Magnetisation (soft iron):

• Place soft iron inside a strong magnetic field (e.g., inside a coil with current). Domains align and the iron becomes magnetised (temporary magnet).
• Wrap the iron with a coil and pass direct current (DC) through the coil — the iron becomes magnetised (electromagnet).
• Stroking method (for some metals): repeatedly stroke the iron with a strong bar magnet from one end to the other in the same direction — aligns domains.

Demagnetisation (soft iron):

• Remove the magnetising field — soft iron usually loses most of its magnetism quickly (soft = easy to demagnetise).
• Heat above the Curie temperature (very high; not used in class) — destroys magnetism.
• Hammering or strong mechanical shock while in a different magnetic environment — randomises domains.
• AC demagnetisation: place the soft iron in an alternating magnetic field and slowly withdraw as the field amplitude is reduced (practical method to demagnetise tools).

What causes emf? When the magnetic flux through a coil changes (by moving a magnet, moving the coil, changing current), an emf is produced across the coil. If the coil is closed, current flows.

Faraday’s law (simple form): emf = -N (dΦ/dt)

Where: N = number of turns; Φ = magnetic flux; the minus sign is from Lenz’s law (direction opposes change).

Magnetic flux: Φ = B · A · cosθ (B = magnetic field strength, A = area of loop, θ = angle between field and normal to loop)

Aim: To show that moving a magnet into or out of a coil induces a current and that the size of induced emf depends on factors like speed and number of turns.

Materials: copper coil (many turns on a cardboard tube), strong bar magnet, sensitive galvanometer or sensitive multimeter (set to mV), connecting wires, ruler, stand, second coil (optional), soft iron core (optional).

Safety: Handle strong magnets carefully (pinch risk). Do not allow leads to short-circuit instruments. Keep electronic devices away from strong magnets.

Procedure:

1. Connect the coil to the galvanometer. Keep the coil steady on a table or clamp it in a stand.
2. Move the north pole of the bar magnet quickly towards the centre of the coil and observe the galvanometer needle (or multimeter reading). Then withdraw the magnet quickly and observe.
3. Repeat with different speeds (slow, medium, fast) and note the change in deflection.
4. Repeat using more turns of wire (add a second coil or wind more turns) and compare readings.
5. Optionally, insert a soft iron core into the coil and repeat to see change in induced emf.

Observations:

• When magnet is pushed in, galvanometer deflects in one direction; when pulled out, it deflects in the opposite direction.
• Faster motion causes larger deflection (larger induced emf).
• More turns or using a soft iron core increases the induced emf.

Explanation: Motion changes the magnetic flux through the coil; according to Faraday’s law, emf is proportional to the rate of change of flux. Lenz’s law explains the direction of induced current: it opposes the change that produced it.

• Rate of change of magnetic flux: faster change → larger emf (move magnet quicker).
• Number of turns (N): more turns → larger emf (emf ∝ N).
• Magnetic field strength (B): stronger magnet → larger flux change → larger emf.
• Area of coil (A): larger area → more flux → larger emf for same change.
• Angle (θ): if coil is turned so field lines pass less through it, flux is less; orientation matters (Φ = B A cosθ).
• Core material: placing soft iron core increases magnetic flux in coil (increases emf).

Simple classroom example calculation:
If a coil of 50 turns sees flux change from 0.02 Wb to 0 Wb in 0.1 s, emf ≈ N ΔΦ/Δt = 50 × (0.02/0.1) = 50 × 0.2 = 10 V (direction given by Lenz’s law).

• Teacher demonstration of magnet field with iron filings on paper (use smooth paper and gently tap to reveal patterns).
• Students work in groups to repeat the coil + magnet experiment, varying speed, number of turns, and core material; record results in a table.
• Make a simple electromagnet: wrap wire on nail, connect to a battery, test lifting paper clips; remove battery to show demagnetisation (soft iron nail loses magnetism fast).
• Field trip or guest talk: visit a nearby electricity substation, small hydro plant, or invite an engineer to explain generators and transformers.
• Class discussion: list appliances at home that use induced emf and discuss how they help daily life and work.
• Homework: draw and label a magnetic field pattern and explain one local application of induction (2–3 paragraphs).