ScienceCBSE Class 10 · Magnetic Effects of Electric Current

Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule

Updated 14 October 2026 · Fact-checked

A current-carrying conductor placed in a magnetic field experiences a force. The force is largest when the current is at right angles to the field, and its direction reverses if either the current or the field is reversed. Fleming's left-hand rule gives the direction: forefinger along the field, middle finger along the current, thumb along the force.

Understand Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule

Where the idea comes from. A current produces a magnetic field that pushes on a nearby magnet. Ampère suggested that, in return, the magnet must exert an equal and opposite force on the current-carrying conductor.

The experiment. A short aluminium rod is hung horizontally between the poles of a strong horseshoe magnet, so that the field passes through it at right angles. When current is passed through the rod, it is pushed sideways. Observations:

reverse the current: the rod moves the opposite way;
reverse the field (swap the poles): the rod again moves the opposite way;
the displacement is largest when the current is at right angles to the field.

So the direction of the force depends on both the direction of current and the direction of the field, and when current and field are perpendicular, the force is perpendicular to both.

Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular.

Forefinger (first finger) → direction of the magnetic field (F for Field).
Middle finger (second finger) → direction of the current (C for Current).
Thumb → direction of force or motion of the conductor.

Moving charges. A beam of charged particles is a current, so it also feels a force in a magnetic field.

For positive particles (protons, alpha particles), current is in their direction of motion.
For electrons, conventional current is opposite to their direction of motion, so point the middle finger opposite to the electrons' motion.

The force is always perpendicular to the particle's motion, so it changes the direction of motion (and so the velocity and momentum) but not the speed or the mass.

Size of the force. The force (and so the rod's displacement) becomes larger if the current is increased, if a stronger magnet is used, or if a longer length of conductor lies in the field.

Where it is used. Devices that use current-carrying conductors in magnetic fields include electric motors, generators, loudspeakers, microphones and measuring instruments. (The working of the electric motor is formative-only in 2026-27 and is not asked in the board exam.)

Do not mix up the rules. The right-hand thumb rule gives the direction of the field produced by a current. Fleming's left-hand rule gives the force on a current placed in a field.

Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule: formulas and key facts

Fleming's left-hand rule: $Forefinger: Field; Middle finger: Current; Thumb: Force (motion)$; Left hand, all three mutually perpendicular.
Condition for maximum force: $current ⊥ field$; No force when current is parallel to the field.
Reversal: $reverse current or field \Rightarrow force reverses; reverse both \Rightarrow same direction$; Direction depends on both.
Electron beams: $current direction = - (direction of electron motion)$; For positive particles, current is along their motion.
Factors increasing force: $larger current, stronger field, longer conductor in field$; As observed with the aluminium rod.

How to solve Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule questions

For direction questions, set up the three directions carefully before using your left hand.

1Draw the situation with clear labels: up/down, left/right, into/out of the page, or north/south/east/west.
2Find the field direction (from N pole to S pole between magnet poles).
3Find the current direction (for electrons, reverse their motion; for positive particles, use their motion).
4Point the forefinger along the field and the middle finger along the current, keeping them perpendicular.
5Read the force from the thumb.
6If the question asks for the field from the force, work backwards: thumb along force, middle finger along current, forefinger gives the field.
7Check: reversing one quantity should reverse the force.

Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Force on a current in a magnetic field

Suppose you open an old loudspeaker and find a coil of wire attached to a paper cone, sitting in the gap of a strong magnet. When music plays, a varying current flows through the coil and the cone moves back and forth.

Explain why the coil moves, why it moves in both directions, and why louder sound (bigger current) makes the cone move more.

Try it first, then show the solutionSolution

The coil carries a current and lies in the magnet's field, so it experiences a force (as a current-carrying conductor in a magnetic field does).
By Fleming's left-hand rule, the force is perpendicular to both the current and the field, so the coil is pushed along its axis, moving the cone.
The music signal makes the current change direction repeatedly; each time the current reverses, the force reverses, so the coil and cone move back and forth.
A larger current gives a larger force, so the cone moves farther, producing louder sound.

Answer: The coil feels a force because it carries current in a magnetic field; the force reverses with the current, making the cone vibrate; bigger current means bigger force and louder sound.

Memory hook: Current in a field gets a push; flip the current, flip the push.

Tricks to remember Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule

Short memory aids. Say each one aloud once, then use it on a question the same day.

FBI on the left hand
Thumb = F (force), forefinger = B (field), middle = I (current): 'FBI', thumb to middle.
Use it when: Assigning fingers quickly.
Left for Force, Right for field
Left hand: force on a current. Right hand: field of a current.
Use it when: Choosing the correct rule.
Electrons go backwards
For electrons, point your middle finger the opposite way.
Use it when: Electron-beam questions.

Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: When is the force largest? · 2 marks · Very short answerBoard level

A wire carrying current sits in a magnetic field. For which orientation of the wire is the push on it greatest? Name the rule that gives the direction of this push.

Try it first, then show the solutionSolution

When the wire, and so the current, is perpendicular to the field lines (1 mark).
Fleming's left-hand rule: with thumb, forefinger and middle finger of the left hand mutually perpendicular, forefinger along field and middle finger along current, the thumb shows the force (1 mark).

Answer: Current perpendicular to field; direction by Fleming's left-hand rule.

Example 2: The aluminium rod · 3 marks · Short answerHarder board level

In the aluminium rod experiment, how will the displacement of the rod be affected if (i) the current is increased, (ii) a stronger horseshoe magnet is used, (iii) the rod is made longer (with more of it in the field)? What happens if both the current and the field are reversed together?

Try it first, then show the solutionSolution

(i) Greater current, greater force: displacement increases (½ mark).
(ii) Stronger field, greater force: displacement increases (½ mark).
(iii) Longer length of conductor in the field, greater force: displacement increases (1 mark).
Reversing both current and field reverses the force twice, so the rod moves in the original direction (1 mark).

Answer: All three increase the displacement; reversing both leaves the direction unchanged.

Example 3: Proton deflection · 3 marks · Short answerHarder board level

A proton moving towards the north enters a magnetic field and is deflected towards the east. Find the direction of the magnetic field. Show your reasoning with Fleming's left-hand rule.

Try it first, then show the solutionSolution

A proton is positive, so the current is in its direction of motion: towards the north (1 mark).
Force (thumb) is towards the east; current (middle finger) towards the north (1 mark).
Holding the left hand this way, the forefinger points vertically upwards. So the field is vertically upwards (1 mark).

Answer: Vertically upwards.

Example 4: Electron beam in a room · 3 marks · Short answerHOTS / competency

You sit with your back to a wall. An electron beam moves horizontally from the back wall towards the front wall and is deflected to your right. Find the direction of the magnetic field.

Try it first, then show the solutionSolution

Electrons move back to front, so the current is from front to back (1 mark).
Force (thumb) is to the right; current (middle finger) points towards the back wall (1 mark).
Arranging the left hand this way, the forefinger points vertically downwards: the field is vertically downward (1 mark).

Answer: Vertically downwards.

Example 5: What changes for a moving charge · 3 marks · Short answerHOTS / competency

A proton moves freely through a uniform magnetic field, at right angles to it. Which of these can change: (a) mass, (b) speed, (c) velocity, (d) momentum? Explain. Why does an electron moving parallel to the field feel no force?

Try it first, then show the solutionSolution

The force is always perpendicular to the motion, so it changes only the direction of motion, not the speed; mass does not change (1 mark).
Velocity and momentum include direction, so (c) and (d) can change (1 mark).
The force exists only when the current has a part across the field; moving parallel to the field, there is no such part, so no force acts (1 mark).

Answer: (c) and (d). Parallel motion gives no force.

Where students lose marks in Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule

The slips examiners see most often, and what to write instead.

Mistake: Using the right hand
The two rules are confused.
Wrong working
Right-hand thumb rule used to find the force on a conductor.
Right working
Fleming's left-hand rule gives the force.
Do this: Left for force.
Usually costs: The full direction mark.
Mistake: Taking electron motion as current
Students forget conventional current.
Wrong working
Electrons move east, so current is east.
Right working
Electrons move east, so current is west.
Do this: Reverse the direction for electrons.
Usually costs: 1-2 marks.
Mistake: Wrong field direction between poles
Students take S to N.
Wrong working
Field from the south pole to the north pole outside the magnet.
Right working
Field from the north pole to the south pole outside the magnet.
Do this: Outside a magnet, field lines go N to S.
Usually costs: 1 mark.
Mistake: Claiming the speed changes
Students think any force speeds things up.
Wrong working
The magnetic force increases the proton's speed.
Right working
The force is perpendicular to motion; it changes direction, not speed.
Do this: Velocity and momentum change; speed and mass do not.
Usually costs: 1 mark in MCQs.

Exam tips

Write the three finger assignments every time before giving a direction.
Use a small 3D sketch (up, into page, right) to avoid confusion.
In 'reverse' questions, remember: one reversal flips the force; two reversals cancel.
Mention that the force is largest when current and field are perpendicular.
Motors and generators are formative-only in 2026-27; don't expect board questions on their working.

Practice questions on Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In Fleming's left-hand rule, the forefinger points in the direction of:
(a) current (b) force (c) magnetic field (d) motion of electrons
Hint
F for Field.
Answer
(c) magnetic field.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): The direction of force on a current-carrying conductor in a magnetic field reverses when the direction of current is reversed.
Reason (R): The force depends on both the direction of current and the direction of the magnetic field.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Recall the rod experiment.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
An electron beam moves towards the east through a vertically upward magnetic field. In which direction is it deflected?
Hint
Current is opposite to electron motion.
Answer
Current is towards the west. Forefinger up, middle finger west: the thumb points north. The beam is deflected towards the north.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a student hangs a light copper rod horizontally between the poles of a horseshoe magnet, with the north pole below and the south pole above the rod. She connects the rod to a battery so that current flows from her left to her right as she faces it.
(a) What is the direction of the magnetic field at the rod? (1 mark)
(b) In which direction will the rod move? (1 mark)
(c) Suggest two changes that make the rod move the other way, and one change that makes it move farther in the same direction. (2 marks)
Hint
Field goes from N to S, so upward here.
Answer
(a) Vertically upward (from the N pole below to the S pole above). (b) Forefinger up, middle finger to her right: the thumb points towards the student, so the rod swings towards her. (c) Reverse the battery connections (current), or swap the magnet's poles (field); increasing the current (or using a stronger magnet) makes it move farther the same way.
Q5 · 3 marks · Short answerHarder board level
State the rule you would use to find the direction of (i) the magnetic field around a straight current-carrying conductor, (ii) the force on a straight conductor carrying current in a perpendicular magnetic field. Explain, in one line each, how to apply each rule.
Hint
Two different hands.
Answer
(i) Right-hand thumb rule: hold the conductor in the right hand with the thumb along the current; curled fingers give the field. (ii) Fleming's left-hand rule: left thumb, forefinger and middle finger perpendicular; forefinger along field, middle finger along current, thumb gives the force.
Q6 · 3 marks · Short answerHOTS / competency
An alpha particle (positively charged) moving horizontally towards the south passes through a vertically downward magnetic field. Predict the direction of the force on it. If an electron moved the same way through the same field, how would its force compare?
Hint
Positive: current along motion. Electron: opposite.
Answer
For the alpha particle, current is towards the south. Forefinger down, middle finger south: the thumb points east, so the force is towards the east. For an electron moving south, the current is towards the north, so the force is reversed: towards the west.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Who suggested that a magnet exerts a force on a current-carrying conductor?Show answer
André-Marie Ampère.
2.Which finger shows current in Fleming's left-hand rule?Show answer
Middle finger.
3.When is the force on a conductor zero?Show answer
When the current is parallel to the field.
4.Direction of current for an electron beam?Show answer
Opposite to the electrons' motion.
5.Name three devices using this force.Show answer
Electric motors, loudspeakers, measuring instruments (also generators, microphones).
6.What does the force on a moving charge change?Show answer
Its direction (velocity, momentum), not its speed.

Force on a Current-Carrying Conductor and Fleming's Left-Hand Rule: frequently asked questions

What is Fleming's left-hand rule?

Stretch the thumb, forefinger and middle finger of the left hand at right angles. If the forefinger points along the magnetic field and the middle finger along the current, the thumb points along the force on the conductor.

When is the force on a current-carrying conductor maximum?

When the current flows at right angles to the magnetic field.

How do I apply the rule to an electron beam?

Take the current as opposite to the direction in which the electrons move, then apply the rule as usual.

What is the difference between the right-hand thumb rule and Fleming's left-hand rule?

The right-hand thumb rule gives the direction of the field produced by a current. Fleming's left-hand rule gives the force on a current placed in an external field.

Is the electric motor in the 2026-27 board exam?

No. The motor, electromagnetic induction and the generator are assessed only formatively this year.