6 Magnetism and electromagnetism

Read each value as a complete measurement: 2.5 A means a current of 2.5 amperes, 12 V means a voltage of 12 volts, and 60 W means a power of 60 watts. Identify the physical quantity first, then attach its matching unit symbol to the numerical value.

Magnetic substances include iron, steel, nickel and cobalt. A magnet attracts them, but many other materials—including plastic, copper and zinc—are not attracted. Being a metal does not automatically make a substance magnetic.

Choose by function: a permanent bar magnet must keep its magnetism, so a hard magnetic material is suitable. A temporary magnet must switch its magnetism on and off readily, so a soft magnetic material is suitable.

Around a bar magnet, field lines leave the north pole, curve through the surrounding space and enter the south pole. Lines never cross: crossing would assign two different field directions to one point.

Line spacing represents field strength. Closely spaced lines show a stronger field; widely spaced lines show a weaker field. The field around a bar magnet is therefore strongest near its poles, where the lines are densest.

The field induces magnetic poles in the material. The end nearest the inducing magnet becomes the opposite pole, so the two nearest poles attract. For example, beside a magnet's south pole, the nearest end of an iron object becomes a north pole and is pulled toward the magnet.

What remains after the field is removed depends on the material. Magnetically soft iron loses most of the induced magnetism, whereas a magnetically hard material retains much more and can become a permanent magnet.

To reveal shape with iron filings: 1. Place the card over the magnet arrangement. 2. Sprinkle a thin, even layer of filings. 3. Tap the card gently so the filings align with the field. The curved or connecting bands show the field pattern, but filings alone do not show its direction.

To map direction with a plotting compass: 1. Place the compass at a chosen starting point and mark the direction of its north-pointing end. 2. Move the compass so its tail is at the previous mark and mark again. 3. Join the marks with a smooth line and add an arrow. 4. Repeat from several starting points to build the pattern.

Place two permanent magnets with unlike poles facing each other across a small gap: north faces south, and the poles are close but not touching. In the central part of the gap, field lines run straight from N to S and are nearly equally spaced, producing an approximately uniform field.

Use three checks together: straight lines show an unchanging direction, parallel lines show that direction is shared across the region, and equal spacing shows constant field strength. Lines curve near the outer edges, so the most uniform region is between the central pole faces.

A plotting compass placed near a wire changes direction when the wire is switched on because the compass needle aligns with the new magnetic field. This provides evidence for the field even though the field itself is not visible.

Reversing the current reverses the direction of the magnetic field. Increasing the current makes the field stronger, while the field becomes weaker farther from the conductor. The exact line patterns for different conductor shapes are developed next.

Construction: 1. Wind many turns of insulated wire in one direction around an iron rod. 2. Connect the wire ends to a d.c. supply, with a switch in series. 3. Close the switch so current flows through the coil and creates a magnetic field that magnetises the core.

Opening the switch stops the current, so the coil's field disappears and the soft iron loses most of its magnetism. This makes the device a controllable magnet rather than a permanently magnetised bar.

Outside a coil or solenoid, field arrows leave its north end and return to its south end. Reversing the current reverses every field arrow and swaps the north and south ends, but the basic field-line shape remains the same.

The force acts at right angles to both the particle's velocity and the magnetic field. It therefore changes the direction of motion rather than directly changing the speed; for perpendicular motion in a uniform field, the continually turning force can produce a circular path.

The reusable chain is: current creates a field → the two fields interact → the conductor experiences a force → the attached component moves. In a motor the component rotates; in a loudspeaker it oscillates.

Method: 1. Align the first finger with the field. 2. Rotate the hand while keeping that alignment until the second finger follows the current. 3. Read the thumb for the force. A dot means a direction out of the page; a cross means into the page.

Orientation also matters: the force is greatest when current and field are perpendicular and zero when they are parallel. Moving stronger magnet poles closer can increase field strength; increasing the supply voltage may increase current if the circuit resistance is unchanged.

Use the causal chain: relative motion or changing field → the conductor cuts magnetic field lines, or the field linkage through the coil changes → a voltage is induced. A current flows only if the conductor is part of a complete circuit; an induced voltage can still exist across an open circuit.

As the coil or magnet rotates, the field linkage changes in one direction and then the other. The induced voltage therefore reverses every half-turn, producing an alternating voltage. If an external circuit is complete, an alternating current flows.

An alternating voltage drives an alternating current in the primary coil → the primary creates a changing magnetic field in the soft iron core → the changing field passes through the secondary coil → a voltage is induced across the secondary coil.

For approximately the same transmitted power, increasing voltage allows a smaller current. A smaller current causes less heating in transmission cables, so less electrical energy is wasted and more reaches consumers.

Vp and Vs are the primary and secondary voltages; Np and Ns are the numbers of turns on the primary and secondary coils.

Method: 1. Write the ratio with primary values on top and secondary values below. 2. Substitute voltages in the same unit. 3. Rearrange before evaluating. 4. Check the direction: if Ns > Np, then Vs must be greater than Vp; if Ns < Np, then Vs must be smaller.

Example: a transformer has 160 primary turns, 45 secondary turns and a 12 V primary supply. Vs = 12 × 45 ÷ 160 = 3.375 V, so the output is about 3.4 V. This is a step-down transformer because the secondary has fewer turns and a lower voltage.

VpIp is the primary input power and VsIs is the secondary output power. Voltage is in volts, current is in amperes and each product is in watts.

Because the two powers are equal, stepping voltage up makes current step down, and stepping voltage down makes current step up. This inverse change explains how a high transmission voltage can carry the same power with a smaller current.

Example: an ideal transformer takes 230 V and 4.5 A, and supplies 0.21 A. Vs = 230 × 4.5 ÷ 0.21 = 4928.6 V, so the output voltage is about 4.9 kV. The higher output voltage is consistent with the lower output current.