4.1 The Magnetic Effect of Current
A Puzzle at the Science Exhibition
At the school science fair, Mohini and Aakarsh stopped at their senior Sumana’s model: an iron nail wrapped in wire and joined to a battery — a tiny lifting crane. When Sumana closed the circuit, the nail grabbed iron paper clips like a magnet. When she opened it, the clips dropped.
But there was no magnet anywhere — only an electric circuit. How could a plain iron nail suddenly behave like a magnet?
The answer is one of the great surprises of physics: an electric current can produce magnetism. Let us see it for ourselves.
Activity 4.1 — Watch the compass jump Set up a simple circuit — a cell, a switch and a wire — and place a magnetic compass just beneath the straight part of the wire. Now switch the current ON while watching the needle: it deflects, swinging away from its usual north–south rest. Switch OFF: the needle swings back. Flip the switch a few times and the needle obeys every time. The current, and only the current, is moving that needle.
What the deflection tells us
A compass needle is itself a tiny magnet — it moves only when another magnetic influence is nearby. So if the needle deflects whenever current flows, the current-carrying wire must be acting like a magnet. When the current stops, that magnetic influence vanishes and the needle relaxes back.
The Key Idea When an electric current flows through a conductor (like a wire), it produces a magnetic field around it — this is the magnetic effect of electric current. The magnetic field is the region around a magnet or a current-carrying wire where its magnetic effect can be felt (for example, by deflecting a compass needle). The field disappears the moment the current stops.
Be a Scientist: Hans Christian Oersted You have just repeated a discovery from 1820! The Danish professor Hans Christian Oersted (1777–1851) noticed during a demonstration that a nearby compass needle deflected every time he opened or closed an electric circuit. He investigated carefully, became certain that a current produces a magnetic field, and published his result. Other scientists repeated his experiment, confirmed it, and a whole new science — electromagnetism — was born. (Notice the pattern from Chapter 1: a chance observation, careful investigation, then sharing for others to check.)
For centuries, electricity and magnetism seemed to be two unrelated things. Oersted’s wobbling needle showed they are deeply connected — a link that today runs electric bells, motors, fans, loudspeakers, and the lifting electromagnet that started this chapter.