The motor effect and force on a conductor

What you'll learn

This revision guide covers the motor effect—the phenomenon where a current-carrying conductor placed in a magnetic field experiences a force. You'll learn how to predict the direction of this force using Fleming's left-hand rule, calculate the size of the force, and understand how the motor effect is applied in electric motors. This topic is essential for AQA GCSE Physics Paper 2 and regularly appears in both multiple-choice and extended response questions.

Key terms and definitions

Motor effect — the phenomenon where a current-carrying conductor placed in a magnetic field experiences a force

Magnetic flux density — a measure of the strength of a magnetic field, measured in tesla (T)

Fleming's left-hand rule — a method using the thumb, first finger and second finger of the left hand to determine the direction of force, magnetic field and current respectively

Electric motor — a device that uses the motor effect to convert electrical energy into kinetic energy

Split-ring commutator — a component in a DC motor that reverses the current direction every half turn to maintain rotation in the same direction

Uniform magnetic field — a magnetic field where the magnetic flux density is the same at all points, represented by equally-spaced parallel field lines

Perpendicular — at 90° to another line or surface; the angle at which the motor effect force is maximum

Core concepts

The basic motor effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This occurs because:

• The current in the conductor creates its own magnetic field around the wire
• This magnetic field interacts with the external magnetic field
• The interaction between the two fields produces a force on the conductor
• The force acts perpendicular to both the magnetic field and the current direction

The force is largest when the conductor is at right angles (90°) to the magnetic field. When the conductor is parallel to the magnetic field, the force is zero.

The motor effect only occurs when:

• There is a current flowing through the conductor
• The conductor is in a magnetic field
• The current direction is not parallel to the magnetic field

If any of these conditions are not met, no force acts on the conductor.

Fleming's left-hand rule

Fleming's left-hand rule allows you to predict the direction of the force on a current-carrying conductor in a magnetic field. Hold your left hand with the thumb, first finger and second finger mutually perpendicular (each at 90° to the others):

• First finger — direction of the magnetic Field (from north to south)
• SeCond finger — direction of conventional Current (from positive to negative)
• ThuMb — direction of Motion/force on the conductor

It is crucial to use your left hand (not right). The right hand is used for Fleming's right-hand rule in electromagnetic induction, which is a different phenomenon.

Practical application:

When applying Fleming's left-hand rule in exam questions:

1. Identify the direction of the magnetic field (usually shown by field lines or stated as north/south pole positions)
2. Identify the direction of conventional current flow (from + to −)
3. Position your left hand so the first finger points along the field and second finger along the current
4. Your thumb now points in the direction of the force/motion

Remember that conventional current flows from positive to negative, which is opposite to electron flow. Always use conventional current for Fleming's left-hand rule.

Calculating the force on a conductor

The size of the force on a current-carrying conductor in a magnetic field can be calculated using:

F = B I L

Where:

• F = force on the conductor (newtons, N)
• B = magnetic flux density (tesla, T)
• I = current flowing through the conductor (amperes, A)
• L = length of conductor in the magnetic field (metres, m)

This equation only applies when the conductor is perpendicular to the magnetic field. If the conductor is at any other angle, the force is less than the maximum value given by this equation.

Factors affecting the size of the force:

The force on a conductor increases when:

• The magnetic flux density increases (stronger magnet)
• The current through the conductor increases
• The length of conductor in the magnetic field increases

The force decreases to zero when:

• The current is switched off
• The conductor is parallel to the magnetic field
• The conductor is removed from the magnetic field

The simple DC electric motor

An electric motor uses the motor effect to convert electrical energy into kinetic energy (rotational motion). The basic components are:

Coil of wire — usually rectangular, mounted on an axle so it can rotate; the current flows through this coil

Permanent magnets — create a uniform magnetic field; positioned with north and south poles on opposite sides of the coil

Split-ring commutator — two halves of a metal ring, each connected to one end of the coil; reverses the current direction every half turn

Brushes — usually carbon contacts that maintain electrical connection to the rotating commutator while allowing it to spin

How a DC motor works:

1. Current flows through the coil, creating forces on opposite sides of the coil
2. Using Fleming's left-hand rule, one side experiences an upward force, the other a downward force
3. These forces create a turning effect (moment) that rotates the coil
4. When the coil reaches the vertical position, the split-ring commutator reverses the current direction
5. This reversal ensures the forces continue to turn the coil in the same direction
6. The coil continues rotating as long as current flows

Increasing motor speed:

The motor rotates faster when you:

• Increase the current (stronger force on each side of coil)
• Increase the magnetic flux density (use stronger magnets)
• Increase the number of turns in the coil (more wire experiencing force)
• Decrease friction in the bearings

Reversing motor direction

The direction of rotation can be reversed by either:

Reversing the current direction — swap the battery connections or switch polarity; this reverses the force direction on both sides of the coil

Reversing the magnetic field — swap the north and south poles of the magnets; this also reverses the force direction on both sides

Doing both simultaneously would result in the motor rotating in the original direction, as two reversals cancel out.

Real-world applications

The motor effect has numerous practical applications beyond simple motors:

Electric vehicles — use powerful electric motors based on the motor effect to drive wheels

Industrial machinery — motors in factories use the motor effect to power conveyor belts, pumps and production equipment

Domestic appliances — washing machines, fans, food mixers and power tools all contain electric motors

Loudspeakers — use the motor effect to convert electrical signals into sound; the varying current through a coil in a magnetic field causes vibrations that produce sound waves

Understanding the motor effect is fundamental to understanding how electrical energy is converted to mechanical energy in countless everyday devices.

Worked examples

Example 1: Calculating force on a conductor

Question: A straight wire of length 0.25 m carries a current of 4.0 A. It is placed at right angles to a magnetic field of magnetic flux density 0.50 T. Calculate the force acting on the wire.

Solution:

Step 1: Write down the equation F = B I L

Step 2: Identify the values from the question

• B = 0.50 T
• I = 4.0 A
• L = 0.25 m

Step 3: Substitute values and calculate F = 0.50 × 4.0 × 0.25 F = 0.50 N

Answer: The force on the wire is 0.50 N (2 marks)

Mark scheme notes: 1 mark for correct equation or correct substitution, 1 mark for correct answer with unit

Example 2: Fleming's left-hand rule application

Question: A vertical wire carries a current upwards through a horizontal magnetic field directed from left to right. Use Fleming's left-hand rule to determine the direction of the force on the wire.

Solution:

Step 1: Identify the directions

• Magnetic field direction: left to right (horizontal) — first finger
• Current direction: upwards (vertical) — second finger

Step 2: Apply Fleming's left-hand rule

• Point first finger (left hand) to the right (field direction)
• Point second finger upwards (current direction)
• Thumb points out of the page/towards you

Answer: The force acts horizontally, out of the page/towards the observer (2 marks)

Mark scheme notes: 1 mark for reference to Fleming's left-hand rule, 1 mark for correct direction

Example 3: Increasing motor speed

Question: A student builds a simple DC motor. Describe two ways the student could increase the speed of rotation of the motor. (4 marks)

Solution:

Step 1: Identify factors that increase the force on the coil

Method 1: Increase the current flowing through the coil

This increases the force on each side of the coil (F = BIL), producing a larger turning effect and faster rotation. (2 marks)

Method 2: Use stronger magnets

This increases the magnetic flux density, which increases the force on the coil sides, resulting in faster rotation. (2 marks)

Alternative acceptable answers: increase number of turns on coil; decrease friction/resistance to rotation

Mark scheme notes: 1 mark for each correct method, 1 mark for each correct explanation linking to force or rotation speed

Common mistakes and how to avoid them

• Using the right hand instead of left hand — Fleming's left-hand rule specifically requires the left hand for motors. The right hand is only used for generators (electromagnetic induction). Remember: "motors use left"

• Confusing current direction — Fleming's left-hand rule uses conventional current (positive to negative), not electron flow. Electrons flow in the opposite direction to conventional current

• Forgetting units — Always include units in calculations: force in newtons (N), magnetic flux density in tesla (T), current in amperes (A), length in metres (m). Convert centimetres to metres before calculating

• Applying F = BIL when conductor is not perpendicular — This equation only gives the maximum force when the conductor is at 90° to the field. If the question specifies a different angle or says "parallel," the force will be different (zero if parallel)

• Not explaining the role of the split-ring commutator — Students often describe what it does (reverses current) but not why this matters. Always explain that reversing current maintains rotation in the same direction

• Confusing which changes reverse motor direction — Remember that reversing either current OR field direction reverses rotation, but reversing both keeps the original direction

Exam technique for "The motor effect and force on a conductor"

• "Describe" questions (2-4 marks) — Give a step-by-step account of what happens. For motor operation, explain: current flows → force acts on coil sides → turning effect produced → commutator reverses current → continuous rotation maintained

• "Calculate" questions — Always show your working: write the equation, substitute values with units, then calculate. Even if your final answer is wrong, you can gain method marks. Use F = BIL only when the conductor is perpendicular to the field

• Fleming's left-hand rule questions — Explicitly state "using Fleming's left-hand rule" in your answer, identify which finger represents which quantity, then give the final direction clearly (e.g., "upwards," "to the left," "out of the page")

• "Explain" questions (3-6 marks) — Link cause and effect using scientific reasoning. For example, "increasing current increases force because F = BIL, so a larger current produces a proportionally larger force." Use the formula to support your explanation when relevant

Quick revision summary

The motor effect occurs when a current-carrying conductor in a magnetic field experiences a force perpendicular to both current and field directions. Use Fleming's left-hand rule (first finger = field, second = current, thumb = motion) to predict force direction. Calculate force using F = BIL when the conductor is perpendicular to the field. Electric motors use this effect with a rotating coil, split-ring commutator and brushes to convert electrical energy to kinetic energy continuously. Reverse motor direction by reversing either current or magnetic field (but not both).