How electric currents create magnetic fields and motion
Oersted discovered that a wire carrying current deflects a nearby compass needle, showing that current produces a magnetic field. Around a straight wire the field lines are concentric circles, and their direction follows the right-hand thumb rule. A long coil called a solenoid produces a uniform field inside, much like a bar magnet, and placing a soft iron core inside makes an electromagnet. In Class 12, the field is calculated using the Biot–Savart law and Ampère’s law; inside a solenoid, B = μ₀nI.
A current-carrying wire in a magnetic field experiences a force F = BIL sin θ, with its direction given by Fleming’s left-hand rule. A moving charge feels the Lorentz force, F = qvB sin θ, which makes it move in a circle of radius mv/qB. An electric motor uses this force to turn a coil, while an electric generator does the reverse, producing current by rotating a coil in a field. In homes, live, neutral and earth wires, fuses and MCBs keep circuits safe.
Why it matters
Electric motors in fans, washing machines, pumps and electric vehicles all work by the force on a current in a magnetic field. Electromagnets lift scrap iron, operate electric bells and form the heart of MRI scanners. Knowing domestic wiring and earthing prevents shocks and fires. This topic leads into electromagnetic induction and alternating current.
Key terms
- Solenoid
- A long coil of wire that produces a nearly uniform magnetic field inside when current flows.
- Fleming’s left-hand rule
- A rule that gives the direction of the force on a current-carrying conductor in a magnetic field.
- Lorentz force
- The force on a charged particle moving through a magnetic field.
- Earthing
- Connecting the metal body of an appliance to the ground so leaked current flows away safely.