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Electromagnetic Induction overview

Topic 2 of 6

Evidence for induction

Compare changes while keeping the coil, leads and viewpoint fixed. A moving meter needle is an observation; the explanation must identify the changed flux linkage, the induced direction and the circuit that carries current.

Connect a fixed coil to a sensitive centre-zero current meter and move a permanent magnet along its axis. The coil and return leads form a complete conducting loop. In the stated winding, A-to-B traversal is clockwise when viewed from the magnet on the left. Keep A connected to the meter's positive terminal and B to its negative terminal throughout the comparison.

A fixed winding and a complete meter circuit

A fixed winding and a complete meter circuitA bar magnet has its north pole on the right and moves right towards a fixed air-core coil. The continuous winding has A at its near left end and B at its far right end. A-to-B traversal is clockwise when viewed from the magnet. In this approach the induced current goes B to A inside the winding, returns from A to the meter positive terminal on the left, crosses the centre-zero current meter and returns from its negative terminal to B. The needle deflects towards the plus side with no numerical calibration. Hidden half-turns are pale dashed paths of the same continuous insulated wire, not separate loops. The motion, winding and lead polarity must be held as stated when comparing the observations.N approachesSNABFixed coilAir core-0++-Sensitive centre-zero current meterCurrent enters its + terminal

This connected current-meter setup shows a current pulse during approach. Stopping gives no sustained deflection; withdrawal reverses it. A high-impedance voltmeter is a different measurement that can show induced e.m.f. with negligible current.

The fixed winding and complete return circuit preserve the A/B and meter references. For north-pole approach, coil current runs B to A and returns from A through the meter's positive terminal to B. Changing leads would change the displayed sign without changing the physical induction.

A deflection in this closed setup indicates a current and supports the presence of induced e.m.f. They are different quantities: e.m.f. is energy supplied per charge, measured in volts, while current is charge flow per time, measured in amperes. A high-impedance voltage measurement can reveal induced e.m.f. with negligible current. An open path prevents sustained loop current, not necessarily induced voltage.

Use controlled observations to infer a relationship

These are comparisons to make and interpret with appropriate apparatus or supplied traces. Record the actual response; an ideal prediction is not itself a measured result.

Induction comparisons, evidence and the inference each supports
Controlled changeObservation or predictionInference
Hold the magnet still, approach the coil, then stop.No sustained deflection when stationary; a pulse during approach, returning to zero when motion stops.Changing linkage produces e.m.f.; a constant field alone does not.
Approach and withdraw the same pole on the same path.Opposite meter deflections.Reversing the linkage change reverses the induced direction.
Pass through the same positions at greater speed.A larger voltage peak with comparable sampling and instrument response.A greater rate of linkage change gives a greater e.m.f.
Repeat the same per-turn field history with twice the linked turns.The ideal model predicts twice the e.m.f.; compare voltage records to test it.Total linkage, including turn count, determines the induced e.m.f.

Changing turn count can also change coil resistance. Therefore twice the e.m.f. does not automatically mean twice the meter current. Use voltage measurements, or account for the known circuit response, before making that inference.

A greater change of B, a larger equally exposed perpendicular area, more linked turns or a faster orientation change can increase the rate of linkage change. Identify what is varied and what remains fixed. A large constant flux still gives zero induced e.m.f.

Infer the opposing direction

View the coil face from the magnet. An approaching north pole increases flux into that face. The induced current produces an outward field, making the near face north and opposing the approach. Its current is anticlockwise from that viewpoint. In the stated winding this is B to A through the coil, with the external return from A through the meter to B.

North approaches: an induced north face

North approaches: an induced north faceThe upper side view shows a north pole facing the coil and moving right towards it. The lower face view looks from that magnet towards the coil. A closed conducting loop carries anticlockwise conventional current. The induced field at its centre points towards this observer, a circled dot, making the near face north. Blue denotes magnet motion, brown loop current and teal the induced magnetic field. The brown loop is a complete circuit; it is not a field line.Magnet motionSNCoilNNear faceLook from the magnet towards the coilN faceAnticlockwise currentInduced B towards you

The approaching N increases flux into the face. An outward induced field opposes that increase. Replacing N by S reverses the current and near pole.

North withdraws: an induced south face

North withdraws: an induced south faceThe upper side view shows a north pole facing the coil and moving left away from it. The lower face view looks from that magnet towards the coil. A closed conducting loop carries clockwise conventional current. The induced field at its centre points away from this observer, a circled cross, making the near face south. Blue denotes magnet motion, brown loop current and teal the induced magnetic field. The brown loop is a complete circuit; it is not a field line.Magnet motionSNCoilSNear faceLook from the magnet towards the coilS faceClockwise currentInduced B away from you

Withdrawing N reduces flux into the face. An inward induced field opposes that decrease. The induced field need not oppose the existing external field.

Both views are from the magnet towards the coil. North-pole approach gives an anticlockwise near-face current and outward induced field; withdrawal gives clockwise current and inward induced field. Motion, current and field symbols have separate meanings.

Withdrawing that north pole makes the induced near face south, attracting the departing pole and opposing the change. The current is clockwise. Replacing north with south reverses the predictions: south approach gives clockwise current; south withdrawal gives anticlockwise current.

The clockwise label depends on the viewing side. Looking from the opposite end reverses its apparent sense. Use the coil field-direction rule and a named viewing side rather than memorising an unlabelled clockwise arrow.

Choose controls and diagnose a limited record

Keep the magnet and pole, coil geometry and turns, relative path, lead polarity and acquisition settings fixed when comparing speed. To compare turns, match the per-turn field-change history instead. Use suitable low-voltage laboratory equipment, secure the coil and record repeated traces without changing the reference.

A short pulse can be missed by a slow measurement, so a near-zero display alone does not establish that nothing changed. Choose a range that contains both signs, adequate response and a short enough sample interval. Check the no-change baseline and leads before interpreting a pulse.

Repetition can reveal a repeatable pattern or random variation. A shifted zero needs a baseline correction, reversed leads need the reference restored, and loose contacts need repair. Preserve the original traces and explain a specific reason before excluding a result; do not remove a reading merely because it disagrees with an expected shape.

Optional check A coil experiences changing magnetic flux linkage. A high-impedance voltmeter connected across its terminals draws negligible current. Which conclusion is justified?
A coil experiences changing magnetic flux linkage. A high-impedance voltmeter connected across its terminals draws negligible current. Which conclusion is justified?