What it does
A wire carrying a current in a magnetic field feels a push. Which way? Hold your left hand with the first finger, the second finger and the thumb all at right angles: the First finger points along the Field, the seCond finger along the Current, and the thuMb gives the Motion (the force). This page shows that as three arrows on a wire between two magnet poles, in a 3D picture you can drag round to look at from any side.
Below it, a DC motor turns: a coil sits between the poles, the forces on its two sides push one up and one down, and a split-ring commutator swaps the current every half turn so the coil keeps turning the same way. A graph shows the turning effect as the coil goes round.
It follows the Class 10 Science topics on the magnetic effects of electric current (force on a current-carrying conductor, Fleming’s left-hand rule, the electric motor) and the Class 12 Physics topics on moving charges and magnetism (F = I L × B, torque on a current loop). Before this, see the field itself in the magnetic field mapper; to design real coils, try the coil field designer.
How to use it
- Pick a level. Class 12 adds the vector form of the force and the torque formula with your numbers.
- Slide the field strength B (tesla), the current I (amperes), the length of wire in the field and the angle θ between the wire and the field.
- Tap Swap the poles or Reverse the current and watch the force arrow flip.
- Drag the 3D picture to turn it. Hold up your own left hand and match it to the arrows.
- In the motor, tap Pause or Play, slide the coil angle by hand, and untick Split-ring commutator to see why a motor needs one.
Worked example
The page starts with B = 0.50 T, I = 3.0 A, L = 10 cm and θ = 90°, with the field from left to right and the current going into the screen at the start view. The force is F = B I L sin θ = 0.50 × 3.0 × 0.10 × sin 90° = 0.15 N, downwards. Slide θ to 30°: sin 30° = 0.5, so F = 0.075 N. At 0° the wire lies along the field and F = 0 N.
For the motor coil (50 turns, 5 cm by 5 cm, so A = 0.0025 m²) the biggest turning effect is N B I A = 50 × 0.50 × 3.0 × 0.0025 = 0.1875 N m, when the coil lies flat along the field. When the coil stands upright (90°) the forces pull straight outwards and the turning effect is zero; that is where the split ring swaps the current.
Tips
- Reversing the current or swapping the poles reverses the force. Doing both leaves it as it was.
- The force is always at right angles to both the wire and the field.
- A real motor uses many turns, a stronger magnet and a soft iron core to make the turning effect bigger.
FAQ
Why the left hand? The left-hand rule is for motors (force from current). For generators (current from motion) Fleming’s right-hand rule is used instead.
What does the split ring do? It reverses the current in the coil every half turn, just as the coil passes upright, so the forces always turn it the same way.
Is anything saved? No. Everything is worked out on this page and nothing leaves your device.
Where the numbers come from
- Force on a current-carrying conductor F = I L × B (size B I L sin θ), Fleming's left-hand rule, and the torque on a coil τ = N B I A sin φ with a split-ring commutator: standard school physics results.
- Checked against numpy.cross for the force vector at several angles, a hand calculation of the coil's largest torque, and the textbook direction case (field to the right, current into the page, force downwards).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.