Magnetic effect of current

A conductor is any material — most commonly a metal wire — that allows electric charge to flow through it easily. A magnetic field is the region around a magnet (or a current-carrying conductor) where its magnetic influence can be detected, for instance by the deflection of a compass needle or the attraction of iron filings. The discovery is credited to the Danish physicist Hans Christian Oersted. In 1820, while demonstrating an unrelated experiment to students, he happened to have a compass lying near a wire connected to a battery. When current flowed, the needle swung away from its usual north-south rest position. When the current stopped, it swung back. When Oersted reversed the direction of the current, the needle deflected to the opposite side. Three simple facts, and together they proved that a flowing current itself generates magnetism — a link between electricity and magnetism that no one had connected before. Two things about this magnetic effect matter most for predicting what you will see in an experiment or exam question.

It exists only while current flows. A wire with no current is magnetically inert — it will not deflect a compass at all. The moment the circuit is completed and current starts, the field appears; the moment the switch is opened, the field vanishes and the needle returns to pointing north-south under Earth's own magnetism alone.

Its direction depends on the current's direction. Reverse the direction in which current flows through the same wire, and the compass needle deflects to the opposite side of its original position instead. This reversibility is direct evidence that the magnetic field itself has a direction tied to the direction of current flow, even though Class 8 does not require you to work out that direction with a rule.

The magnetic effect of current is not just a curiosity — it is the working principle behind electromagnets, coils of wire wound around an iron core that behave as strong magnets only while current flows through them, and that can be switched off simply by opening a switch. Electromagnets built this way are used in cranes at scrapyards and steel factories to lift and drop heavy iron loads on command, and they appear inside electric bells, loudspeakers, and many other devices you will meet later in this chapter.

There is no formula to derive at this level, but it is worth reasoning through why the compass behaves the way it does, because the same logic answers almost every question this topic can ask, including questions with two circuits, reversed currents, or a switch that is opened partway through an experiment. A compass needle is itself a tiny magnet, free to rotate, that normally aligns with Earth's own weak magnetic field and so settles pointing roughly north-south. Any additional magnetic field placed near it adds to Earth's field, and the needle rotates to align with the combined field rather than with Earth's field alone. Before current flows, the only field acting on the needle is Earth's, so it simply points north-south as usual. The instant current starts flowing in a nearby wire, a second magnetic field appears around that wire (Oersted's discovery), and the needle rotates away from north-south to line up with the new combined field — this rotation is the "deflection" you observe. Because the wire's field exists only for as long as current flows, switching the current off removes that second field entirely, leaving only Earth's field, and the needle rotates back to north-south. Reversing the current's direction reverses the direction of the wire's field, so the needle now rotates to the opposite side instead of its original deflected side. Every observed behaviour in this chapter's activities — deflection on closing the switch, return to normal on opening it, opposite deflection on reversing the current — follows directly from this one chain of reasoning about a needle responding to the combined field around it.

Question: A student sets up a straight wire connected to a cell and a switch, with a compass placed directly below the wire so that the wire runs north-south, matching the compass needle's rest direction. Predict what the compass needle does in each of these three stages: (a) switch open, (b) switch closed, (c) the current in the same circuit is now reversed by swapping the cell's terminals, switch closed again.

Step 1. With the switch open, no current flows in the wire, so the wire produces no magnetic field of its own. The only field acting on the needle is Earth's magnetic field, so the needle points north-south, exactly as it would with no wire present at all.

Step 2. Closing the switch allows current to flow through the wire. By Oersted's finding, this current-carrying wire now produces its own magnetic field in the space around it, including at the compass's position just below it.

Step 3. The needle no longer feels Earth's field alone — it feels the combined effect of Earth's field and the wire's new field — so it rotates away from north-south and settles at a new, deflected angle. Opening the switch again at this point would remove the wire's field and let the needle swing back to north-south.

Step 4. Now the cell's terminals are swapped, reversing the direction of current through the wire, and the switch is closed again. The wire again produces a magnetic field, but because the current direction is reversed, this field points the opposite way to before.

The needle deflects to the side opposite its Step 3 deflection — same amount of turn, mirrored, because reversing the current reverses the direction of the magnetic effect while leaving Earth's field unchanged.

Competitive stretch

If two students, one with current flowing left-to-right and another with an identical circuit but current flowing right-to-left, close their switches at the same instant, their needles will both deflect (since both wires now carry current and produce fields) but towards opposite sides of the north-south line — the same reasoning as Step 4, applied to two separate circuits instead of one circuit at two different times.

• Treating the wire as permanently magnetic, like a bar magnet. It only produces a magnetic field while current actually flows through it; with the switch open, it has no magnetic effect at all.
• Attributing the needle's deflection to heat or vibration of the wire. The deflection is caused purely by the magnetic field the current creates — not by any warming or physical shaking of the conductor.
• Assuming a coil or a special wire shape is required to see the effect. Even a single straight wire produces a magnetic field; coiling the wire (as in an electromagnet) simply concentrates and strengthens the effect, it does not create it from nothing.
• Forgetting that reversing the current's direction reverses the direction of deflection, and incorrectly predicting the needle will deflect the same way (or not deflect at all) after the terminals are swapped.

Ask the tutor: Want to see how this same current-produced magnetic field, made stronger by coiling the wire around an iron nail, becomes a working electromagnet you could build on a table?