Electromagnetic induction: the experiments of Faraday and Henry
A changing magnetic field through a coil induces an emf and a current in it. Faraday and Henry showed this with magnets and coils.
What electromagnetic induction is
In 1820 Oersted showed that an electric current produces a magnetic field. Faraday in England and Henry in the USA then asked the reverse question: can a magnet produce a current? Their experiments, around 1830 to 1831, showed that it can, but only when something is changing. Electromagnetic induction is the production of an emf in a circuit when the magnetic flux linked with it changes. An emf (electromotive force) is the energy given to each unit of charge, measured in volts. If the circuit is closed, the emf drives an induced current. A galvanometer (a sensitive meter for small currents) shows this current and its direction.
Experiment 1: a bar magnet and a coil
You need a coil of insulated copper wire, a galvanometer G and a bar magnet. There is no battery.
- 1Join the two ends of the coil C₁ to the galvanometer G.
- 2Push the north pole of the bar magnet towards the coil. The pointer of G deflects, showing a current.
- 3Hold the magnet still, even inside the coil. The deflection drops to zero.
- 4Pull the magnet away. G deflects in the opposite direction.
- 5Bring the south pole towards the coil. The deflection is opposite to that for the north pole.
- 6★Move the magnet faster. The deflection is larger.
- 7Keep the magnet still and move the coil instead. The same effects appear, so only relative motion matters.
★ key step
Experiments 2 and 3: two coils
- Experiment 2: replace the magnet by a second coil C₂ joined to a battery. A steady current makes C₂ act like a magnet.
- Moving C₂ towards or away from C₁ deflects G. Moving C₁ instead gives the same result. Again, relative motion changes the flux.
- Experiment 3: keep both coils still. Join C₂ to a battery through a tapping key.
- Pressing the key gives a momentary deflection in G. Holding it pressed gives no deflection.
- Releasing the key gives a momentary deflection in the opposite direction.
- Putting a soft iron rod through both coils makes the deflection much larger, because iron strengthens the field.
- Conclusion: a change of current in one coil, without any motion, also induces an emf in a nearby coil.
What causes a deflection in G?
| Action | Deflection in G | Reason |
|---|---|---|
| Magnet pushed towards the coil | Yes, one way | Flux through the coil increases |
| Magnet held still | None | Flux does not change |
| Magnet pulled away | Yes, the opposite way | Flux decreases |
| Magnet moved faster | Larger | Flux changes at a greater rate |
| Key in C₂ pressed or released | Momentary | Current in C₂, and so the flux in C₁, changes |
| Key held pressed (steady current) | None | A steady current gives a constant flux |
How to draw the magnet and coil experiment
- Draw the coil as a helix of 5 or 6 loops and label it C₁.
- Join its two ends to a circle with G inside: the galvanometer.
- Draw a bar magnet beside the coil. Mark N on the end facing the coil and S on the far end.
- Draw an arrow from the magnet towards the coil and label it direction of motion.
- Show the galvanometer pointer turned to one side and write deflection.
- Give the title: Faraday's experiment with a magnet and a coil.
Almost all the electricity in India, from coal plants to hydro plants on rivers like the Sutlej, is made by electromagnetic induction. An induction cooktop also uses a rapidly changing magnetic field to heat the base of the vessel.
1 marks: “What is electromagnetic induction?” The production of an emf (and a current, if the circuit is closed) in a circuit when the magnetic flux linked with it changes.
2 marks: “A bar magnet is held at rest inside a coil joined to a galvanometer. What does the galvanometer show? What happens when the magnet is pulled out quickly?” Zero deflection at rest, as the flux does not change. Pulling it out quickly gives a large deflection, opposite to that seen when the magnet was pushed in, because the flux falls rapidly.
3 marks: “Describe an experiment with two coils to show that a changing current in one coil induces a current in the other. What happens if a soft iron rod is placed inside the coils?” C₂ with battery and tapping key, C₁ with galvanometer; momentary deflection on pressing, none while held, opposite deflection on release; the iron rod increases the deflection because it strengthens the flux.
No change, no current: a magnet or a current induces an emf only while the flux is changing.