Exam practice

In a magnet the molecular magnets are lined up in open chains, all pointing the same way. Each blow of the hammer jolts them out of line. After many blows they point in random directions or form closed chains, their effects cancel, and the magnet loses its magnetism.

A dip needle lines up with the Earth’s magnetic field lines. At the magnetic poles the field lines go straight down into the Earth (or come straight up), at right angles to the ground. So the needle stands vertical, making an angle of 90° with the horizontal.

So that the live, neutral and earth wires can be told apart at a glance. Then switches and fuses can be put in the live wire, each wire can be joined to the right pin of a plug or socket, and if something goes wrong, the damaged wire and its place can be found easily and safely.

The live wire is the one at a high voltage. If the fuse is in the live wire, then when it blows, the appliance is cut off from the live supply and is safe to touch. If it were in the neutral wire, a blown fuse would stop the current, but the appliance would still be joined to the live wire, and touching it could give a shock.

A magnet is demagnetized by:

• i.heating it,
• ii.hammering it or dropping it again and again,
• iii.passing alternating current through a coil around it,
• iv.keeping like poles of two magnets together for a long time,
• v.natural loss over time, especially without keepers.

Heating: when a magnet is heated, its molecular magnets vibrate strongly. Their parallel rows (open chains) break up and they fall back into closed chains or random directions, so their effects cancel and the magnet loses its magnetism.

The molecular theory of magnetism was given by Wilhelm Weber and improved by James Alfred Ewing. It says:

• i.Every magnet and magnetic material is made of tiny molecular magnets, each with a north and a south pole.
• ii.In an unmagnetized magnetic material, the molecular magnets are arranged in closed chains, so their poles cancel and the material shows no magnetism.
• iii.When they are lined up in open chains, all pointing the same way, the material becomes a magnet: free north poles at one end make its N-pole and free south poles at the other its S-pole.
• iv.When all of them are lined up, the magnetism is the greatest possible: magnetic saturation.

Diagram: draw Figure 8.6: (i) closed loops of small arrows for an unmagnetized bar, (ii) rows of arrows all pointing one way, with N at one end and S at the other, for a magnet.

• i.In electric motors, fans, generators and loudspeakers.
• ii.In a magnetic compass, to find directions.
• iii.As electromagnets in cranes, to lift heavy iron and scrap in factories and ports.

(Others: MRI machines in hospitals, computer hard disks, electromagnetic door locks, induction cookers, electric bells.)

The magnetism of the Earth is called terrestrial magnetism (geomagnetism). The Earth behaves like a huge magnet, with a magnetic pole near each geographic pole, because of electric currents in the molten iron and nickel moving in its outer core. Its field makes a freely hung magnet point north–south.

• i.It works the magnetic compass, used for finding directions on ships, submarines and airplanes.
• ii.It shields the Earth from the harmful charged particles of the solar wind, turning them aside or steering them to the polar regions.
• iii.The magnetism frozen into igneous rocks helps scientists estimate their age, and magnetic surveys help find ores of metals.

Bringing the electricity made at a power station into people’s houses through transmission lines, and wiring it safely to the lamps, fans and appliances, is household electrification.

Phase (live) wire: brings the current from the supply; is at a high voltage (about 230 V) compared with the ground; gives a dangerous shock if touched; switches and fuses are put in it; red (older) or brown (newer) in color.

Neutral wire: takes the current back to the supply; is at about zero voltage; the current flows because of the voltage difference between live and neutral; no switch or fuse is put in it; black (older) or blue (newer); must still not be touched.

Magnetic materials that lie in the Earth’s magnetic field for a long time slowly get their molecular magnets lined up along it, and become magnets. Examples:

• i.An iron rod buried in the ground in the north–south direction for a long time becomes magnetized.
• ii.Old iron railings, poles and pillars that have stood for years often turn out to be weak magnets.
• iii.Natural magnets such as lodestone, and the magnetic minerals in igneous rocks, which line up with the Earth’s field as lava cools, are found in the ground.

So these materials, which would otherwise be unmagnetized, are magnetized by the Earth’s field. (Lightning strikes may also magnetize some lodestones strongly.)

• i.Unscrew the cover of the plug.
• ii.Cut away about 3–4 cm of the cable’s outer covering and separate the three wires.
• iii.Identify the live, neutral and earth wires and strip about 1–1.5 cm of insulation from each.
• iv.Twist the bare strands of each wire and bend the tips slightly.
• v.Connect the live wire (red or brown) to the right-hand pin, the neutral (black or blue) to the left-hand pin, and the earth (green, or green-and-yellow) to the top pin, tightening each screw. (Right and left as seen from the back of the plug.)
• vi.Tighten the cord grip over the cable.
• vii.Put the cover back and tighten its screw. Check before use.

Earthing is joining the metal bodies of appliances (and the earth pins of sockets), by a low-resistance earth wire, to a metal plate buried in damp ground, so that any leaking current flows safely into the ground.
It is necessary because if a live wire inside an appliance touches its metal body, the body becomes live. With earthing, a large current flows into the ground at once, the fuse blows or the MCB trips, and the appliance is cut off. This protects people from electric shocks, and protects appliances and the wiring from damage and fire.

Yes. A magnetic substance always has molecular magnets; only their arrangement differs. If they are lined up in open chains (for example, by stroking the substance with a magnet or putting it in a coil carrying current), it becomes a magnet. If they are disturbed back into closed chains or random directions (by heating, hammering or alternating current), it loses its magnetism. So the same piece of iron can be changed from non-magnetized to magnetized and back again. (A non-magnetic substance such as wood or copper has no molecular magnets to arrange, so it can’t be made a magnet.)

Yes. An electric current produces magnetism.
Activity: wind insulated copper wire around an iron nail, and connect its ends to a cell through a switch (Activity 8.2). With the switch on, the nail picks up pins; with it off, they drop. The current in the coil has made the nail a magnet, an electromagnet.
The same link is why alternating current in a coil can demagnetize a magnet (Activity 8.3): the current’s magnetism keeps flipping and jumbles the molecular magnets.
The link works the other way too: moving a magnet near a coil of wire makes a current flow in it. That is how the generators in hydropower stations make electricity.

A month of 30 days is twice as long as 15 days.

Units in 15 days = 75
Units in 30 days = 75 × 2 = 150 units
Bill = units × rate = 150 × Rs 9.50 = Rs 1,425

$P$ = 25 W = 25/1000 kW = 0.025 kW
$N$ = 8
$t$ = 4 h 40 min = 4 + 40/60 h = 4.667 h (exactly 14/3 h)
E.C. = $P \times N \times t$ = 0.025 × 8 × 14/3 = 0.2 × 14/3 = 0.933 units (about 0.93 kWh)

Units in a day = 8.5 ÷ 30 = 0.2833 kWh
$P$ = 65 W = 0.065 kW, $N$ = 1
From E.C. = $P \times N \times t$: $t$ = E.C. ÷ ($P \times N$) = 0.2833 ÷ 0.065 = 4.36 hours a day (about 4 hours 22 minutes)

$P$ = 1,500 W = 1.5 kW, $N$ = 14, $t$ = 6.5 h
Units a day = 1.5 × 14 × 6.5 = 136.5 units
Units a month = 136.5 × 30 = 4,095 units
Bill = 4,095 × Rs 12.50 = Rs 51,187.50