Magnetism & Motors
Year 10 (IGCSE) ⚡ Electricity & Magnetism Describe the motor effect, F = BIL, and the DC motor.
🧲 The Motor Effect
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the motor effect — the principle behind every electric motor!
Fleming's Left-Hand Rule: Point your left hand so that the First finger points along the field (N→S), seCond finger along the Current, and your THumb shows the direction of the Thrust (force) on the wire! (FBI rule: F=B×I)
⚡ Motor Effect Force
$$F = BIL$$
$F$ = force (N) · $B$ = magnetic flux density (T) · $I$ = current (A) · $L$ = length of conductor in field (m)
📐 F = BIL Calculations
The force F = BIL is maximum when the conductor is perpendicular to the magnetic field, and zero when parallel.
🔬 Example 1: A wire of length 0.1 m carries 2 A in a 0.5 T field (at 90°).
F = BIL = 0.5 × 2 × 0.1 = 0.1 N
F = BIL = 0.5 × 2 × 0.1 = 0.1 N
🔬 Example 2: Find current needed to produce 0.3 N force on a 0.15 m wire in a 2 T field.
I = F ÷ (BL) = 0.3 ÷ (2 × 0.15) = 1 A
I = F ÷ (BL) = 0.3 ÷ (2 × 0.15) = 1 A
Increasing B, I, or L all increase the force. Reversing the current OR reversing the field reverses the force direction!
⚙️ The DC Electric Motor
A DC motor uses the motor effect to create continuous rotation.
| 🔩 Component | 📝 Function |
|---|---|
| Coil (armature) | Carries current; experiences force from motor effect |
| Permanent magnets | Provide the external magnetic field |
| Split-ring commutator | Reverses current every half turn — keeps rotation going the same way |
| Brushes (carbon) | Maintain electrical contact while the coil rotates |
Without the commutator, the coil would oscillate back and forth instead of rotating continuously — the commutator is the key invention!
Ready to test yourself? Click the Quiz tab above to answer questions on this topic!
🧲 Motor Force Calculator (F = BIL)
Force on a current-carrying conductor in a magnetic field: F = BIL