PhysicsICSE Class 10 · Magnetic Effect of Current and Electromagnetic Induction

Force on a Current-Carrying Conductor: Fleming's Left-Hand Rule and the DC Motor (ICSE Class 10)

Updated 16 October 2026 · Fact-checked

A current-carrying conductor placed across a magnetic field experiences a force. The force increases with the field strength, the current and the length of conductor in the field; it is greatest when the conductor is perpendicular to the field and zero when parallel. Its direction is given by Fleming's left-hand rule.

Understand Force on a Current-Carrying Conductor and the DC Motor

The motor effect. A conductor carrying a current has its own magnetic field. Placed in the field of a magnet, the two fields interact and the conductor feels a force. A simple demonstration: a light copper rod is placed across the poles of a horseshoe magnet and connected to a cell through a key. When the key is closed, the rod moves (rolls or swings) out of the gap.

What happens if you change things.

Change	Effect on the rod
Reverse the current	Moves the opposite way
Reverse the field (swap poles)	Moves the opposite way
Reverse both	Same direction as before
Rod parallel to the field	No force
Rod perpendicular to the field	Largest force

Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand mutually at right angles:

Forefinger: direction of the magnetic field (N to S);
Middle finger: direction of the current (+ to −);
Thumb: direction of the force (motion).

Factors affecting the size of the force. $F \propto B$ , $F \propto I$ and $F \propto l$ (for a conductor perpendicular to the field):

strength of the magnetic field $B$ ;
current $I$ in the conductor;
length $l$ of the conductor inside the field.

Indirect effects: think through the current. A quantity that is not in this list can still change the force by changing the current. Replace the rod with a thicker one of the same metal and length on the same supply: its resistance is lower ( $R = ρ \frac{l}{A}$ ), so the current is larger ( $I = \frac{V}{R}$ ), so the force is larger. In the same way, adding a resistor in series lowers the current and the force; increasing the supply voltage raises them.

The DC motor (main parts and energy transfer; working not required). It converts electrical energy into mechanical (rotational kinetic) energy.

Armature coil: a rectangular coil of many turns of insulated wire, wound on a soft iron core, free to rotate on an axle.
Field magnet: a strong magnet whose poles face each other with the coil between them.
Split-ring commutator: two halves of a ring, joined to the ends of the coil; it reverses the current in the coil every half rotation, so the coil keeps turning in the same direction.
Carbon brushes: press lightly on the split ring and connect the coil to the battery.

The two long sides of the coil carry current in opposite directions, so (by the left-hand rule) they are pushed in opposite directions. The pair of forces makes a turning effect, and the coil rotates.

Factors affecting the speed of rotation of a DC motor.

the current through the coil;
the strength of the magnetic field;
the number of turns in the coil;
the area of the coil.

(These are the motor's factors. Do not mix them with the factors for the induced current in a generator.)

Uses of the motor effect. Electric motors in fans, mixers, pumps, washing machines and electric vehicles; loudspeakers (a coil in a magnet's field vibrates with the changing current).

Force on a Current-Carrying Conductor and the DC Motor: formulas and key facts

Fleming's left-hand rule: Forefinger field
Middle finger current
Thumb force (motion); Motor effect.
Factors (qualitative): $F \propto B, F \propto I, F \propto l$; Conductor perpendicular to the field; zero force when parallel.
Current through the rod: $I = \frac{V}{R}, R = ρ \frac{l}{A}$; Thicker rod, smaller R, larger I, larger F.
DC motor speed depends on: Current
Field strength
Number of turns
Area of coil; Energy: electrical → mechanical.

Forefinger	field
Middle finger	current
Thumb	force (motion)

How to solve Force on a Current-Carrying Conductor and the DC Motor questions

Direction questions need the left-hand rule applied carefully; size questions need you to trace every change through the current.

1Mark the field direction (N to S) and the conventional current direction (+ to −) on the diagram.
2Point the forefinger along the field and the middle finger along the current; the thumb gives the force.
3If both current and field reverse, the force does not change direction; if one reverses, it does.
4For a change in rod, supply or resistance, find what happens to the current first ( $I = \frac{V}{R}$ ), then to the force.
5For a motor, name the part and its job (coil, magnet, split ring, brushes) and state 'electrical to mechanical'.

Force on a Current-Carrying Conductor and the DC Motor 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.

Reversing a toy car

Suppose your toy car's motor makes it run forwards, and you want it to run backwards without opening the motor.

What simple change will reverse the car, and why does it work?

Try it first, then show the solutionSolution

Turn the battery round (reverse its connections).
The current in the armature coil reverses, so by Fleming's left-hand rule the forces on its sides reverse.
The motor turns the other way.

Answer: Reverse the battery connections; the current reverses, so the force and rotation reverse.

Memory hook: Flip the current, flip the force.

How a loudspeaker makes sound

Imagine taking apart an old loudspeaker and finding a light coil attached to a paper cone, sitting in the gap of a strong ring-shaped magnet.

Explain how the speaker produces sound from an electrical signal.

Try it first, then show the solutionSolution

The signal is a current that keeps changing in size and direction.
The coil is in a magnetic field, so it feels a force that changes in the same way, pushing it back and forth.
The cone moves with the coil and makes the air vibrate: sound.

Answer: A changing current in a field gives a changing force, which vibrates the cone.

Memory hook: Current wiggles, cone wiggles.

Tricks to remember Force on a Current-Carrying Conductor and the DC Motor

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

FBI, left hand
Thumb = Force, First finger = B (field), seCond finger = I (current).
Use it when: Fleming's left-hand rule.
Left for motors
We drive on the LEFT in India: LEFT hand for MOTORS.
Use it when: Choosing the right rule.
BIL makes the force
Force grows with B, I and L.
Use it when: Factors affecting the force.
Split for motor, slip for generator
The DC motor's split ring keeps it turning one way.
Use it when: Motor parts.

Force on a Current-Carrying Conductor and the DC Motor: solved examples

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

Example 1: The middle finger · 1 mark · MCQBoard level

In Fleming's left-hand rule, the middle finger points in the direction of the:
(a) magnetic field
(b) current
(c) force
(d) induced current

Try it first, then show the solutionSolution

Forefinger: field; middle finger: current; thumb: force.

Answer: (b) current

Example 2: Rod between the poles · 3 marks · Short answerBoard level

A light aluminium rod hangs from two flexible wires between the poles of a horseshoe magnet and swings outwards when a current is passed. (a) Name the rule that gives its direction of motion. (b) What happens if the current is reversed? (c) What happens if the current and the magnet's poles are both reversed?

Try it first, then show the solutionSolution

(a) Fleming's left-hand rule. (1)
(b) The rod swings the opposite way (inwards). (1)
(c) It swings outwards again, as before: reversing both leaves the force unchanged. (1)

Answer: (a) Fleming's left-hand rule (b) Opposite direction (c) Same direction as at first

Example 3: Finding the direction · 2 marks · Short answerHarder board level

A vertical wire carries a current upwards in a horizontal magnetic field that points towards the north. In which direction is the force on the wire?

Try it first, then show the solutionSolution

Left hand: forefinger pointing north (field), middle finger pointing up (current). (1)
The thumb then points west. (1)

Answer: Towards the west

Example 4: A thicker rod · 4 marks · Long answerHarder board level

A rod of resistance 2 Ω lies across a magnetic field and is connected to a 6 V supply of negligible internal resistance. (a) Find the current. (b) It is replaced by a rod of the same metal and length but twice the cross-sectional area. Find the new current. (c) How does the force change? (d) What would happen to the force if, instead, the magnet were replaced by one twice as strong?

Try it first, then show the solutionSolution

(a) $I = \frac{6}{2} = 3$ A. (1)
(b) Double area halves the resistance: 1 Ω, so $I = \frac{6}{1} = 6$ A. (1)
(c) $F \propto I$ , so the force doubles. (1)
(d) $F \propto B$ , so the force would also double. (1)

Answer: (a) 3 A (b) 6 A (c) Doubles (d) Doubles

Example 5: Angle matters · 2 marks · Short answerHarder board level

A straight wire carrying a current is placed in a uniform magnetic field. When is the force on it (a) greatest, (b) zero?

Try it first, then show the solutionSolution

(a) When the wire is perpendicular to the field. (1)
(b) When the wire is parallel (or antiparallel) to the field. (1)

Answer: (a) Perpendicular (b) Parallel

Example 6: Double the diameter · 3 marks · Short answerHOTS / competency

Rods A and B are of the same metal and length, but B has twice the diameter of A. Each in turn is placed in the same position across a magnetic field and connected to the same cell of negligible internal resistance. Find the ratio of the force on B to the force on A, explaining each step.

Try it first, then show the solutionSolution

Area ∝ diameter², so B has 4 times the area of A; $R \propto \frac{1}{A}$ , so $R_{B} = \frac{R _{A}}{4}$ . (1)
Same voltage: $I = \frac{V}{R}$ , so $I_{B} = 4 I_{A}$ . (1)
$F \propto I$ (same B and l), so $F_{B} : F_{A} = 4 : 1$ . (1)

Answer: 4 : 1

Example 7: Case study: the toy motor · 4 marks · Case-basedHOTS / competency

A student builds a small DC motor with a rectangular coil, a strong magnet, a split ring and two brushes, powered by a battery.
(a) State the energy conversion.
(b) What is the job of the split-ring commutator?
(c) The motor turns too slowly. Suggest two changes to the coil or magnet that will make it turn faster.
(d) Why do the two long sides of the coil make it rotate rather than move sideways?

Try it first, then show the solutionSolution

(a) Electrical energy → mechanical (rotational kinetic) energy. (1)
(b) It reverses the current in the coil every half rotation, so the coil keeps turning in the same direction. (1)
(c) Any two: more turns on the coil; a stronger magnet; a larger coil area (or a larger current). (1)
(d) The two sides carry current in opposite directions, so they feel equal and opposite forces along different lines; together these turn the coil. (1)

Answer: (a) Electrical to mechanical (b) Reverses the coil current every half turn (c) More turns, stronger magnet, larger area (any two) (d) Opposite forces on the two sides form a turning pair

Where students lose marks in Force on a Current-Carrying Conductor and the DC Motor

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

Mistake: Wrong rule named for the rod
The force on a current is found with Fleming's left-hand rule; the right-hand rule is for induced current.
Wrong working
'Fleming's right-hand rule' or 'right-hand thumb rule' for a rod moving in a field.
Right working
'Fleming's left-hand rule.'
Do this: Make a table: thumb rule (field round a wire), left-hand (force), right-hand (induced current), clock rule (poles).
Usually costs: Reported by CISCE examiners for 2025 Q9(iii).
Mistake: 'No effect' for a thicker rod
Thickness lowers resistance, which raises current and so the force.
Wrong working
'F = BIl sin θ has no area, so the force does not change.'
Right working
'Resistance decreases, current increases, so the force increases.'
Do this: Always ask how a change affects the current.
Usually costs: Reported by CISCE examiners for 2025 Q9(iii).
Mistake: Motor speed factors muddled
The examiners want current, field strength, number of turns or area of the coil.
Wrong working
'Number of coils', 'magnet', or 'induced current'.
Right working
'Number of turns of the coil', 'strength of the magnetic field'.
Do this: Use the exact terms and give only the number of factors asked.
Usually costs: Reported by CISCE examiners for 2024 Q3(iii).
Mistake: Electron flow used as current
Fleming's rules use conventional current, from + to −.
Wrong working
Pointing the middle finger along the flow of electrons.
Right working
Middle finger from the positive terminal round to the negative.
Do this: Mark + and − on the diagram before using the rule.
Usually costs: Usually costs the direction mark.

Exam tips

Write the meaning of each finger when stating Fleming's left-hand rule; a bare name may not earn full marks.
When a diagram is given, keep its orientation and mark field and current directions on it.
For a DC motor sketch, label armature coil, field magnet (N, S), split-ring commutator, carbon brushes and battery.
Justify 'force increases' or 'decreases' with the change in current.

Practice questions on Force on a Current-Carrying Conductor and the DC Motor

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
The force on a current-carrying wire in a magnetic field is greatest when the angle between the wire and the field is:
(a) 0°
(b) 30°
(c) 60°
(d) 90°
Hint
Perpendicular.
Answer
(d) 90°
Q2 · 1 mark · Very short answerBoard level
Which parts of a DC motor connect the rotating coil to the battery?
Hint
They press on the split ring.
Answer
The carbon brushes, which press on the split-ring commutator.
Q3 · 1 mark · Very short answerBoard level
Name two devices that work on the force on a current-carrying conductor in a magnetic field.
Hint
Motor effect.
Answer
An electric motor and a loudspeaker.
Q4 · 2 marks · Short answerHarder board level
A wire carries a current towards the north in a magnetic field that points vertically upwards. Find the direction of the force on it.
Hint
Forefinger up, middle finger north.
Answer
Towards the east.
Q5 · 2 marks · Short answerHarder board level
In a motor-effect experiment, both the current and the magnetic field are reversed. What happens to the direction of motion of the rod? Explain.
Hint
Two reversals.
Answer
It stays the same: reversing the current alone reverses the force, and reversing the field reverses it back.
Q6 · 3 marks · Short answerHarder board level
A DC motor in a model runs too slowly. Suggest three changes that would make it run faster.
Hint
What makes the force larger?
Answer
Any three: increase the current (higher supply voltage), use a stronger magnet, increase the number of turns on the coil, increase the area of the coil.
Q7 · 1 mark · Very short answerHarder board level
The current in a wire placed across a magnetic field is doubled and the field is halved. How does the force change?
Hint
$F \propto B I$ .
Answer
It stays the same ( $2 \times \frac{1}{2} = 1$ ).
Q8 · 1 mark · Very short answerHarder board level
Why is the armature coil of a motor wound on a soft iron core?
Hint
Strength of the field.
Answer
Soft iron concentrates the magnetic field through the coil, making the field stronger and the turning force larger.
Q9 · 3 marks · Short answerHOTS / competency
A rod of resistance 3 Ω placed across a magnetic field is connected to a 6 V cell of negligible internal resistance. How does the force on it change if a 3 Ω resistor is connected (a) in series with the rod, (b) in parallel with the rod?
Hint
Find the current through the rod in each case.
Answer
Originally $I = \frac{6}{3} = 2$ A. (a) Series: $I = \frac{6}{6} = 1$ A, so the force halves. (b) Parallel: the rod still has the full 6 V across it, so its current stays 2 A and the force is unchanged (the extra current flows through the resistor).
Q10 · 2 marks · Short answerHOTS / competency
A student says, 'A thicker rod feels no extra force, because thickness does not appear among B, I and l.' Explain why the student is wrong for a rod connected to the same supply.
Hint
What does thickness change?
Answer
A thicker rod has a smaller resistance, so on the same supply it carries a larger current ( $I = \frac{V}{R}$ ). Since the force increases with current, the thicker rod feels a larger force.

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.Rule for the direction of force on a current?Show answer
Fleming's left-hand rule.
2.Force when the wire is parallel to the field?Show answer
Zero.
3.Part of a DC motor that reverses the coil current?Show answer
Split-ring commutator.
4.Effect of a thicker rod on the same supply?Show answer
Larger current, larger force.
5.Energy conversion in a DC motor?Show answer
Electrical to mechanical.

Force on a Current-Carrying Conductor and the DC Motor: frequently asked questions

What is Fleming's left-hand rule?

Stretch the thumb, forefinger and middle finger of the left hand mutually at right angles. With the forefinger along the field and the middle finger along the current, the thumb gives the direction of the force on the conductor.

On what factors does the force on a current-carrying conductor depend?

On the strength of the magnetic field, the current in the conductor and the length of conductor in the field. It is greatest when the conductor is perpendicular to the field and zero when parallel.

What are the main parts of a DC motor?

An armature coil on a soft iron core, a field magnet, a split-ring commutator and carbon brushes. It converts electrical energy into mechanical energy.