The Motor Effect

This lesson covers the motor effect — the force on a current-carrying conductor in a magnetic field — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand Fleming's left-hand rule, the equation F = BIl (Higher tier), and how a DC motor works.

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What Is the Motor Effect?

When a current-carrying conductor (a wire with current flowing through it) is placed in a magnetic field, it experiences a force. This force can cause the conductor to move. This is called the motor effect.

Conditions for the Motor Effect

For the motor effect to occur, you need:

1. A current flowing through a conductor.
2. An external magnetic field (from a permanent magnet or another electromagnet).
3. The current must not be parallel to the magnetic field — the force is greatest when the current is perpendicular (at 90°) to the field.

Exam Tip: If the current is parallel to the magnetic field lines, there is no force on the conductor. The force is at its maximum when the current is at right angles to the field. This is a common question in the Edexcel exam.

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Fleming's Left-Hand Rule

Fleming's left-hand rule allows you to determine the direction of the force (motion) on a current-carrying conductor in a magnetic field.

How to Use It

Hold your left hand with the thumb, first finger and second finger all at right angles to each other (like the three axes of a 3D graph):

Finger	Represents	Mnemonic
First finger	Field direction (N to S)	F for Field
Second finger	Current direction (conventional: + to −)	C for Current
Thumb	Motion (force / thrust)	M for Motion

graph TD
    A["Fleming’s Left-Hand Rule"] --> B["First Finger → Field<br/>(North to South)"]
    A --> C["Second Finger → Current<br/>(Conventional: + to −)"]
    A --> D["Thumb → Motion / Force<br/>(Direction of movement)"]
    B --> E["All three fingers<br/>at RIGHT ANGLES<br/>to each other"]
    C --> E
    D --> E

    style A fill:#2c3e50,color:#fff
    style B fill:#c0392b,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#8e44ad,color:#fff

Applying the Rule — Worked Example

A wire carries a current flowing to the right and is placed in a magnetic field pointing into the page. What direction is the force?

1. Point your first finger into the page (field direction).
2. Point your second finger to the right (current direction).
3. Your thumb points upward — the force is upward.

Exam Tip: Practise using Fleming's left-hand rule with your actual hand before the exam. Many students get confused when the field or current is in an unusual direction. Remember — it is always the LEFT hand, and you must use conventional current (positive to negative). If the question gives electron flow, reverse it to get conventional current direction.

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Factors Affecting the Size of the Force

The force on a current-carrying conductor in a magnetic field depends on three factors:

Factor	Effect
Magnetic flux density (B)	A stronger magnetic field produces a greater force
Current (I)	A larger current produces a greater force
Length of conductor in the field (l)	A longer wire in the field experiences a greater force

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The Force Equation — F = BIl (Higher Tier)

For Higher tier students, the relationship is expressed as an equation:

F = B × I × l

Where:

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

Rearranging the Equation

To Find	Formula
Force (F)	F = BIl
Magnetic flux density (B)	B = F / (Il)
Current (I)	I = F / (Bl)
Length (l)	l = F / (BI)

Worked Example 1

A wire of length 0.25 m carries a current of 4.0 A in a magnetic field of flux density 0.30 T. Calculate the force on the wire.

F = BIl F = 0.30 × 4.0 × 0.25 F = 0.30 N

Worked Example 2

A wire experiences a force of 0.48 N when carrying a current of 6.0 A in a magnetic field of flux density 0.20 T. Calculate the length of wire in the field.

l = F / (BI) l = 0.48 / (0.20 × 6.0) l = 0.48 / 1.2 l = 0.40 m

Worked Example 3

A force of 1.5 N acts on a 0.50 m wire in a field of flux density 0.60 T. What is the current?

I = F / (Bl) I = 1.5 / (0.60 × 0.50) I = 1.5 / 0.30 I = 5.0 A

Exam Tip: Always check the units before substituting into the equation. Length must be in metres (not cm), current in amps and force in newtons. If the length is given in cm, divide by 100 first.

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The DC Motor

The motor effect is the principle behind the electric motor. A simple DC motor consists of:

• A rectangular coil of wire mounted on an axle so it can rotate.
• A permanent magnet (or pair of magnets) providing a magnetic field.
• A split-ring commutator — a cylinder split into two halves, connected to the two ends of the coil.
• Carbon brushes — stationary contacts that press against the commutator and connect it to the power supply.

How the DC Motor Works

1. Current flows through the coil via the brushes and commutator.
2. One side of the coil carries current upward and the other side carries current downward (because the coil is a loop).
3. Using Fleming's left-hand rule, one side of the coil experiences a force upward and the other side experiences a force downward.
4. These two forces create a turning effect (moment) — the coil rotates.
5. When the coil reaches the vertical position, the split-ring commutator swaps the connections — reversing the current direction in the coil.
6. This ensures the forces continue to push the coil in the same rotational direction, so it keeps spinning.

The Role of the Split-Ring Commutator

The split-ring commutator is essential because:

• Without it, when the coil passes the vertical position, the forces would reverse and the coil would push back the other way — it would just oscillate.
• The commutator reverses the current every half turn, ensuring continuous rotation in one direction.