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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

The loudspeaker and a simple motor 2 of 3

Building a simple motor

Activity 13.4 builds a simple motor from a coil of copper wire, a dry cell and a small magnet.

Activity 13.4

You will need: a dry cell, insulated copper wire, two long pins with large heads, clay, sellotape, a knife that can cut wire, and a small round magnet

  1. First make the coil. Starting from the middle of the wire, wind about 30 turns round a cylinder such as a fairly thick pen (picture 1). To stop the coil coming loose, wrap the free ends of the wire a few times round the coil.
  2. With the blade, strip the insulation from the two free ends (picture 2). Strip only one half of each end, and both halves must be on the same side of the wire.
  3. Pass the two ends through the heads of the pins and rest the coil level on the pins (picture 3).
  4. Hold the pins against the two terminals of the dry cell with sellotape (picture 4), and fix the cell with clay so it does not move.
  5. Finally fix the round magnet on the cell with clay (picture 5).

Figure to be added

Activity 13.4, pictures 1-5 — the wound coil; stripping half of each end with a knife; the coil resting in the pin heads; the pins taped to the cell's terminals; the magnet fixed on the cell with clay under the coil.

The observation

You will see the copper coil turn. If it does not, start it turning gently with your hand; it will then keep on turning.

Why it may need a push

Look at when current can flow. The pins touch the wire at the ends, and only the stripped half of each end conducts. So current flows only while the coil is turned with its bare side down on the pins; for the other half of the turn the insulation breaks the circuit. If the coil happens to come to rest in the insulated position, no current flows, there is no force, and nothing happens — so a small push is needed to bring the bare side round. Once going, the coil carries on through the dead half turn on its own momentum. The next page explains why half the wire is left insulated at all.

It gets warm

The coil short-circuits the cell through very little resistance. Disconnect it when you have finished, or the cell runs flat and the wire gets hot.

A coil, a cell and a magnet make a motor

A current-carrying coil in a magnet's field turns; half-stripped ends switch the current on for only half of each turn.