Experiment 18: Build and Test an Electromagnet

Wind wire around a nail, connect a battery, and you own a magnet you can switch off — an electromagnet. Cranes that lift cars and the motors in everything run on this trick. In this experiment you will build one, measure its strength in the honest unit of *paperclips held*, and show that strength scales with turns × current — in the sim's own readout, B ∝ N·I.

The physics behind the practical

A current flowing in a wire creates a magnetic field circling the wire. Coil the wire into a helix and the loops' fields add along the axis, producing a strong, directed field like a bar magnet's — with two upgrades: it vanishes when the current stops, and its strength is adjustable. The field scales with turns × current (for the sim's readout in these units: strength rises directly with the product N·I: 80 turns × 1.5 A = 120 units). Sliding an iron core into the coil multiplies the effect enormously: the coil's field aligns the iron's domains, and the iron's own magnetisation adds to the coil's. Iron is soft magnetically — its domains randomise when the current stops, so the clips fall. Steel is harder: it keeps some magnetisation (remanence), which is why steel is used for permanent magnets and iron for electromagnets that must let go.

One cost is heat: the coil's resistance converts electrical energy to thermal energy (P = I²R), so a hard-driven coil warms up, its resistance rises, and its current — and strength — sag during a long test. Method discipline (short connections, rest between trials) is physics, not fussiness.

Variables and controls

In the virtual lab

Set Coil turns (N) to 80 and Current (I) to 1.5 A and switch the Power ON: the Field strength readout shows the N·I product at work (80 × 1.5 = 120) and the Paperclips held count climbs. Now trade variables: halve the turns to 40 and double the current to 3.0 A — same product, same strength, same clips: the product is what matters. Push to 200 turns × 3 A (product 600) for the maximum clip count, then switch power OFF and watch every clip drop — soft iron forgets instantly. The Data Visualization's current–strength line at fixed N is straight through the origin: strength ∝ I. Log N, I, their product and clips held for at least five settings.

Hands-on: build and test

Wrap insulated wire tightly around a large iron nail in neat counted turns (start with 20), leaving both ends free; strip the last centimetre of insulation from each end. Touch the ends to a battery pack's terminals — the nail's tip now lifts paperclips. Count the longest chain it holds. Test the upgrades one at a time: 40 turns; two battery packs (more current); the nail removed (air core). Disconnect between trials — the coil gets warm in use (that is expected I²R heating, and a reason to work in short bursts, not a fault), never use mains electricity, never leave the coil connected unattended, and if anything smells hot or the battery swells, disconnect and tell an adult.

A worked investigation

Sim results (strength readout = N·I units) with the bench alongside:

Bench: 20 turns with one battery held 3 clips; 40 turns held 6; 40 turns with two batteries held 11; 40 turns with the nail removed held just 1.

Step 1 — The product rule. Compare rows 2 and the "half-and-double" run in the sim: 80 turns × 0.5 A = 40 units → 3 clips; 40 turns × 1.0 A is also 40 units → also 3 clips. N and I are interchangeable currencies for field strength.

Step 2 — Approximate linearity — with a ceiling. Clips rise with N·I (20→1, 40→3, 120→6, 300→9) but the rise slows and caps at 10 — the sim's chain limit, and a reminder that "clips held" is a saturating proxy: each extra clip hangs off the previous one, farther from the pole, in weaker field. Doubling the product does not promise double the chain.

Step 3 — The core's contribution. Bench: identical windings with and without the nail — 6 clips vs 1. The iron multiplies strength several-fold by aligning its domains with the coil's field. That −83% drop is the single largest effect in the whole investigation, larger than any affordable change in turns or batteries.

Step 4 — Heat drift. Holding connections for 30 s straight made later bench counts sag by a clip or two: I²R heating raises the wire's resistance, dropping the current at fixed battery voltage. Disconnecting between trials and letting the coil cool restored the counts — an evaluation point worth a mark in any write-up.

Conclusion and evaluation

Strength scaled with turns × current, the iron core multiplied it, and switching off released everything — the electromagnet model confirmed on every axis a school bench can test. The clip-count measure is honest but coarse and saturating; a spring-balance pull-force test would give newtons instead. Battery sag and coil heating both masquerade as "weaker magnet", and only the disciplined disconnect-and-rest method keeps them from polluting the data.

Safety

Battery packs only — never mains. Disconnect between trials: coils and batteries heat in use. If anything becomes hot, smells, or a battery swells, disconnect immediately and involve an adult. Mind dropped nails and clip chains on the floor.

Common mistakes

• Leaving the coil connected while chatting — heating sags the current, and the "mystery weakening" is just I²R.
• Uncounted, messy turns — turns that overlap randomly or slip down the nail change N between trials.
• Comparing chains carelessly — clips cling to each other; count only the stable hanging chain, with the same clip size.
• Losing insulation at the contact ends or shorting turns, changing the effective circuit.
• Using a steel bolt and expecting a clean switch-off — remanence keeps it slightly magnetic and confuses the on/off test.

Exam Experience

Question 1 — calculation and prediction [4 marks]. In the simulation, 80 turns at 1.0 A (product 80) hold 5 clips. Predict the clips held by (a) 80 turns at 2.0 A and (b) 160 turns at 1.0 A, and explain why the two predictions are the same. State one reason the actual counts might fall short.

Scoring guide:

• Both settings have N·I = 160, double the reference product [1]
• Prediction: about 10 clips if scaling is linear [1]
• Same product → same field strength, because B ∝ N·I regardless of how the product is made [1]
• Shortfall reason: chain saturation (clips far from the pole in weaker field) or coil heating sagging the current [1]

Question 2 — evaluation and design [4 marks]. A student tests two core materials — iron and steel — and finds the steel-core magnet "stronger because it still holds 2 clips after switch-off". Evaluate this conclusion and design the corrected comparison.

Scoring guide:

• Evaluation: residual clips after switch-off show remanence (kept magnetisation), not powered strength — the comparison conflates two different properties [1]
• Powered strength must be compared with the current ON, counting clips held then [1]
• Corrected design: same coil, same current, same turns; swap only the core; count clips while powered [1]
• Then test remanence separately (clips held after switch-off) and report both properties distinctly [1]

Multiple choice 1. When the current is switched off, an iron-core electromagnet drops its paperclips because:

• A. The iron has been used up
• B. Iron is magnetically soft — its domains randomise without the coil's field
• C. Gravity increases
• D. The clips lose their charge

Answer: B. Soft iron magnetises easily and demagnetises easily — precisely the property that makes switchable magnets possible.

Multiple choice 2. Doubling both the turns and the current multiplies the field strength by:

• A. 2
• B. 3
• C. 4
• D. It stays the same

Answer: C. B ∝ N·I, so 2 × 2 = 4 — the product rule again.

Going deeper: electromagnets at work

The switchable magnet runs more of the world than the permanent kind. A relay is an electromagnet doing office work: a small current in a coil pulls a switch closed, letting a large current flow elsewhere — your car's starter motor is commanded this way, sparing the ignition switch the heavy current. Scrapyard cranes lift car bodies with electromagnets and drop them by letting go electrically — try that with a permanent magnet. Door locks, doorbells, loudspeakers, MRI scanners all trade on strength you can dial or kill, and the MRI's superconducting cousin keeps its field for years on one charge of current because its coils have no resistance to bleed.

The heating warning in this lesson scales with the job: a coil carrying 3.0 A with 2 Ω of resistance dissipates P = I²R = 9 × 2 = 18 W as heat — a soldering iron's worth, inside your windings. Real electromagnet design is half magnetics, half plumbing for that heat. When your bench coil sags after long connections, you are meeting the same engineering wall the professionals do, at kinder temperatures.

One more angle: two ceilings on strength

Even with unlimited batteries, an electromagnet meets two ceilings. First, the battery's internal resistance: as you demand more current, the cell's own resistance drops more voltage internally, so terminal voltage and coil current sag — the supply quietly refuses the arms race. Second, core saturation: once every domain in the iron points the same way, the core has no more alignment to offer, and extra current adds only the coil's unaided field — the multiplication stops. Engineers read both ceilings off the same curve you sketched with paperclips: linear while life is simple, bending over where the materials have given all they have.