Strength of an electromagnet
A scrapyard crane can lift an entire car body made of steel, yet a coil of wire wound around a nail by a student in a school lab can barely hold a few pins. Both are electromagnets — a magnet that exists only while current flows through a coiled wire — so why is one so much stronger than the other? Not magic, and not a different kind of wire. The difference comes down to two things a student can control directly on a lab bench: how much current pushes through the coil, and how many times the wire is wound around the core. Add a soft iron core inside the coil and the effect grows further still. This article stays close to what a Class 8 student actually builds and tests — a cell, a coil, an iron nail, a handful of pins — and asks, precisely, what happens to the pull of the electromagnet as each of these is changed one at a time.
An electromagnet gets stronger when more current flows through its coil and when the coil has more turns of wire, with an iron core boosting the effect further, while reversing the current only swaps the poles and does not change the strength.
The core idea
The strength of an electromagnet is a measure of how powerfully it attracts magnetic materials such as iron filings, pins, or nails. Unlike a permanent bar magnet, this strength is not fixed — it can be dialled up or down by changing the circuit, and two factors matter most.
Current through the coil. A magnetic field exists around any wire carrying current, and a bigger current makes a stronger field. In a simple cell-driven circuit, connecting more cells in series increases the push driving the current, which increases the current flowing through the coil, which in turn increases the field around every part of the wire.
Number of turns in the coil. Each loop, or turn, of wire produces its own small magnetic field. When a wire is wound into a coil, all these individual loop-fields line up and add together inside the coil. Wind the same current-carrying wire into more turns and the combined field grows stronger, even though nothing about the current itself has changed. A third, related factor is the core material. Slipping a soft iron core inside the coil, instead of leaving the centre empty (an air core), makes the electromagnet noticeably stronger, because the iron itself becomes magnetized by the coil's field and adds its own contribution on top. This is why real electromagnets — in cranes, bells, relays — are almost always coils wound around an iron core, not just bare wire. It helps just as much to know what does not affect strength. Reversing the direction of current through the coil swaps which end becomes the north pole and which becomes the south pole, but it changes nothing about how strongly the electromagnet pulls. Strength is set by how much current flows and how many turns the coil has — the direction of that current decides polarity, not power.
Why it works
There is no formula on the Class 8 syllabus for electromagnet strength, but the qualitative reasoning behind both factors follows directly from what a magnetic field is and how fields combine. A current-carrying wire is always surrounded by a magnetic field, circling the wire. This is the starting fact behind everything else here: no current, no field, no electromagnet. Push more current through the same wire and the field around it grows stronger in direct proportion — this is why adding a second cell, which drives more current through the circuit, makes the coil pull harder. When that same wire is bent into a loop, the circular field lines from every part of the loop line up through the centre of the loop, reinforcing one another. A coil is simply many such loops stacked side by side, and the field each loop produces inside the coil adds to the fields from all the other loops. This is exactly like several people pushing together in the same direction: each turn contributes its own push, and more turns means more combined push. Doubling the number of turns, while keeping the current unchanged, roughly doubles the strength of the field along the coil's axis. An iron core boosts this further because iron is a material whose own atoms behave like tiny magnets that are easily lined up by an outside field. Placed inside the coil, the coil's field lines the iron's own atomic magnets up in the same direction, turning the iron into an extra magnet that adds its pull to the coil's. Non-magnetic materials such as wood, plastic, or aluminium have no such atomic magnets to line up, so they add nothing when used as a core — only materials that are themselves attracted to a magnet, like soft iron, work.
Worked example
Question. A student builds a coil with 20 turns of wire around an iron nail, connects it to a single cell, and finds it can pick up 4 pins. The student then rewinds the same length of wire into a coil with 40 turns around the same nail, keeping the same single cell. Later, still using 20 turns and the iron nail, the student swaps in two identical cells connected together in place of one. Predict what happens to the number of pins picked up in each change, and explain why.
Step 1. Compare the 20-turn and 40-turn trials. Both use the same single cell, so the current through the coil is essentially unchanged between the two — only the number of turns has changed, from 20 to 40.
Step 2. Apply the turns rule. Each turn adds its own share of magnetic field, and doubling the turns from 20 to 40 roughly doubles the combined field along the coil, since the current supplying each turn has not changed.
Step 3. A stronger field means a stronger pull on nearby iron objects, so the 40-turn coil should pick up noticeably more than 4 pins — a reasonable prediction is around 8 pins, since the field has roughly doubled.
Step 4. Now compare the 20-turn, one-cell trial with the 20-turn, two-cell trial. The number of turns and the iron core are unchanged, so only the current differs: connecting two cells together increases the push driving the current, so more current flows through the coil.
Step 5. More current through the same 20 turns produces a stronger magnetic field than one cell did, so this coil should also pick up more than 4 pins, though the exact increase depends on how much the current actually rises.
Answer: both changes are expected to increase the number of pins picked up above the original 4 — doubling the turns (to roughly 8 pins) and, separately, increasing the current with a second cell — because current and number of turns are the two factors that independently control an electromagnet's strength.
Common traps
- Believing that only one factor — current or number of turns — controls strength; both matter, and changing either one on its own changes the strength.
- Assuming that reversing the current direction makes the electromagnet stronger or weaker; it only swaps which end is the north pole and which is the south pole.
- Thinking any core material boosts strength; only materials attracted to a magnet, such as soft iron, work — wood or plastic cores add nothing.
- Changing the number of turns and the number of cells at the same time in an experiment, which makes it impossible to tell which factor caused the observed change in strength.
Ask the tutor: Want to see how these two factors combine to make the giant electromagnets used in scrapyard cranes and MRI machines so powerful?
Does the length of the wire itself affect the electromagnet's strength?
Not directly. What matters is how the wire is arranged — specifically the number of turns wound around the core and the current flowing through it — not the raw length of wire used. If the same total length of wire is rewound into more, tighter turns around the same core, the strength increases even though the wire length has not changed, because more turns now contribute their fields to the same core.
Can I use any metal as the core to make the electromagnet stronger?
No. Only materials that are themselves attracted to a magnet, such as soft iron, work well as cores, because the coil's field can line up their internal atomic magnets to add extra pull. Non-magnetic materials such as wood, plastic, copper, or aluminium have no such atomic magnets to align, so using them as a core adds nothing over an air-filled coil.
If I use more cells to increase the current, will the electromagnet keep getting stronger forever?
At the simple classroom level, more cells generally mean more current and a stronger electromagnet, but this cannot continue without limit in practice — too much current can overheat the wire, drain the cells quickly, or exceed what the coil and circuit are safely built for. For the purposes of this topic, though, the key qualitative point to remember is simply that more current through the coil increases its strength, all else being equal.