid, with current out of the page in the upper parts of every turn and into the page in the lower parts, giving internal field to the right and a north pole at the right end; and the motor-effect directions for field right, current out of the page and force up.Straight wireSolenoidMotor effectcurrent out of pagethumb points out; fingers curlanticlockwise around the wireLarger current → stronger field.Further away → weaker field.Section through the coil axisupper parts: I out of pageSNlower parts: I into pageinside field: S → NEach upper/lower pair belongsto the same coil turn.Curl fingers with current;thumb points right, to N.Return field outside not shown.B: N → SBIout of pageFupFleming’s left handfirst finger → field Bsecond finger → current Ithumb → force FReverse B or I → F reverses.Reverse both → F is unchanged.
Scroll diagram horizontally to read all labels.
Use the right-hand grip rule for fields made by currents; use Fleming’s left-hand rule for the force on a current in an external magnetic field.

C. What happens when you reverse directions?

  • Reverse the current → force reverses.
  • Reverse the magnetic field → force reverses.
  • Reverse both → force stays the same direction.

D. Experiment with a current-carrying conductor

  1. Place a light horizontal conductor between the poles of a strong magnet so the conductor is perpendicular to the magnetic field. Connect it to a low-voltage d.c. supply and a switch.
  2. Close the switch briefly. Observe the conductor move or deflect, showing that a force acts when current crosses the magnetic field.
  3. Reverse the supply connections but keep the magnet fixed. The conductor deflects in the opposite direction, so reversing current reverses the force.
  4. Restore the current and turn the magnet around. The deflection reverses again, so reversing the field reverses the force.

Keep the same current magnitude, conductor position and magnet separation when comparing directions. Switch off between trials to limit heating. If the conductor is parallel to the field, it should not deflect; this provides a useful comparison trial.

E. Experiment with a beam of charged particles

An electron beam (for example, in an electron-beam tube) is a stream of moving negative charges. Conventional current is opposite to the direction of electron motion, so convert the electron direction before applying Fleming’s left-hand rule.

In a classroom electron-beam tube or an equivalent teacher demonstration:

  1. Produce a narrow electron beam and note its undeflected path on the fluorescent screen.
  2. Apply a magnetic field across the beam and observe that its path curves or shifts.
  3. Reverse the magnetic field while keeping the beam setting unchanged. The deflection reverses.
  4. If the apparatus allows the beam direction to be reversed safely, reverse that direction while keeping the field fixed. The force reverses again.

These observations show that a magnetic force acts on moving charges and that its direction depends on both the field direction and the direction of conventional current. Electron-beam equipment uses high voltage and must be operated only as instructed by a teacher or laboratory technician.

Check your understanding

In each experiment, why must you reverse only one direction at a time? State what you would conclude if both current and magnetic field were reversed together and the force direction stayed unchanged.

Simulation checkpoint

In the Magnetism & Induction Lab, choose the motor-effect view. Set field to the right and current out of the page (⊙), predict the force, then check it. Reverse only the current, only the field, and finally both; record the force direction each time.

Link

Magnetic field direction is N → S outside a magnet: Magnetic Field & Magnetic Field Lines.

4. Common Mistakes

  • Using electron flow instead of conventional current in Fleming’s left-hand rule.
  • Forgetting the condition for no force (current parallel to field).
  • Mixing up right-hand grip rule (field around a wire) with left-hand rule (force on a wire).
  • Treating electron motion as conventional current; the two directions are opposite.

5. Exam Tips

  1. Start by drawing arrows for field and current.
  2. Apply Fleming’s left-hand rule to get the force direction.
  3. If the question mentions reversing current/field, state clearly what happens to the force direction.

6. Worked Examples

Modelled example 1

Finding force direction

Core

Problem

The magnetic field is into the page and conventional current is to the right. Determine the force direction on the wire.
Study the worked solution

  1. Set the field finger

    Method

    Point the left first finger into the page.

    Reason

    The first finger represents magnetic field from N to S.

    Working

    Field: ⊗.

  2. Set the current finger

    Method

    Point the second finger to the right.

    Reason

    The second finger represents conventional current.

    Working

    Current: →.

  3. Read the thumb

    Method

    State that the force is upward.

    Reason

    The thumb gives the force direction when field and current are set.

    Working

    Force: ↑.

Guided practice 2

Reversing current

About 4 min

Problem

Keep the field from Example 1 into the page, but reverse current so it points left. What happens to the force?

Reverse one input direction only

Hints

Hint 1: one reversal
Reversing either current or field reverses force.
Hint 2: starting force
The original force was upward.
View solution step by step

  1. Reverse the force

    Method

    State that the force becomes downward.

    Reason

    Current reverses while magnetic field remains fixed.

    Working

    I reverses, B fixed ⇒ F reverses: ↓

Common misconception 3

No-force case

Find and correct the mistake

Learner response

A wire carries current upward in a magnetic field that is also upward. A learner tries to force the two fingers of Fleming’s rule into these directions and predicts a sideways force. Diagnose the method.

Check the angle condition before using the hand rule

Unit: N

View solution step by step

  1. Check relative directions

    Method

    Recognise that current is parallel to the field.

    Reason

    The perpendicular component of current is zero.

    Working

    Angle between I and B is 0°.

  2. State the force

    Method

    Give zero magnetic force.

    Reason

    The motor-effect force is zero for parallel current and field.

    Working

    F = 0 N

Examiner practice 4

Perpendicular case (maximum idea)

2 marks

Examination question

For fixed current and field strength, compare the force when the conductor is parallel and perpendicular to the field. [2 marks]

State both limiting cases

View solution step by step

  1. Compare the limits

    2 marks

    Method

    State that force is zero when parallel and greatest when perpendicular.

    Reason

    Only the current component across the field produces the force.

    Working

    I∥ B: F = 0; I⊥ B: F maximum

Challenge 5

Force direction check

Minimal support

Charged-particle transfer

An electron beam travels into the page through a magnetic field directed left to right. Determine the beam’s deflection direction.

Convert electron motion before using Fleming's rule

Hints

Hint 1: current conversion
Conventional current is opposite to electron motion, so it is out of the page.
Hint 2: left-hand rule
Set field right and conventional current out of the page.
View solution step by step

  1. Convert the beam direction

    Method

    Represent electron motion into the page as conventional current out of the page.

    Reason

    Electrons carry negative charge, so conventional current is opposite their motion.

    Working

    Electrons: ⊗ ⇒ conventional current: ⊙.

  2. Apply the left-hand rule

    Method

    Set field right and current out of the page.

    Reason

    These are the two inputs to Fleming’s left-hand rule.

    Working

    First finger →; second finger ⊙.

  3. Read the deflection

    Method

    State that the beam deflects upward.

    Reason

    The thumb points upward for those directions.

    Working

    Force: ↑.

7. Mind Stretchers

Mind stretcher 1: Reverse bothExtension

If both the current direction and the magnetic field direction are reversed at the same time, what happens to the force direction?

Show Answer

The force direction stays the same (reversing both cancels out).

Mind stretcher 2: Identify the ruleExtension

When should you use the right-hand grip rule, and when should you use Fleming’s left-hand rule?

Show Answer

Right-hand grip rule: to find the direction of the magnetic field around a current-carrying wire/solenoid.
Left-hand rule: to find the direction of the force on a current-carrying conductor in a magnetic field.

8. Practice and next step

Reverse current and field independently in the Motor–Generator–Transformer Lab, then apply the force pair to the D.C. Motor.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027