Magnetism and electromagnetism for NDA

The NDA syllabus lists natural and artificial magnets, the properties of a magnet, and the Earth as a magnet, plus the magnetic effect of current and the mariner's compass in the devices list. In practice, questions spread a little wider, into motors, generators and transformers, because they are the everyday face of electromagnetism.

This chapter is mostly conceptual. You need clean one-line answers: which rule gives the direction of force on a wire, what a commutator does, why a transformer will not work on DC, why power lines carry very high voltages. The only regular numerical is the transformer turns ratio, which takes one line.

Typical question types are:

• properties of magnets and the surest test of magnetism;
• field lines and their properties;
• ferromagnetic, paramagnetic and diamagnetic materials; soft iron versus steel;
• the Earth's magnetism: declination, dip and the compass;
• Oersted's experiment, solenoids and electromagnets;
• the motor effect, Fleming's left-hand rule and the commutator;
• electromagnetic induction, Fleming's right-hand rule and generators;
• transformers, the turns ratio and long-distance transmission.

Magnets and their properties

Natural magnets are pieces of lodestone (magnetite, Fe₃O₄). Artificial magnets are made in useful shapes: bar magnets, horseshoe magnets, compass needles, and electromagnets.

• Attractive property: a magnet attracts iron, and the attraction is strongest at the poles.
• Directive property: a freely suspended magnet comes to rest roughly along the geographic north–south line. The end pointing north is its north pole.
• Like poles repel, unlike poles attract.
• Poles always exist in pairs. Break a magnet into pieces and each piece becomes a complete magnet with its own north and south poles.
• Repulsion is the surest test of magnetism. An unmagnetised iron bar is attracted by either pole of a magnet, so attraction proves nothing; only a magnet can be repelled.
• Induced magnetism: an iron nail near a magnet becomes a temporary magnet itself, which is how a magnet picks up a chain of pins.

Magnetic field lines

• Outside a magnet they run from north to south; inside, from south to north. They form closed loops.
• They never cross, since the field has only one direction at any point.
• They are crowded where the field is strong (near the poles) and spread out where it is weak.

Magnetic materials

Soft iron is easy to magnetise and easy to demagnetise, so it is used for electromagnets and transformer cores. Steel is harder to magnetise but keeps its magnetism, so it is used for permanent magnets; alloys such as alnico make strong ones.

A magnet loses its magnetism if it is heated strongly (above the Curie temperature, about 770 °C for iron), hammered or dropped repeatedly, or placed in a coil carrying AC. Magnets are stored in pairs with soft-iron keepers across the poles to preserve them. A soft-iron ring or box shields whatever is inside from outside magnetic fields.

The Earth as a magnet

The Earth behaves as if a giant bar magnet lay inside it, tilted from the axis of rotation. The field is thought to arise from electric currents in the molten iron of the outer core. Because the north pole of a compass needle points to the geographic north, the Earth's magnetic pole in the northern hemisphere is, in magnetic terms, a south pole.

• Declination: the angle between the geographic meridian (true north) and the magnetic meridian (the direction a compass points). Navigators correct for it.
• Dip (inclination): the angle the Earth's field makes with the horizontal. It is 0° at the magnetic equator and 90° at the magnetic poles. A freely pivoted dip needle stands vertical at a magnetic pole.
• The magnetic poles are not at the geographic poles, and they drift slowly over the years.
• The mariner's compass is a magnetised needle pivoted over a direction card, long used for navigation at sea.
• The Earth's field steers charged particles from the Sun towards the polar regions, where they light up the sky as auroras.

Magnetic effect of current

In 1820 Oersted noticed that a compass needle deflects near a wire carrying current. A current produces a magnetic field.

• Around a straight wire, the field lines are circles. The right-hand thumb rule: grip the wire with the right thumb along the current, and the fingers curl in the direction of the field.
• A long coil (solenoid) carrying current behaves like a bar magnet.
• An electromagnet is a solenoid wound on a soft-iron core. It is magnetic only while current flows. Its strength increases with the current, the number of turns and the presence of a soft-iron core.
• Uses: cranes that lift scrap iron, electric bells, relays, loudspeakers, the old telegraph, MRI scanners and maglev trains.

The motor effect

A current-carrying conductor placed in a magnetic field experiences a force. The force is F = BIL when the wire is perpendicular to the field (B is the field strength, I the current, L the length), and zero when the wire is parallel to the field.

Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand at right angles. Forefinger = Field, seCond finger = Current, thuMb = Motion (force).

The electric motor converts electrical energy into mechanical energy. A coil in a magnetic field feels forces on opposite sides that turn it. A split-ring commutator reverses the current in the coil every half turn, so the turning force always acts the same way and the coil keeps rotating. Fans, mixers, pumps, washing machines and electric vehicles all use motors. Moving-coil meters and loudspeakers work on the same effect.

Electromagnetic induction

In 1831 Faraday showed the reverse of Oersted's effect: a changing magnetic field through a coil induces a current in it. Moving a magnet into a coil, or a coil near a magnet, deflects a galvanometer; holding them still gives nothing.

• The induced current is larger with faster motion, more turns and a stronger magnet.
• Lenz's law: the induced current opposes the change that produces it. This is energy conservation: you must do work to push a magnet into a coil.
• Fleming's right-hand rule gives the direction of the induced current in a moving conductor (same finger scheme on the right hand).

The generator (dynamo) converts mechanical energy into electrical energy by rotating a coil in a magnetic field. An AC generator uses slip rings; a DC generator uses a split-ring commutator. Other uses of induction: induction cooktops, metal detectors at airports, and wireless phone chargers.

Transformers and power transmission

A transformer has two coils, the primary and the secondary, wound on a laminated soft-iron core. The changing current in the primary produces a changing field that induces a voltage in the secondary. It works only on AC, because a steady DC gives no change and so no induction.

Vs/Vp = Ns/Np. A step-up transformer has more turns on the secondary and raises the voltage; a step-down transformer lowers it. In an ideal transformer, power in = power out, so VpIp = VsIs: when the voltage goes up, the current comes down in the same ratio. Laminating the core reduces energy lost to eddy currents.

Why transmit at high voltage? Power lost as heat in the lines is I²R. For the same power, raising the voltage lowers the current, and the loss falls with the square of the current. Doubling the voltage halves the current and cuts the loss to a quarter. So power is stepped up at the station, sent over long lines, and stepped down in stages before it reaches homes.

Worked NDA-style MCQs

Q1. A transformer has 200 turns on the primary and 1,000 on the secondary. With 220 V AC on the primary, the secondary voltage is:
(a) 44 V (b) 1,100 V (c) 220 V (d) 4,400 V

Vs = 220 × 1,000/200 = 1,100 V. It is a step-up transformer. Answer: (b).

Q2. A step-down transformer converts 220 V to 11 V. If the primary has 2,000 turns, the secondary has:
(a) 100 turns (b) 200 turns (c) 40,000 turns (d) 1,100 turns

Ns = 2,000 × 11/220 = 100. Answer: (a).

Q3. An ideal transformer steps up voltage 10 times. The current in the secondary compared with the primary is:
(a) 10 times (b) the same (c) one-tenth (d) 100 times

VpIp = VsIs, so the current falls in the same ratio. Answer: (c).

Q4. A wire 0.5 m long carries 2 A at right angles to a magnetic field of 0.4 T. The force on it is:
(a) 0.4 N (b) 1.6 N (c) 4 N (d) 0

F = BIL = 0.4 × 2 × 0.5 = 0.4 N. Parallel to the field, it would be zero. Answer: (a).

Q5. The angle of dip at the magnetic equator is:
(a) 90° (b) 45° (c) 0° (d) 180°

The field is horizontal there. Answer: (c).

Q6. For the same power, the transmission voltage is doubled. The heat lost in the line becomes:
(a) double (b) half (c) one-quarter (d) four times

Current halves, and loss ∝ I², so it becomes ¼. Answer: (c).

Common mistakes

• Using attraction as proof of magnetism. Only repulsion proves it.
• Swapping Fleming's rules. Left hand for motors (force), right hand for generators (induced current).
• Saying a transformer increases power. It trades voltage for current; power stays the same at best.
• Using steel for an electromagnet's core. Soft iron, because it must lose its magnetism when the current stops.
• Calling the Earth's northern magnetic pole a north pole. Magnetically, it behaves as a south pole.

Practice set

1. Which of these is diamagnetic? (a) iron (b) nickel (c) bismuth (d) cobalt
2. The core of an electromagnet is made of: (a) steel (b) soft iron (c) copper (d) aluminium
3. The direction of force on a current-carrying wire in a magnetic field is given by: (a) Fleming's right-hand rule (b) Fleming's left-hand rule (c) the right-hand thumb rule (d) Lenz's law
4. In a DC motor, the split-ring commutator: (a) increases the current (b) reverses the current in the coil every half turn (c) stores energy (d) produces the magnetic field
5. A transformer works on: (a) DC only (b) AC only (c) both AC and DC (d) static charge
6. The angle of dip at the magnetic poles is: (a) 0° (b) 45° (c) 60° (d) 90°
7. Electromagnetic induction was discovered by: (a) Oersted (b) Faraday (c) Ampere (d) Ohm
8. A permanent magnet loses its magnetism when: (a) cooled (b) heated strongly (c) stored with keepers (d) kept in a wooden box

Answers:

1. (c). Bismuth is weakly repelled by a magnet.
2. (b). Easily magnetised and demagnetised.
3. (b). The right-hand rule is for induced current.
4. (b). So the coil keeps turning the same way.
5. (b). Induction needs a changing current.
6. (d). The field is vertical there.
7. (b). Oersted found the magnetic effect of current.
8. (b). Heating above the Curie temperature destroys it.

What to do next

• Make a three-row table (motor, generator, transformer) with principle, energy change and rule, and say it aloud until it is automatic.
• Solve eight turns-ratio questions and four transmission-loss questions.
• Revise electricity for NDA for the circuits side, then continue to modern physics basics.