Cutting a magnetic field with a wire
Practical Activity for 14-16
Students should previously have inserted a magnet into a coil and measured the e.m.f. generated. This is a simpler version showing first of all only one coil (or loop) of wire.
Apparatus and Materials
- Mild steel yoke
- Copper wire, insulated with bare ends, 200 cm
- Magnadur magnets, 2
- Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)-
Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).
Health & Safety and Technical Notes
- Attach two Magnadur magnets to the steel yoke with opposite poles facing each other.
- Connect a long lead of insulated copper wire to the galvanometer.
- Move the wire through the field between the permanent magnets.
- Try the effect of a coil of many turns (see picture) and see how this changes the deflection.
Students will find that:
- There is only a current when the wire and magnet are moving relative to each other
- The faster the magnet or wire is moved then the greater the current
- The current changes direction when the relative motion of the wire and the magnetic field changes direction
- The effect is greater when the wire is formed into a coil (because there is more wire moving across the magnetic field)
This experiment was safety-tested in April 2006