Complete each stage to unlock the next one.
On the syllabus: GCSE PhysicsA-Level Physics
Inside an MRI machine, patients are surrounded by a magnetic field 60,000 times stronger than Earth's — yet they feel nothing, and the machine doesn't push them around. But if a paper clip gets close to that same magnet, it shoots across the room like a bullet. How can a magnetic field be lethal to a paper clip and harmless to a human being at the same time?
Inside an MRI machine, patients are surrounded by a magnetic field 60,000 times stronger than Earth's — yet they feel nothing, and the machine doesn't push them around. But if a paper clip gets close to that same magnet, it shoots across the room like a bullet. How can a magnetic field be lethal to a paper clip and harmless to a human being at the same time?
Electric motors, MRI machines, particle accelerators, and the Earth's own magnetic field all operate on the same principle: moving charges create magnetic fields, and magnetic fields exert forces on moving charges. Mastering these two ideas unlocks the mechanism behind virtually all electrical technology.
A long straight wire carrying current I produces a circular magnetic field around it. The field strength B depends on current and distance but not on the charge itself. A charge moving through a magnetic field experiences a force perpendicular to both its velocity and the field — the Lorentz force.
The right-hand rule encodes the direction: for a wire carrying current upward, wrap your right hand around it with the thumb pointing up — your fingers curl in the direction of B (counterclockwise when viewed from above). Magnetic forces never do work on charges because F is always perpendicular to v — they change direction but not speed. This is why a compass needle aligns with Earth's field without energy input. Electromagnets, speakers, and particle accelerators all rely on the Lorentz force F = qv×B.