The Invisible Push
Force on a current in a field — Fleming's left hand
What this lesson covers
Why it matters
Hang a loose wire between magnet poles, switch on the current — and the wire JUMPS like something kicked it. Nothing touched it. Learn to predict which way it kicks, every time.
The idea in plain words
The rule of the motor age. Tap the fingers.
Fleming's LEFT hand: F·B·I ⊥
A horizontal wire carries current EAST; Earth's field points NORTH. Force direction?
Upward — currents can levitate, feebly
- index: First finger — Field direction (N → S)
- middle: seCond finger — Current direction (+ → −)
- the answer: thuMb — Motion — the force on the conductor
- the factors: F = BIl — stronger field, more current, longer wire → bigger force (qualitative in ICSE)
- First finger north, second finger east
- Thumb points… vertically UP
Predict first
Current flows RIGHT, field points INTO the page. The wire is pushed…
Fleming's left hand: First finger = Field (in), seCond = Current (right), thuMb = Motion → up. The force is always perpendicular to both.
- upward — correct
- downward
- along the current
What you do
The push on the wire is always at 90° to both B and I. Three rounds: point your LEFT-hand fingers along B and I — your thuMb calls the jump.
Check yourself
The force on a current-carrying wire in a field is greatest when…
Parallel to the field the force is ZERO; at 90° it's maximum — hence motor coils sit across the field.
Reverse BOTH the current and the field. The force…
Two flips cancel: (−B)(−I) keeps F. Reverse only ONE to flip the push.
Fleming's LEFT hand is for motors; the RIGHT hand is for…
Left = force from current (motor). Right = current from motion (generator/dynamo). Two hands, two machines.
- the wire is perpendicular to the field — correct
- the wire is parallel to the field
- orientation doesn't matter
- stays the same direction — correct
- reverses
- becomes zero
- generators — induced current from motion — correct
- stronger forces
- AC circuits only